Chapter 8 - Head and Neck 2015 (5)

N. = Plate Reference in Netter, F.H., Atlas of Human Anatomy, 6th ed., Saunders/Elsevier, 2014 CD = The Gross Anatomy Laboratory Assistant CD-ROM

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8. HEAD AND NECK

THE SCALP AND CALVARIA

SCALP From the surface to the bones of the cranial vault five layers of the scalp can be identified (N. 103 top)

Dense Sub Galea

SCALP

kin utaneous tissue poneurotica oose connective tissue eriosteum

Since the scalp is commonly subjected to trauma, some of its characteristics are of special interest. The skin is firmly bound down to the galea aponeurotica by dense connective tissue septa within the subcutaneous tissue. The subcutaneous tissue contains the major blood vessels and nerves of the scalp. The dense connective tissue septa tend to hold the blood vessels open when they are lacerated so that bleeding is usually profuse. Further, the denseness of the subcutaneous tissue inhibits the injection and diffusion of local anesthetic within this layer for the repair of lacerations. The major arteries and veins entering the scalp from below include the supraorbital vessels anteriorly, the superficial temporal and posterior auricular vessels laterally, and the occipital vessels posteriorly (N. 3). These are usually accompanied by nerves including anteriorly the supraorbital and supratrochlear nerves (from the ophthalmic division of the trigeminal nerve); laterally the zygomatic nerves (from the maxillary division of the trigeminal nerve), auriculotemporal nerve (from the mandibular division of the trigeminal nerve) and lesser occipital nerve (from the cervical plexus); and posteriorly the dorsal rami of C2 (greater occipital nerve) and C3 spinal nerves (N. 2). An appreciation of the location of these neurovascular structures permits local anesthetic block and pressure point control of bleeding in scalp laceration. The galea aponeurotica is an intermediate tendon between the frontal and occipital bellies of the epicranius muscle (N. 25). It largely overlies the parietal bones. Between the galea and the periosteum of the cranial vault there is a very loose connective tissue interval where fluid accumulation can be extensive (N. 103 top), e.g., the huge hematomas or edematous fluid accumulations that can occur in the newborn as a result of birth trauma. This interval also provides a good fascial plane for raising neurosurgical scalp flaps. SUPERIOR ASPECT OF THE SKULL The superior part of the skull serves as the roof of the cranial cavity and is called the calvaria or cranial vault (N. 9, 103). The relatively flat bones which form it are trilaminar with outer and inner tables of compact bone and an intermediate diploe containing spongy bone and blood producing marrow. There are large valveless diploic veins running within the diploe (N. 101, 103). These drain both internally into dural venous sinuses and externally into scalp veins. These veins may serve to transmit infection in either direction and they are at times normally visible as radiographic lucencies. The calvaria (N. 9, 103) is formed anteriorly by the unpaired frontal bone although occasionally a midsagittally placed frontal or metopic suture may be present as an embryonic remnant of the line of fusion of the two laterally placed ossification centers of this bone (N. 14). The frontal bone articulates behind with the paired parietal bones at the coronal suture. The paired parietal bones articulate with each other midsagittally to form the sagittal suture. Posteriorly the parietal bones articulate with the unpaired occipital bone to form an L-shaped lambdoidal suture. In the fetus and newborn (N. 14) the point of intersection of the coronal, sagittal and frontal sutures is not completely ossified by the peripherally expanding paired ossification centers of the frontal and parietal


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bones. This leaves a large diamond-shaped "soft spot" called the anterior fontanelle which is normally closed to palpation by around one year of age. Likewise, at the intersection of the sagittal and lambdoidal sutures the paired parietal bone ossification centers fail to meet the single ossification center of the superior (squamous) part of the occipital bone, thereby forming a smaller triangle-shaped soft spot," the posterior fontanelle, which is normally closed to palpation shortly after birth. During delivery of the fetus the obstetrician can palpate the fontanelles and use their shape to determine the position of the fetal head in the birth canal, thereby providing information that can predict the ease of delivery and the type of delivery assist steps which must be taken. The fontanelles have postnatal diagnostic significance to the pediatrician, for their routine palpation provides information concerning the cerebrospinal fluid pressure of the infant and the normalcy of the infant's local and general bone development. The inner aspect of the calvaria shows a midsagittally running groove which enlarges as it courses posteriorly (N. 9). This is the groove for the superior sagittal dural venous sinus and the edges of the groove serve for the upper attachment of the dural septum called the falx cerebri. The inner aspect of the calvaria also shows grooves occupied by branches of the middle meningeal arteries. In operatively opening the calvaria (trephine) the position of the underlying brain, dural venous sinuses and middle meningeal arteries must be taken into consideration.

CRANIAL CAVITY In describing the cranial cavity and its contents a major emphasis will be placed upon the relationships of the brain and cranial nerves (1) to each other, (2) to the bony cranium and its openings (3) to the meningeal and vascular structures within the cranial cavity and (4) to the extracranial sources of encroachment upon these intracranial neural structures. An appreciation of these relationships is necessary to understand the signs and symptoms produced by impingement upon the brain and cranial nerves by intracranial and extracranial disease processes. SUPERIOR VIEW OF THE BASE OF THE SKULL (FLOOR OF THE CRANIAL CAVITY) The bones forming the skull are often divided into those forming the facial skeleton and those that form the skeletal housing for the brain, sometimes described as the cranium. However, no rigid distinction can be made between cranial and facial bones, since many bones contribute to both the walls of the cranial (brain) cavity and the facial skeleton. The floor of the cranial cavity is divided into three cranial fossae separated by prominent ridges (N. 8, 11, 13). The anterior cranial fossa is separated from the middle cranial fossa medially by the anterior margin of the prechiasmatic groove of the sphenoid bone and laterally by the lesser wings of the sphenoid. The middle cranial fossa is separated from the posterior cranial fossa medially by the dorsum sellae of the sphenoid bone and laterally the superior margins of the petrous portion of the temporal bone. ANTERIOR CRANIAL FOSSA (N. 5, 7-8, 11, 13; CD H&N Sections 1 to 3; CD Normal Skull Radiographs 1 to 4) The frontal bone has a relatively flat vertical portion underlying the forehead region, called its squama, and a horizontal portion forming the roof of the orbit called its orbital part. The anterior cranial fossa is largely formed by the orbital part of the frontal bone. Medially the sieve-like cribriform plate of the ethmoid bone with its sagittal vertical protrusion, the crista galli, forms part of the fossa. Posteriorly, the fossa is completed medially by the anterior part of the body of the sphenoid bone and laterally by the lesser wings of the sphenoid bone. The anterior cranial fossa contains the anterior portion of the frontal lobes of the brain and the olfactory tracts, bulbs and nerve rootlets (N. 106-108). The cribriform plate of the ethmoid bone forms bilateral olfactory grooves for the olfactory tracts and bulbs, and the perforations in this plate transmit


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numerous olfactory nerve rootlets from the olfactory bulbs down into the nasal cavity (N. 39). The anterior and posterior ethmoidal arteries (from the ophthalmic artery) and nerves (from the ophthalmic division of the trigeminal nerve) pass across the olfactory groove on their way from the orbit to the nasal cavity. At this point the anterior ethmoidal artery gives off its anterior meningeal artery branch (N. 102). One of the important extracranial relationships of the anterior cranial fossa is the nasal cavity lying immediately beneath the cribriform plate of the ethmoid bone (N. 8, 43, 102). The ethmoid paranasal air sinuses are located on either side of the olfactory grooves (N. 43-44). Lateral to this the frontal paranasal air sinus extends a variable distance back into the orbital part of the frontal bone (N. 43-44). Posteriorly the sphenoid paranasal air sinuses lie within the body of the sphenoid bone (N. 44). The orbit lies beneath both the orbital part of the frontal bone and the lesser wing of the sphenoid bone (N. 4, 43). All of these extracranial regions may be sites of tumors or infections which can erode the intervening bone to impinge upon the olfactory and frontal lobe contents of this fossa. Further, anterior cranial fossa fractures with meningeal laceration can cause leakage of blood (from the anterior meningeal arteries) and/or cerebrospinal fluid into the nasal cavity directly, or indirectly through the paranasal sinuses (cerebrospinal fluid rhinorrhea). Bleeding into the loose tissues of the orbit can also produce a black eye. The findings of cerebrospinal fluid rhinorrhea and a black eye when coupled with loss of smell (olfactory bulb, tract or rootlet lacerations) and the emotional changes of frontal lobe involvement can be useful diagnostic indicators of a fracture of the anterior cranial fossa. MIDDLE CRANIAL FOSSA (N. 7-8, 11, 13; CD H&N Sections 4, 5; Normal Skull Radiographs 1 to 4) The middle cranial fossa may be divided into three parts: a central and two lateral portions (N. 11, 13) . The sphenoid and temporal bones form the parts of this fossa. The sphenoid bone is a multiwinged bone composed of a body, greater and lesser wings, and pterygoid (=winged) processes. The CENTRAL PORTION OF THE MIDDLE CRANIAL FOSSA is formed by the major part of the body of the sphenoid bone which has an overall surface contour much like a turkish saddle and is therefore called the sella turcica. Anteriorly there is a transversely placed prechiasmatic groove, so named because it is anterior to the optic chiasm. The prechiasmatic groove leads laterally to the optic canal which transmits the optic nerve and ophthalmic artery into the orbit. Where the lesser wings of the sphenoid join the body lateral to the optic canal there arises a posteriorly projecting anterior clinoid process to which the free edge of the tentorium cerebelli dural septum is attached most anteriorly. The internal carotid artery emerges from the cavernous venous sinus into the subarachnoid space immediately medial to the anterior clinoid process (N. 105). At this point the anterior clinoid process tends to hold the internal carotid artery tightly against the optic chiasm so that any aneurysmal dilation of the artery will tend to affect the visual pathway. Behind the prechiasmatic groove a shallow concavity, the hypophyseal fossa, supports the inferior surface of the hypophysis. Posterior to this fossa the platelike dorsum sellae (=dorsum of the saddle, i.e., like the board on the back of an arabian or turkish saddle) rests against the hypophysis (N. 11, 105, 107 top). It is commonly eroded by hypophyseal tumors. A small posterior clinoid process protrudes from each lateral corner of the dorsum sellae. These processes serve as additional attachment for the tentorium cerebellii. The sphenoid paranasal air sinus occupies much of the interior of the body of the sphenoid and its disease processes may encroach superiorly upon the hypophysis or optic chiasm (N. 105 top). Hypophyseal tumors can be removed most safely via a nasal cavity-transphenoidal approach. Each LATERAL PORTION OF THE MIDDLE CRANIAL FOSSA is formed anteriorly by the greater wing of the sphenoid bone, laterally by the squamous part of the temporal bone, posteriorly by the anterior surface of the petrous part (pyramid) of the temporal bone and medially by the lateral aspect of the sphenoid body (N. 11). Each lateral part of the middle cranial fossa is largely filled by the anterior portion of the temporal lobe of the brain, has the cavernous venous sinus in its medial portion and is perforated by a number of important apertures. Anteriorly the superior orbital fissure lies between the


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lesser and greater wings of the sphenoid bone (N. 11, 13). It transmits the oculomotor, trochlear and abducens nerves, and the branches of the ophthalmic division of the trigeminal nerve into the orbit; it conveys the superior ophthalmic vein from the orbit to the cavernous venous sinus (N. 105). Just inferior to the medial end of the superior orbital fissure a round aperture, the foramen rotundum, conducts the maxillary division of the trigeminal nerve to the pterygopalatine fossa (described with the nasal cavity). Posterior and a little lateral to the foramen rotundum lies an oval aperture, the foramen ovale. This transmits the mandibular division of the trigeminal nerve into the infratemporal fossa. Posterolateral to the foramen ovale a small opening, the foramen spinosum, conveys the major meningeal artery, the middle meningeal artery, into the cranial cavity. The grooves for the frontal and parietal branches of the middle meningeal artery can be followed from the foramen spinosum onto the internal surface of the calvaria (N. 97). Posterior to the foramen ovale there is usually a dehiscence in the roof of the anteromedially directed carotid canal. This canal transmits the internal carotid artery and its surrounding sympathetic plexus from the upper end of the carotid sheath into the cavernous venous sinus. On the anterior face of the petrous pyramid in the area of the dehiscence in the roof of the carotid canal there is a depression which will just lodge the tip of one's finger. This is the impression of the trigeminal ganglion (N. 105). At this point the trigeminal ganglion is exposed to the pulsations of the immediately adjacent internal carotid artery which can be a cause of trigeminal neuralgia (discussed with the face). The superior margin of the petrous pyramid is grooved for the superior petrosal dural venous sinus (N. 105) and serves as an anterior attachment for the tentorium cerebelli. The important extracranial relations of the lateral portions of the middle cranial fossa are the orbit anteriorly, the sphenoid paranasal air sinus medially, and the middle ear cavity located posteriorly within the petrous portion of the temporal bone. From these sites tumors and infection can encroach upon the temporal lobe, or upon the third through sixth cranial nerves, which traverse this fossa mostly within the cavernous sinus. POSTERIOR CRANIAL FOSSA (N. 7-8, 11, 13; CD H&N Sections 5 to 7; CD Normal Skull Radiographs 1 to 4) Anterior to the foramen magnum the medial portion of the posterior cranial fossa is formed by the posterior aspect of the dorsum sellae, posterior portion of the body of the sphenoid and the basilar part of the occipital bone (N. 8, 11, 13). Collectively, these structures form an inclined plane called the clivus which extends from the dorsum sellae to the foramen magnum. Anterolaterally, the posterior cranial fossa is formed by the posterior surface of the petrous pyramid of the temporal bone. The rest of the fossa lateral and posterior to the foramen magnum is formed by the broad curved squamous parts of the occipital bone as high as the grooves for the transverse dural venous sinuses (N. 8, 11, 13, 105). The tentorium cerebelli gains its posterior attachment to the edges of these grooves. . From below upward the medulla, pons and midbrain rest against the clivus (N. 107). The midbrain is located at about the level of the dorsum sellae within the incisure of the tentorium cerebelli. The areas lateral to and behind the foramen magnum are filled by the cerebellum. All of the cranial nerves except the olfactory and optic nerves emerge from the brain stem within the posterior cranial fossa.The tentorium cerebelli roofs over the posterior cranial fossa and partially separates its contents from the overlying occipital lobes and the posterior part of the temporal lobes of the brain (N. 105, 107). The last six cranial nerves emerge from the cranial cavity through three paired apertures in the posterior fossa. On the posterior surface of the petrous pyramid there is a laterally directed cul-de-sac, the internal acoustic meatus (N. 13). It conveys the facial and vestibulocochlear nerves and the labyrinthine artery into the interior of the temporal bone (N. 105). Between the petrous portion of the temporal bone and the occipital bones is the large irregular jugular foramen. This transmits the sigmoid dural venous sinus (seen grooving the bone lateral to the foramen) and the inferior petrosal dural venous sinus (seen grooving the petrooccipital fissure anteromedial to the foramen) into the beginning of the


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internal jugular vein. It also conducts the glossopharyngeal, vagus and accessory nerves into the neck. In the anterolateral margin of the foramen magnum the hypoglossal canal conveys the hypoglossal nerve into the upper neck region. The large unpaired foramen magnum admits into the cranial cavity the spinal cord and its meningeal coverings, the spinal part of the accessory nerve, the vertebral artery, and connections between the internal vertebral venous plexuses and the dural venous sinuses (N. 105, 107). It also transmits the odontooccipital ligaments and the anterior and posterior spinal arteries. The important extracranial relations of the posterior cranial fossa are to the mastoid and the middle and inner ear structures within the petrous pyramid. The mastoid air cells and the middle ear are potential sources of posterior cranial fossa abscesses. These can produce symptoms by encroaching on the medulla, pons, midbrain, cerebellum and their associated cranial nerves. CRANIAL MENINGES AND MENINGEAL VESSELS The brain and spinal cord are surrounded by three membranes called the meninges. From without they are the tough fibrous dura mater which is attached to the inner aspect of the bones forming the cranial cavity, the thin delicate arachnoid which is in contact with dura and the highly vascularized pia mater which is attached to the brain and spinal cord. DURA MATER (N. 101-105) The spinal dura (see page 34) is separated from the periosteum by an epidural space containing fat and the internal vertebral venous plexus. However, at the foramen magnum the spinal dura fuses with the periosteum lining the cranial cavity so that the normal spinal epidural space is eliminated at this point (N. 104, 107). Therefore, the cranial dura serves as the periosteal lining of the cranial cavity and it is sometimes described as having two layers: an inner meningeal layer and an outer periosteal layer. However, these layers are firmly bound together and are only separable in the areas of dural venous sinuses and dural septa (N. 101, 103). Generally the dura has only a moderate attachment to the overlying bone. So it can be separated from the overlying bone by hemorrhage from the meningeal vessels lying within the dura, such as occurs secondary to a skull fracture which crosses these vessels. This is called an epidural hematoma, but it is important to recognize that such a hematoma is not formed within a preexisting epidural space (N. 101, 103, 107). Hence, this hematoma and its signs and symptoms develop slowly as the dura is gradually stripped away from the bone by the pressure of the accumulating blood. DURAL SEPTA (N. 101, 103-105) In certain regions the meningeal layer of the dura leaves the periosteal layer to form a reduplication on itself in the form of septa which project between parts of the brain and therefore partially subdivide the cranial cavity. Dural septa act as internal extensions of the skull and provide additional support for some of the major subdivisions of the brain. There are four dural septa: the falx cerebri, tentorium cerebelli, falx cerebelli and diaphragma sellae. The falx cerebri is a sickle-shaped septum suspended from the inner aspect of the calvaria in the midsagittal plane. It is attached anteriorly to the crista galli of the ethmoid and posteriorly to the tentorium cerebelli. It extends deeply into the longitudinal fissure between the cerebral hemispheres. The tentorium cerebelli is a transverse septum that is tented up in the midline. It extends into the horizontal fissure between the superior surface of the cerebellum below and the occipital and posterior temporal lobes of the cerebrum above (N. 107). It is attached posteriorly to the edges of the sulcus for the transverse sinus and anterolaterally to the superior margin of the petrous pyramid. It is broadly notched anteromedially to form the tentorial notch (incisure) which surrounds the midbrain. The free edge of the tentorial notch attaches to the posterior clinoid process and can often be traced forward to the anterior clinoid process.


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Intracranial space taking lesions like tumors, abscesses, hematomas or obstruction of the upper ventricular system of the brain can displace parts of the brain against these dural septae and thereby produce signs of encroachment of neural structures located at a distance from the primary lesion. If such a space taking lesion is located supratentorially it will produce increased supratentorial pressure. This can cause the inferomedial corner of the temporal lobe (the temporal uncus) (N. 107-108), which rests above the tentorial notch, to herniate down between the notch and midbrain and compress the oculomotor nerve as it enters the dura above the posterior clinoid process. It is this compression of the oculomotor nerve which causes the classic fixed dilated pupil (see oculomotor nerve) of increased supratentorial pressure. There is a small falx cerebelli midsagittally located behind the foramen magnum. This extends between the cerebellar hemispheres on their posteroinferior surface. About the hypophyseal stalk the dura is purse-stringed to produce a diaphragma sellae above the hypophysis as it rests in the hypophyseal fossa part of the sella turcica. In the region of the trigeminal ganglion impression on the anterior surface of the petrous pyramid the dura forms a pocket-like evagination around the upper part of the trigeminal ganglion called the trigeminal (Meckel's) cave. DURAL VENOUS SINUSES (N. 101, 103-105) Dural venous sinuses are most commonly located either at the attached margins of the dural septa, i.e., where the meningeal layer of the dura reflects away from periosteal layer, or at the free edge of dural septa, i.e., where the meningeal layer of the dura reflects back on itself. Hence, these sinuses are intradural and have as walls only their endothelial lining and the surrounding dural connective tissue. Where the dural venous sinuses are located between the periosteal and meningeal layers of the cranial dura they are in a position homologous to the position of the internal vertebral venous plexus between the spinal dura and the periosteum lining the vertebral canal. The dural venous sinuses receive the venous drainage from the brain via cerebral veins which drain into the most closely adjacent venous sinus. They also receive the venous drainage from the meninges through the meningeal veins and from the diploic marrow via the diploic veins. They establish connections with extracranial veins through emissary veins. Emissary veins communicate between extracranial veins and the intracranial dural venous sinuses. While flow in these veins is usually from intracranial to extracranial any increase in thoracic cavity pressure, as in coughing or straining, may reverse their flow since neither these veins nor the dural venous sinuses have valves. Therefore, infection can spread from the extracranial veins to the intracranial venous sinuses via the emissary veins. The superior sagittal sinus is located along the attached margin of the falx cerebri to the inner aspect of the calvaria. It receives superior cerebral veins from the adjacent cerebral hemispheres and conveys blood posteriorly toward the confluence of sinuses. The confluence of sinuses is located at the posterior extent of the attachment of the falx cerebri to the tentorium cerebelli and, as its name implies, serves as a potential confluence site for several sinuses. The inferior sagittal sinus is located in the inferior free margin of the falx cerebri. At its posterior end it is joined by the great cerebral vein (of Galen), which drains the interior of the brain, to form the straight sinus. The straight sinus is located along the line of attachment of the falx cerebri to the tentorium cerebelli. It conducts its blood posteriorly to the confluence of sinuses. A small occipital sinus is located in the attached margin of the falx cerebelli. It begins around the foramen magnum where communications are established with the internal vertebral venous plexuses, through which abdominopelvic cancer can be transmitted directly to the cranial cavity (see page 34). The occipital sinus drains its blood superiorly into the confluence of sinuses. While there is much variability within the confluence of sinuses, most commonly the blood in the superior sagittal sinus is diverted into the right transverse sinus and that in the straight sinus into the left transverse sinus. The paired transverse sinuses are located within the posterior attachment of the tentorium cerebelli. They direct blood laterally and forward toward the lateral end of the petrous pyramid where they become continuous


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with the sigmoid sinuses. At this point the paired sigmoid sinuses also receive the superior petrosal dural venous sinuses and then pursue a gentle S-shaped course along the inferior aspect of the posterior surface of the petrous pyramid to empty through the jugular foramen into the upper end of the internal jugular vein. The paired cavernous sinuses are formed on either side of the body of the sphenoid bone and hypophysis by a separation of the meningeal and periosteal layers of the dura (N. 105). Each is subdivided into many interconnecting channels by dense dural connective tissue bundles. From superior to inferior its lateral wall contains the oculomotor and trochlear nerves and the ophthalmic and maxillary divisions of the trigeminal nerve. The internal carotid artery and its surrounding sympathetic plexus run through the interior of this sinus, as does the abducens nerve which lies lateral to the artery. This sinus receives the superior ophthalmic veins from the orbit via the superior orbital fissure. The right and left cavernous sinuses are connected together by intercavernous sinuses which lie in the free margin of the diaphragma sella. From the cavernous sinus blood may enter either the superior or inferior petrosal sinuses. The superior petrosal sinus courses along the superior margin of the petrous pyramid to the beginning of the sigmoid sinus, while the inferior petrosal sinus runs along the petrooccipital fissure to the jugular foramen, through which it empties directly into the internal jugular vein. The cavernous sinus and its contained neurovascular structures may be encroached upon by laterally expanding hypophyseal tumors, by tumors or infections arising within the adjacent sphenoid paranasal air sinus, by temporal lobe tumors or abscesses, or by an infectious thrombosis of this sinus which has emissary vein connections with many areas that have a high incidence of infection. For example, the superior ophthalmic veins communicate anteriorly with the angular vein of the face (see page 243) so that infection from a pimple around or below the eye can drain back into the cavernous sinus. The labyrinthine character of the cavernous sinus and the slow percolation of blood through it predispose to infectious thrombosis of this sinus. The structures of the cavernous sinus can also be encroached upon by an aneurysmal dilation of the intracavernous portion of the internal carotid artery. Whether the cavernous sinus is encroached upon from within or without, the signs which point the physician toward the cavernous sinus are signs of increased venous pressure in the organs which normally drain into the cavernous sinus (e.g., swollen orbital structures and dilated retinal veins) and signs of involvement of the oculomotor, trochlear, and abducens nerves, the ophthalmic and maxillary divisions of the trigeminal nerve and the orbital sympathetics. MENINGEAL ARTERIES (N. 102-103, 105) The dura also contains the meningeal arteries, the most widely distributed of which are the middle meningeal arteries. The middle meningeal arteries are branches of the maxillary artery. They enter the middle cranial fossa through the foramen spinosum. Within a short distance they divide into frontal and parietal branches, which distribute respectively to anterior and posterior regions of the calvaria. The small anterior meningeal arteries supply the medial portion of the anterior cranial fossa dura. They arise from the anterior ethmoidal branches of the ophthalmic artery. The dura of the posterior cranial fossa is supplied by small posterior meningeal arteries which arise from the occipital and ascending pharyngeal branches of the external carotid artery and the vertebral arteries. Lacerations of these meningeal arteries by skull fracture can cause posterior cranial fossa epidural hematoma (N. 102). ARACHNOID AND PIA (N. 101, 103, 110) The avascular arachnoid is a thin delicate membrane which is typically held into contact with the dura mater by the pressure of the cerebrospinal fluid in the subarachnoid space. The dura and arachnoid are separated by only a thin layer of fluid. The real but very narrow space between them is the subdural space and it is analogous to the space between the pages of a closed book (N. 101, 103) However, in head trauma blood can accumulate in the subdural space to form a subdural hematoma. A subdural hematoma is commonly caused by laceration of the bridging cerebral veins which must penetrate all the meninges to reach the dural venous sinuses (N. 101, 103). It can also be caused by laceration of the meningeal vessels.


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The pia mater is adherent to the brain and dips into all of its sulci and fissures. It contains the branches of the major arteries and veins of the brain. The subarachnoid space is the interval between the arachnoid and pia. It is interlaced by many very fine trabeculae that extend between these membranes. It is filled with cerebrospinal fluid which functions as a water cushion about the central nervous system and normally maintains a constant pressure within the closed bony containers of the cranial cavity and spinal canal. Cerebrospinal fluid is produced by the choroid plexuses of the ventricles of the brain and enters the subarachnoid space through apertures in the fourth ventricle (N. 110). It can circulate freely through the subarachnoid space about the brain and spinal cord and is mostly reabsorbed into the superior sagittal sinus through the arachnoid granulations. The arachnoid granulations are evaginations of the arachnoid which protrude into lateral outpouchings of the superior sagittal sinus, called venous lacunae (N. 103, 110). At this point the wall between the cerebrospinal fluid and the venous system is thinned out for the reabsorptive process. In areas where the pia dips into deep fissures between major parts of the brain, while the arachnoid remains in contact with the dura, the subarachnoid space is enlarged to form subarachnoid cisterns. The largest of these is the cerebellomedullary cistern (cisterna magna) which is located in the interval between the inferior aspect of the cerebellum and the posterior surface of the medulla (N. 110). It is into this cistern that the cerebrospinal fluid exiting from the fourth ventricle passes. These cisterns can be visualized by CT or MRI. The cerebrospinal fluid is normally clear and colorless with a low protein and cellular content and a recumbent pressure of about 100 ml. of water. It can be withdrawn for diagnostic analysis at spinal cord levels by lumbar puncture or occasionally by tapping the cerebellomedullary cistern through the foramen magnum. If the fluid is bloody it indicates a subarachnoid hemorrhage which may be caused, for example, by a bleeding aneurysm involving one of the arteries of the base of the brain that protrude into the subarachnoid space. Increased white blood cells indicate infection and increased protein can be a sign of tumor. An increase in pressure can be caused by anything which takes up space within the closed bony containers of the central nervous system, e.g. tumor, abscess, hemorrhage (epidural, subdural, subarachnoid or intracerebral) or hydrocephalus (dilation of the ventricles of the brain by blockage of the ventricular system). MAJOR TOPOGRAPHIC FEATURES OF THE BRAIN (N. 106-108, 115-116, 151) The brain is made up of five major levels: medulla, pons and cerebellum, midbrain, diencephalon, and cerebrum or cerebral hemispheres (N. 107). If the cerebral hemispheres and cerebellum are removed the remaining medulla, pons, midbrain and diencephalon are called the brain stem (N. 115-116). The lowest level of the brain is the relatively cylindrical medulla or medulla oblongata. Its caudal boundary with the spinal cord is defined either by the level of the foramen magnum or by the plane across the interval between the uppermost emerging rootlet of C1 spinal nerve and the lowest rootlet of the hypoglossal (XII) cranial nerve. Its rostral boundary with the pons is marked by a transversely situated inferior pontine sulcus. The medulla has two major ventral surface elevations. The paramedian longitudinal elevations on either side of a ventral median sulcus are the pyramids. Just lateral to the pyramids the elliptical elevations of the olives (olivary eminences) are visualized. Each olive is demarcated anteriorly by a preolivary sulcus and posteriorly by a postolivary sulcus. Multiple hypoglossal (XII) nerve rootlets emerge from each preolivary sulcus (N. 115). The multiple rootlets emerging from the postolivary sulcus will aggregate to form, from above downward, the glossopharyngeal (IX), vagus (X), and cranial root of the accessory (XI) nerves. These can only be approximately identified by relative location in an isolated brain where the aggregation of rootlets into the definitive nerves is not usually visualized. The important generalization to be recognized about the medulla is that the last four cranial nerves originate or terminate there, and medulla lesions will be characterized by deficits involving one or more of these nerves. Hence, CRANIAL NERVES IX, X, CRANIAL XI AND XII ARE THE “LESION-LEVEL-LOCATORS” OF THE MEDULLA.


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The base of the pons (= bridge) is visualized as a transverse elevation connecting the two cerebellar hemispheres. It is separable caudally from the medulla by the inferior pontine sulcus and cranially from the midbrain by the superior pontine sulcus. The middle four cranial nerves have their nuclei of origin or termination mostly within the pons. The abducens (VI) nerve emerges from the inferior pontine sulcus in line with the preolivary sulcus of the medulla (N. 115). The facial (VII) and vestibulocochlear (VIII) nerves emerge from the inferior pontine sulcus in line with the postolivary sulcus of the medulla, with the facial nerve just medial to the vestibulocochlear nerve. At this point they lie in the angle between the lateral part of the pons and the inferior aspect of the cerebellum, the cerebellopontine angle. The trigeminal (V) nerve emerges from the lateral aspect of the midpons base, where the pons is connected to the cerebellum by the middle cerebellar peduncles. So CRANIAL NERVES VI THROUGH VIII ARE THE “LESION-LEVEL-LOCATORS” OF THE LOW PONS and CRANIAL NERVE V IS THE “LESION-LEVEL-LOCATOR” OF THE MIDPONS. Ventrally the midbrain is demarcated from the pons below by the superior pontine sulcus and from the diencephalon above by a plane through the posterior border of the small paired rounded mammillary bodies (N. 108, 115-116). The ventral midbrain presents two large diverging cerebral peduncles which disappear into the cerebral hemispheres above. Between the cerebral peduncles there is an interpeduncular fossa where the oculomotor (III) nerve emerges from the medial aspect of each peduncle (N. 115). The dorsal aspect of the midbrain contains four rounded elevations called the corpora quadrigemina. They are the paired superior and inferior colliculi (N. 115 top, 116 top). The trochlear (IV) nerve is the only cranial nerve to emerge from the dorsal aspect of the brain stem where it emerges just below the inferior colliculus. CRANIAL NERVES III AND IV ARE THE “LESION-LEVEL-LOCATORS” OF THE MIDBRAIN. Except for its ventral aspect, the diencephalon is largely hidden from surface visualization by the huge overlying cerebral hemispheres. The ventral part of the diencephalon that is visualizable includes from posterior to anterior the mammillary bodies, hypophyseal stalk and optic chiasm (N. 108, 115). These are all parts of the hypothalamus subregion of the diencephalon. The optic (II) nerves end grossly at the level of the optic chiasm, though their nerve fibers continue through the chiasm into the optic tracts which encircle the rostral end of the midbrain. The OPTIC NERVE IS THE “LESION-LEVEL-LOCATOR” OF THE VENTRAL DIENCEPHALON. Each cerebral hemisphere is composed of four major lobes which are visualized on lateral or medial view (N. 106-107). The external surface of the cerebral hemispheres is characterized by irregular elevations called gyri which are separated by depressions called sulci (N. 106). Very deep depressions are sometimes called fissures The lobes of the cerebral hemispheres are partly named and defined by their relationships to the overlying bones of the calvaria, but are more accurately defined by prominent separating sulci and fissures. They also have unique functional differences. The temporal lobe is separated from the frontal and parietal lobes above by the very deep lateral cerebral sulcus (Sylvian fissure) (N. 106). The frontal lobe is separated by the obliquely running central (Rolandic) sulcus from the parietal lobe posteriorly. The central sulcus can typically be identified from other relatively vertical sulci by the fact that it usually just crosses the superior margin of the hemisphere, but does not quite reach the lateral fissure. Further, it is usually continuous and it is parallelled by two flanking gyri, the precentral and postcentral gyri. The occipital lobe is best defined on the medial surface of the hemisphere where it is separated from the parietal lobe in front by a deep parietooccipital sulcus (fissure). On the inferior surface of the frontal lobes the olfactory bulbs and tracts can be located about a cm parasagittal to the longitudinal fissure, which separates the hemispheres (N. 108). Multiple olfactory (I) nerve rootlets arise from the olfactory bulb (N. 121). THE OLFACTORY NERVE IS A “LESION-LEVEL-LOCATOR” OF DISEASE INVOLVING THE INFERIOR ASPECT OF THE FRONTAL LOBES.


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THE INTRACRANIAL COURSE OF THE CRANIAL NERVES (N. 105, 108, 115) A knowledge of the intracranial course and relationships of each cranial nerve is essential to localizing either a lesion of that nerve itself or a lesion involving related structures which might impinge upon that nerve. To fully appreciate each nerve's course and relationships one must identify (1) its site of origin from the brain where it will pick up a pial layer, (2) its course through the subarachnoid space, (3) the point of penetration of the arachnoid and entry into the dura, (4) the intradural course if this is significant in length, and (5) the bony aperture of emergence from the cranial cavity. In general, the frequency with which cranial nerves are involved by intracranial disease is dependent upon three factors. First, the overall length of their intracranial course is of import, since the longer the course the greater the likelihood of involvement by disease in various parts of the cranial cavity. Second, the length of a nerve's intradural course is important, since when a nerve is encroached upon by lesions while it is firmly bound down within the dura it cannot readily slip out of harm's way, as it can to some extent during a subarachnoid course. Thirdly, the degree of hazard of the specific terrain traversed by a given cranial nerve is significant, i.e., the frequency of disease developing within structures along the course of a given nerve. Because of the intimate relationship of the olfactory bulb to the cribriform plate of the ethmoid bone, the multiple rootlets of the olfactory nerve have a very short subarachnoid course and they penetrate the arachnoid and dura and exit the cranial cavity via the cribriform plate in quick succession (N. 120). Because of their limited intracranial course, lesions involving the olfactory nerves are usually localized to the undersurface of the frontal lobe within the anterior cranial fossa. The optic nerves grossly emerge from the brain at the level of the optic chiasm within the central part of the middle cranial fossa. Their further course is unique, since they exit the cranial cavity through the optic canal while they are still within the subarachnoid space. Each nerve carries a sleeve of dura, arachnoid, pia and subarachnoid space through the orbit to the posterior aspect of the eyeball where the meninges become continuous with the outer layer of the eye, the sclera (N. 85, 89). Hence, increased CSF pressure can be transmitted along the optic nerve to its entry into the eye. The oculomotor nerves emerge from the medial aspect of the cerebral peduncles of the midbrain (N. 115). They course across the anterior part of the tentorial incisure to pass superior to the posterior clinoid process and enter the dura along the reflection of the tentorium from the posterior to the anterior clinoid process (N. 105). At this point the nerve can be encroached upon by transtentorial herniation of the medial portions of the temporal lobes when there are supratentorial space-taking lesions. It then runs forward in the upper part of the lateral wall of the cavernous sinus to exit the cranial cavity through the superior orbital fissure. The trochlear nerve exits the brain just below the inferior colliculus on the dorsal aspect of the lowest midbrain (N. 115-116). It then pursues a long subarachnoid course around the side of the midbrain to enter the dura just lateral to the posterior clinoid process (N. 105). It courses forward in the lateral wall of the cavernous sinus, just below the oculomotor nerve, to exit the cranial cavity through the superior orbital fissure.

The trigeminal nerve arises from the lateral aspect of the midpons within the posterior cranial fossa (N. 115). It then passes over the superior margin of the medial part of the petrous pyramid portion of the temporal bone (N. 105). Here it carries a pocket-like evagination of the arachnoid and subarachnoid space into a cleft in the dura to form the trigeminal (Meckel's) cave. Near the distal end of this cave the nerve enters its trigeminal ganglion, which is largely intradural. From here the mandibular division of the trigeminal nerve (mandibular nerve, V3) has a short intradural course to the foramen ovale where it exits the cranial cavity. Both the ophthalmic division (ophthalmic nerve, V1) and maxillary division (maxillary nerve, V2) of the trigeminal nerve run forward in the lower lateral wall of the cavernous sinus. The ophthalmic division exits the cranial cavity through the superior orbital fissure, which it enters after dividing into its three terminal branches (see orbit). The maxillary division of the trigeminal nerve (maxillary nerve, V2) exits the cranial cavity through the foramen rotundum.


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The abducens nerve leaves the brain in the inferior pontine sulcus in line with the medulla's preolivary sulcus (N. 115). It then ascends the clivus of the posterior cranial fossa within the subarachnoid space to midpons level, where it enters the dura of the upper clivus (N. 105). It ascends within this dura and then crosses the petrous apex to enter the interior of the cavernous sinus, wherein it runs forward to exit the cranial cavity through the superior orbital fissure. The abducens nerve has the longest intracranial course and longest intradural course of any cranial nerve. Further, it pursues a high hazard course in terms of the incidence of disease developing within structures related to its course. So it is not difficult to see why it is frequently involved by intracranial disease. The facial and vestibulocochlear nerves traverse the cranial cavity together. They arise from the brain within the cerebellopontine angle in the lateral part of the inferior pontine sulcus in line with the postolivary sulcus of the medulla (N. 115). Then they traverse the subarachnoid space within the cerebellopontine angle to enter the medial end of the internal acoustic meatus (N. 105). They carry a sleeve of pia, subarachnoid space, arachnoid and dura with them to the distal end of this meatus where they enter the interior of the temporal bone. During their intracranial course both nerves may be encroached upon by the relatively common vestibular schwannoma which develops in the vestibular portion of the vestibulocochlear nerve. The glossopharyngeal, vagus and cranial root of the accessory nerve leave the medulla in the postolivary sulcus by multiple rootlets (N. 115). After a relatively short subarachnoid space course they penetrate the arachnoid and dura in quick succession to exit the cranial cavity through the jugular foramen (N. 105). The spinal root of the accessory nerve ascends the posterior cranial fossa from the foramen magnum and after a temporary adherence to the cranial root separates to exit the jugular foramen as an independent nerve (see accessory nerve; N. 128). Hence, intracranial lesions involving these nerves can be pretty well localized to the lower anterolateral portion of the posterior cranial fossa. The hypoglossal nerve exits the brain in the preolivary sulcus of the medulla by multiple rootlets (N. 115). During a relatively short subarachnoid course it is stretched across the anterolateral part of the foramen magnum where it penetrates the arachnoid and dura in rapid succession to exit the cranial cavity through the hypoglossal canal (N. 105). As it is stretched across the foramen magnum it can be encroached upon by a herniation of the cerebellum down through the foramen magnum when there is any type of increased intracranial pressure. ARTERIES AT THE BASE OF THE BRAIN AND THEIR CRANIAL NERVE RELATIONSHIPS (N. 137, 139, 140-144) The course and relationships of the major arteries of the base of the brain is important because of (1) their physiological distribution to various brain regions, (2) their frequency of involvement in cerebral vascular accidents or strokes (caused by occlusion of or hemorrhage from their branches), and (3) their potential for forming aneurysms which can produce signs and symptoms of encroachment upon adjacent cranial nerves even before rupture. The brain is supplied by the paired vertebral and internal carotid arteries, which usually anastomose widely with each other. The two vertebral arteries enter the cranial cavity through the foramen magnum lateral to the low medulla. They course ventromedially around the medulla to join at the medulla-pons junction to form the basilar artery. As they encircle the medulla they lie against the ventral aspect of the hypoglossal nerve roots, where they give off posterior and anterior spinal artery branches and the important posterior inferior cerebellar arteries. Each posterior inferior cerebellar artery courses dorsally around the medulla, in a very serpentine fashion, amongst the rootlets of the last four cranial nerves, to reach the posterior inferior surface of the cerebellum (N. 140, 142, 144). An aneurysm of the vertebral or posterior inferior cerebellar artery could involve any of these nerves. The basilar artery ascends the midline of the base of the pons to terminate at the pons-midbrain junction by dividing into the two posterior cerebral arteries. At low pons levels the basilar artery gives off the anterior inferior cerebellar arteries which course dorsolaterally around the low pons to achieve the


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anterior inferior aspect of the cerebellum (N. 140, 142, 144). The anterior inferior cerebellar artery may run either superior or inferior to the abducens nerve and then usually has a close but variable relationship to the facial and vestibulocochlear nerves. So an aneurysm of the anterior inferior cerebellar artery could encroach any of these cranial nerves. Likewise, an aneurysm of the lower basilar artery could impinge the abducens nerve which ascends the subarachnoid space just lateral to this artery. Just before it terminates at high pons levels the basilar artery gives off the superior cerebellar arteries. These pass dorsolaterally around the high pons to reach the superior aspect of the cerebellum (N. 140, 142, 144). As the oculomotor nerve emerges from the interpeduncular fossa of the midbrain it is sandwiched between the upper basilar artery medially, the posterior cerebral artery superiorly and the superior cerebellar artery inferiorly. So aneurysmal dilation of any of these vessels may cause oculomotor nerve signs and symptoms. The posterior cerebral arteries typically each give off a posterior communicating artery of variable size, which runs forward along the lateral aspect of the diencephalon to join the internal carotid artery (N. 140-141, 144). The posterior cerebral arteries then encircle the low midbrain to supply the inferomedial aspect of the posterior temporal and occipital lobes. The internal carotid arteries have cervical, petrous, cavernous sinus and subarachnoid portions, any of which can be occluded to cause stroke. All portions and their branches are well visualized by arteriography and magnetic resonance angiography. The cervical portion ascends the neck just posterolateral to the oropharynx and nasopharynx (N. 137). The petrous portion is directed anteromedially within the carotid canal of the petrous temporal bone (N. 138). At the medial end of the carotid canal the internal carotid artery has an intimate relationship to the trigeminal ganglion through a dehiscence in the roof of the canal (N. 105). On exiting the anteromedial end of the canal, the artery first ascends into the cavernous sinus. It then runs forward and downward within the sinus, closely related to the cranial nerves traversing the sinus, especially the abducens nerve. It again ascends in the anterior part of the sinus to pass medial to the anterior clinoid process. So in lateral view the cavernous portion of the

internal carotid artery has the form of an S lying on its side ( ) (N. 137). The severity of the bends of the S will vary with its tortuosity. Intracavernous sinus aneurysms of the internal carotid artery can encroach upon any of the nerves in the sinus to produce characteristic signs and symptoms. As the internal carotid artery ascends medial to the anterior clinoid process it exits the cavernous sinus and enters the subarachnoid space as its subarachnoid portion. At this point the anterior clinoid process holds the artery in tight apposition to the lateral aspect of the optic chiasm (N. 105, 140). So aneurysms here will cause the typical visual defects of lateral chiasmal compression. Just below this, the internal carotid artery gives off the ophthalmic artery which accompanies the optic nerve through the optic canal (N. 137). It then receives (or gives off) the posterior communicating artery and gives off an anterior choroidal artery which will accompany the optic tract around the upper midbrain (N. 140). The internal carotid artery ends by dividing into anterior and middle cerebral arteries (N. 137, 139-144). The middle cerebral arteries pass laterally within the lateral cerebral sulcus to supply most of the lateral aspect of the cerebral hemisphere. They or their branches are the most frequent intracranial stroke arteries. The anterior cerebral arteries run forward and medially superior to the optic nerves or chiasm and inferior to the posterior end of the olfactory tracts where aneurysms can encroach both structures, particularly the optic nerves. Where the two anterior cerebral arteries approach each other as they enter the longitudinal fissure they have a short anterior communicating artery bridging between them. From here the anterior cerebral arteries supply the medial aspect of the frontal and parietal lobes. The potential polygonal anastomosis between the upper end of the basilar artery and the right and left internal carotid arteries forms the cerebral arterial circle (of Willis) (N. 139-141). This is formed from posterior to anterior by the terminal end of the basilar, the posterior cerebral, posterior communicating, terminal internal carotid, anterior cerebral and anterior communicating arteries. Its development is frequently asymmetrical and some parts may even be absent. If the circle is complete it provides potential collateral anastomotic channels between the right and left internal carotid arteries and between the carotid and vertebral-basilar systems (N. 139-141). The junction of the vertebral arteries to form the basilar artery


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provides a potential collateral channel between the right and left vertebral arteries. Stroke mortality and morbidity is often determined by the adequacy of these collateral channels. SUMMARIZATION OF FUNCTIONAL NEURAL COMPONENTS AND CRANIAL NERVES Prior to the study of the distribution of the cranial nerves it is useful to overview the general functional fiber types contained in each cranial nerve, as summarized in Table 8-1. This table will be especially valuable for review after the detailed study of the subregions of the head and neck where each cranial nerve has its major distribution.

TABLE 8-1 OUTLINE OF THE FUNCTIONAL FIBER TYPES IN EACH CRANIAL NERVE

I. OLFACTORY NERVE

Smell, from the olfactory epithelium.

II. OPTIC NERVE

Vision, from the retina

III. OCULOMOTOR NERVE

Parasympathetic fibers to the sphincter pupillae and ciliary muscles.

Motor to the levator palpebrae superioris, the superior, medial and inferior recti, and the inferior oblique muscles of the eye.

IV. TROCHLEAR NERVE

Motor to the superior oblique muscle of the eye.

V. TRIGEMINAL NERVE

General sensation from the skin of the face, mucous membrane of oral and nasal cavities, teeth, anterior two-thirds of tongue, eye and orbit, and other structures in the anterior half of the head;

also proprioception from muscles innervated by V and from skeletal muscles innervated by other cranial nerves.

Motor to the muscles of mastication (masseter, temporalis, lateral and medial pterygoids) and the tensor tympani, tensor veli palatini, anterior digastric and mylohyoid muscles.

VI. ABDUCENS NERVE

Motor to the lateral rectus muscle of the eye.

VII. FACIAL NERVE

General sensation from skin of central part of auricle, mastoid region and posterior wall of external auditory meatus.

Taste from the anterior two-thirds of tongue.

Parasympathetic fibers to submandibular and sublingual glands, lacrimal gland, and the glands of nasal cavity, paranasal sinuses and palate.

Motor to muscles of facial expression, and to the stapedius, stylohyoid, and posterior digastric muscles.


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VIII. VESTIBULOCOCHLEAR NERVE

Hearing, from the organ of Corti in the cochlear duct.

Equilibrium, from the utricle, saccule and semicircular canals of the inner ear.

IX. GLOSSOPHARYNGEAL NERVE

General sensation from skin of central part of the auricle, mastoid region and posterior wall of external auditory meatus.

Visceral sensation from middle ear, parotid gland, pharynx, posterior one-third of the tongue, carotid sinus and body.

Taste from posterior one-third of the tongue.

Parasympathetic fibers to the parotid gland.

Motor to the stylopharyngeus muscle.

X. VAGUS NERVE

General sensation from the skin of the central part of auricle, mastoid region and posterior wall of external auditory meatus.

Visceral sensation from many organs including the heart, esophagus, much of the G.I. tract and its glands, trachea, bronchi, lungs, carotid and aortic body chemoreceptors.

Taste from the region of the epiglottis and valleculae.

Parasympathetic fibers to many visceral structures including the heart, pulmonary system, esophagus, and the smooth muscle and glands of the G.I. tract down to the level of the left portion of the transverse colon.

Motor to all skeletal muscles of the larynx, pharynx, palate and upper esophagus except the tensor veli palatini and the stylopharyngeus muscles. Receives cranial root of accessory nerve.

XI. ACCESSORY NERVE

Cranial root (vagal part)

Joins vagus to help innervate skeletal muscles of larynx, pharynx and palate.

Spinal root (part)

Motor to the sternocleidomastoid and trapezius muscles.

XII. HYPOGLOSSAL NERVE

Motor to extrinsic and intrinsic muscles of the tongue (except the palatoglossus)


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ORBIT OSTEOLOGY OF THE ORBIT (N. 4 lower, 5, 7; CD Normal Skull 1 to 4) The bony orbit is approximately cone shaped with an apex posteriorly at the optic canal and a base anteriorly at the orbital margin. However, the orbit achieves its greatest diameter about a cm posterior to the orbital margin, in order to accommodate the greatest diameter of the eyeball. For purposes of description it is convenient to describe the bony orbit as having four bony walls: medial, lateral, superior and inferior. The medial walls are nearly parallel to each other and to the midsagittal plane (N. 4, 85 top; CD H&N Sections 3,4). Each lateral wall diverges laterally from the medial wall at an angle of about 45o. The orbital axis, is the line drawn from the center of the optic canal to the center of the orbital margin. It will diverge from the medial wall by an angle of about 23o. The importance of the orbital axis lies in the fact that the optic nerve course approximates this axis, and two extraocular muscles, the superior and inferior rectus muscles, will parallel this axis (N. 86) from their eyeball (bulbar) to their orbital apex attachment. Therefore, these muscles will exert a substantial medially directed pull upon the eyeball (see orbital muscles). The superior wall or roof of the orbit is largely formed by the orbital part of the frontal bone which extends from the superior orbital margin to within a cm of the orbital apex. The lesser wing of the sphenoid bone forms the most posterior part of the orbital roof and at this point forms the superior boundary of the superior orbital fissure and the superior, lateral and inferior boundaries of the optic canal. About one-third of the way laterally along the superior orbital margin there is either a supraorbital notch or foramen for the transmission of the supraorbital nerve and vessels onto the forehead. The notch is palpable in the living subject and can be used as a point to locally anesthetize the forehead and anterior scalp. About a cm posterior to the lateral part of the superior orbital margin, palpation of a skull reveals a large shallow depression which descends onto the lateral wall. This is the fossa for the lacrimal gland. A few mm behind the area where the superior and medial orbital margins merge there is a tiny shallow depression where the trochlea is attached (N. 86). The trochlea (=pulley) is a ring-like fibrocartilaginous pulley through which the tendon of the superior oblique muscle will pass to execute an acute angle bend. The trochlea is palpable in the living subject as a small elevation. The important superior extraorbital rela-tionships of the orbital roof are to the anterior cranial fossa and its contained frontal lobes of the brain and to that portion of the frontal paranasal air sinus that extends a variable distance into the orbital part of the frontal bone. Therefore, anterior cranial fossa and frontal sinus disease can encroach into the orbit. Likewise, orbital disease can encroach into the anterior cranial fossa. While the orbital part of the frontal bone forms the uppermost part of the medial orbital wall and the orbital face of the maxilla forms its lowermost part, there are four major bony contributors to the medial orbital wall. The medial orbital margin is formed by the frontal process of the maxilla. Posterior to this the small thin quadrilateral lacrimal bone separates the orbit from the nasal cavity. Both the frontal process of the maxilla and the lacrimal bone form the fossa for the lacrimal sac which is bounded by vertical crests. The fossa for the lacrimal sac leads inferiorly into the nasolacrimal canal which conveys the nasolacrimal duct from the lacrimal sac to the inferior meatus of the nasal cavity. Posterior to the lacrimal bone the ethmoid bone forms the largest part of the medial wall. The ethmoid air cells can usually be visualized through this paper-thin bony plate. Near the suture between the frontal and ethmoid bones there are ethmoidal foramina which transmit the ethmoidal nerves and vessels. The body of the sphenoid bone forms the most posterior part of the medial wall of the orbit including the medial wall of the optic canal. At this point the sphenoid paranasal air sinus is closely related to the optic nerve and ophthalmic artery (CD H&N Section 3). So disease of the sphenoid sinus, ethmoid air cells and nasal cavity can invade the orbit through its medial wall. The inferior wall of the orbit is formed medially by the maxilla and laterally by the zygomatic bone. The orbital face of the maxilla forms the anterior margin of the inferior orbital fissure. An infraorbital sulcus extends forward from the inferior orbital fissure and is roofed over anteriorly to form the infraorbital canal. These will convey the infraorbital nerves and vessels onto the face. The major


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inferior relation of the orbit is the maxillary paranasal air sinus, disease of which can erode the inferior orbital wall. The lateral wall of the orbit is formed anteriorly by the zygomatic bone and posteriorly by the greater wing of the sphenoid bone. The greater wing of the sphenoid forms both the posterior margin of the inferior orbital fissure and the inferior margin of the superior orbital fissure. The inferior orbital fissure transmits the zygomatic nerves and the infraorbital nerve and vessels into the orbit and permits some venous communications between the ophthalmic veins and the pterygoid venous plexus. The superior orbital fissure transmits the oculomotor, trochlear, abducens, and branches of the ophthalmic division of the trigeminal nerves into the orbit and permits communications between the ophthalmic veins and the cavernous sinus. The anterior part of the lateral orbital wall is related to the temporal fossa and face, where external trauma to this protuberant part of the face can easily fracture the orbit. The posterior part of the lateral orbital wall is related to the lateral part of the middle cranial fossa. So middle cranial fossa disease can encroach the orbit through its lateral wall and orbital disease can encroach the middle cranial fossa. ORBITAL FASCIA The periosteum of the orbit is called the periorbita. At the orbital apex it is thickened to form the common tendinous ring (N. 85-86) which encircles both the optic canal and the medial part of the superior orbital fissure. The six voluntary orbital muscles which originate from the orbital apex arise from or near the common ring tendon. From here the fascia is continuous along the muscles to the eyeball where it forms a sheath about the eyeball called the fascial sheath of the eyeball (Tenon’s capsule) (N. 85, 89). This fascia attaches to the medial and lateral orbital walls near the insertion of the medial and lateral rectus muscles and thereby acts as a hammock-like support for the eyeball. The eyeball can swivel in various directions within this fascial sheath because it is separated from this surrounding fascial sheath by a fascial plane. Orbital fat fills all of the available space between the fascia-covered structures, nerves and vessels of the orbit. EYELIDS AND LACRIMAL APPARATUS (N. 83-84) From anterior to posterior each eyelid (=palpebra) is formed by skin, the palpebral portion of the orbicularis oculi muscle, the tarsus and its related attaching structures, and the conjunctiva. These layers are separated by very loose connective tissue which permits easy swelling of the lids as in infection, edema, or trauma (black eye). The curved form of the lids is maintained by dense connective tissue plates, the superior and inferior tarsi (N. 83). These have straight edges at the lid margins where some glands contained in these plates open. At their opposite margins they are convex. They are attached at their ends to the medial and lateral orbital walls by medial and lateral palpebral ligaments. They are attached to the superior and inferior orbital margins by the fascial orbital septa which are continuous with the periorbita. The superior tarsus has the involuntary and voluntary elevators of the upper eyelid attaching to it. The support the tarsi provide the lids allows an examining physician to turn the lids back upon themselves at the convex margins of the tarsi to examine both the lids and eyeball for foreign bodies. The skin of the outer surface of the eyelid is continuous at the lid margin with the conjunctiva on its inner surface. As the conjunctiva ascends the upper eyelid it reflects off the eyelid onto the eyeball. The superior recess where the palpebral conjunctiva reflects onto the eyeball is called the superior conjunctival fornix (N. 83). There is a similarly formed inferior conjunctival fornix. It is because of the attachment of the conjunctiva to the eyeball that, e.g., examination of the inferior fornix can be facilitated by asking the patient to look up, which will have the effect of bringing the inferior fornix up into view. Similarly examination of the superior fornix, with the eyelid turned back on itself, is facilitated by asking the patient to look down. The lacrimal gland is situated in the superolateral part of the orbit just behind the orbital margin (N. 84). It is mostly superior to the orbital muscles and adjacent to the periorbita of the fossa for the lacrimal gland. It's multiple ducts open into the lateral part of the superior conjunctival fornix. The constant tears


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they produce stream across the surface of the eyeball with their movement facilitated by blinking. They then move along the lid margins to the point where the upper and lower eyelids meet medially, the inner canthus. At the medial end of each eyelid there is a minute elevation called the lacrimal papilla where a lacrimal canaliculus opens. The superior and inferior lacrimal canaliculi drain medially into the lacrimal sac which is located within its bony fossa. From here the tears descend into the inferior meatus of the nose via the nasolacrimal duct which is situated within the bony nasolacrimal canal. EYEBALL (N. 89-92) The eyeball (bulb) is made up of three layers: an outer fibrous tunic composed of sclera and cornea; a middle vascular tunic (the uvea) composed of choroid, ciliary body and iris; and an inner or sensory tunic, the retina (N. 89). The sclera is a white tough fibrous protective outer coating into which the extraocular muscles attach. It covers approximately the posterior five-sixths of the eyeball. It is continuous anteriorly with the transparent and refractive cornea which is more highly curved than the rest of the eyeball. The very vascular choroid underlies the sclera and receives its blood supply from the posterior ciliary arteries (N. 92-93). Deep to the most anterior part of the sclera this layer thickens and contains smooth muscle to form the ciliary body, which completely encircles the biconvex lens (N. 89-91). The ciliary body has internal projections, the ciliary processes, to which zonular fibers attach peripherally. The zonular fibers attach centrally to the peripheral regions of the lens, which they suspend from the ciliary processes like the spokes of a wheel (N. 89-91). The ciliary smooth muscle has its major attachment anteriorly to a thickening of the inner aspect of the sclera next to the cornea. From this point most of the fibers of the ciliary muscle run posteriorly into the choroid, though some run circularly and have a sphincteric function (N. 90). When the ciliary muscle contracts the choroid is pulled forward into the ciliary body. This plus the contraction of its sphincteric fibers narrows the inner diameter of the ciliary body. This, in turn, takes the normal tension off of the zonular fibers to allow the lens to round up by virtue of its own intrinsic elasticity. This process is necessary for near vision and it is called accommodation. As an object is moved nearer to the eye the light rays emanating from it diverge more widely. Hence, to keep the image focused on the retina the eye must increase its refractive capabilities. It accomplishes this by the accommodative process which increases the refractive power of the lens by allowing its anterior and posterior surfaces to become more highly curved. When the ciliary muscle is relaxed the intrinsic elasticity of the choroid (which must exceed that of the lens) pulls the system back to its at rest distant vision posi-tion. So accommodation for near vision is an active, but involuntary muscular process which is under the control of parasympathetic nerve fibers derived from the oculomotor nerve (see oculomotor nerve description). In contrast, the deaccommodative mechanism which returns the system to the distant vision mode appears to be a passive elastic process. The iris is the pigmented thin flat washer-like structure which projects centrally in the coronal plane from the anterior part of the ciliary body (N. 89-91). It is situated anterior to the lens and its central aperture is the pupil. It functions like the diaphragm of a camera and its diameter is under the active control of two antagonistic muscles, the sphincter and dilator pupillae muscles (N. 90). The sphincter pupillae muscle is smooth muscle arranged circularly about the inner margin of the pupil. Its contraction causes a narrowing of the pupil, called pupilloconstriction or miosis. It is under the control of parasympathetic fibers from the oculomotor nerve which synapse in the ciliary ganglion. The dilator pupillae muscle is made up of radially arranged smooth muscle fibers which are fixed peripherally to the iris so that when they contract they will dilate the pupil, a process called pupillodilation or mydriasis. This is under sympathetic control. The iris subdivides the space anterior to the lens and zonular fibers into an anterior chamber between the cornea and the iris and a posterior chamber between the iris and the lens apparatus (N. 89-91). These chambers contain a thin watery fluid called the aqueous humor which is produced in the posterior chamber by the epithelium over the ciliary processes. It is absorbed in the anterior chamber at the


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iris-cornea junction. An imbalance between aqueous production and resorption causes glaucoma, wherein increased intraocular pressure may cause retinal degeneration. The internal sensory tunic of the eye is the retina. Between the retina and lens is the vitreous chamber of the eye which is filled with a gelatinous vitreous body. The retina has a thick visually receptive portion called the optic (neural) part of the retina, which covers more than half of the posterior portion of the eyeball. It is made up of about ten named layers with the receptive rods and cones most externally placed (N. 121). The area of greatest visual acuity is the macula, a depressed area with its deepest central part called the fovea centralis (N. 89, 92). The depression is caused by the divergence of all the internal retinal layers to give the externally placed receptive cells of this area (cones) direct access to incoming images. When the eye is functioning in distant vision (looking at an object more than 20 feet away) the fovea is at the posterior end of an anteroposterior visual axis through the eyeball. Generally, eye movements are generated to keep the image of interest falling upon the fovea. Several mm medial to the fovea the retinal nerve fibers converge to form the optic nerve disc (nerve head, nerve papilla) (N. 89, 92). This normally has a raised periphery and a central depression (like looking into a soup bowl or a moon crater). When the center of the optic nerve disc is elevated so that the entire disc bulges into the vitreous chamber it is called papilledema or choked disc. This is a serious sign of increased intracranial pressure which can be detected by fundoscopic examination (see optic nerve). The posterior part of the inner aspect of the eyeball is called the fundus. Examination of the fundus with an ophthalmoscope provides not only an opportunity to examine the retina and optic nerve disc, but it also gives an indication of the normalcy of the refractive media of the eye which must be looked through. Further, it provides an opportunity to examine the condition of the body's smaller arterial and venous channels as these are reflected in the retinal vasculature. The central artery and vein of the retina enter the eyeball through the optic nerve disc (N. 89, 92). They break up into branches which ramify on the internal aspect of the retina. Examination of these vessels not only provides information about the vascular integrity of the eye, but gives an indication of the progress of many systemic vascular diseases, e.g., hypertension or diabetes. MUSCLES OF THE ORBIT (Table 8-2, Figs. 8-1, 8-2 and 8-3; N. 85-86; CD H&N Sections 3, 4) There are seven voluntary skeletal muscles within the orbit of which six produce movement of the eyeball and the seventh is a voluntary elevator of the upper eyelid. There is also an involuntary elevator of the upper eyelid. The levator palpebrae superioris muscle is the most superiorly situated muscle in the orbit. It arises from the common ring tendon region and runs forward just below the periorbita of the superior orbital wall. Anteriorly, it fans out into a broad aponeurosis which inserts into the tarsus and dermis of the upper eyelid (N. 83, 85-86). It is the voluntary elevator of the upper eyelid, and it is innervated by the superior division of the oculomotor nerve. In the anterior part of the orbit a thin sheet of smooth muscle, the superior tarsal muscle (of Muller) arises from the inferior aspect of the fascial covering about the levator palpebrae superioris (N. 83). This smooth muscle sheet inserts into the upper border of the superior tarsus. It is the involuntary elevator of the upper eyelid. It is innervated by the sympathetic nerve fibers which ascend into the head about the internal carotid artery. If either the voluntary or involuntary elevator of the upper eyelid is paralyzed the upper lid will droop (ptosis). Ptosis is a sign of either an oculomotor or a sympathetic nerve lesion. To determine which, the other functions of the oculomotor and sympathetic nerves must be tested. Although the eyeball can be moved in any oblique direction, for convenience of description it is considered to be movable about three mutually perpendicular axes situated in the three major planes of anatomic space. Movements are typically considered to start from an at rest distant vision position of the eyeball when the visual axes of the two eyes will be essentially parallel to each other and to the sagittal plane. Movement about a mediolaterally directed axis through the center of the eyeball results in elevation and depression of the corneal aspect of the eyeball. Movement about a vertical axis through the center of the eyeball results in adduction and abduction of the corneal aspect of the eyeball. Finally, movement can occur about an anteroposterior axis through the center of the eyeball with the reference point being the 12


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o'clock position of the cornea or iris. When this point rotates medially it is called intorsion and when it rotates laterally it is termed extorsion. There are six muscles which attach to and move the eyeball. Four of these arise from the region of the common ring tendon and run forward in linear fashion to gain attachments to the anterior portion of the eyeball. These are called the rectus (= straight) muscles. Since they are spaced about 90o apart from each other on the lateral, medial, superior and inferior aspects of the orbit and eyeball, they are named lateral, medial, superior and inferior rectus muscles. The other two muscles approach the eyeball very obliquely on its superior and inferior aspects to attach to its posterior portion, and hence they are called superior and inferior oblique muscles. From its common ring tendon origin the lateral rectus muscle runs forward immediately adjacent to the periorbita of the lateral orbital wall where it can be easily impaled by or pinched between fracture fragments. It attaches to the lateral aspect of the anterior part of the eyeball. Acting about the vertical axis it is the primary abductor of the eyeball. Since it acts across the mediolateral axis and parallel to the anteroposterior axis it will have no functions about these axes with the eyeball in a distant vision position (see Table 8-2). It is the only muscle innervated by the abducens nerve. From its common ring tendon origin the medial rectus muscle runs forward adjacent to the periorbita of the medial orbital wall to gain an attachment to the medial aspect of the anterior portion of the eyeball. It is the primary adductor of the eyeball, and like the lateral rectus has no functions about the mediolateral or anteroposterior axes with the eyeball in a distant vision position. It is innervated by the inferior division of the oculomotor nerve. The superior rectus muscle is situated immediately inferior to the levator palpebrae superioris muscle. It extends from the common ring tendon to an attachment on the superior aspect of the anterior part of the eyeball. It parallels the obliquely situated orbital axis (N. 86). Hence, its posterior pull on the superior aspect of the anterior eyeball will have a medially directed component. So in addition to being an elevator by pulling the front of the eyeball upward and backward about the mediolateral axis, it is an adductor about the vertical axis because of its medial pull on the front of the eyeball, and an intorter about the anteroposterior axis because of its medial pull upon the superior aspect of the eyeball (See Table 8-2). It is innervated by the superior division of the oculomotor nerve. The inferior rectus muscle arises from the common ring tendon and runs forward in the inferior part of the orbit to attach to the inferior aspect of the anterior part of the eyeball (N. 86). Hence, its downward and backward pull upon the front of the eyeball will cause depression about the mediolateral axis. But since it follows the orbital axis it will exert a medial pull upon the inferior aspect of the front of the eyeball. The medial pull upon the front of the eyeball will cause adduction about the vertical axis like the superior rectus does. However, its medial pull upon the inferior aspect of the eyeball will cause extorsion about the anteroposterior axis. This muscle is innervated by the inferior division of the oculomotor nerve. The superior oblique muscle arises in the region of the common ring tendon and runs forward in the superomedial part of the orbit just above the medial rectus. Near the anterior orbital margin its tendon passes through a fibrocartilaginous pulley, the trochlea. From here it angles acutely posteriorly and laterally to gain an attachment on the superior aspect of the posterior part of the eyeball (N. 86). About the mediolateral axis this muscle will pull the posterior part of the eyeball upward and forward to cause depression of the corneal pole of the eye. Acting about the vertical axis it will pull the posterior part of the eyeball medially and this causes the corneal pole to be abducted. Its medial pull on the superior aspect of the eyeball will cause it to be an intorter about the anteroposterior axis. It is the only muscle innervated by the trochlear nerve. The inferior oblique muscle is the only voluntary orbital muscle which doesn't arise from the vicinity of the common ring tendon at the orbital apex. It arises from the medial part of the inferior orbital wall just behind the orbital margin (N. 86). From here its fibers pass posteriorly and laterally beneath the eyeball, pretty much paralleling the pull of the superior oblique tendon above the eyeball. It attaches to the


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inferolateral aspect of the posterior part of the eyeball. Acting about the mediolateral axis it tends to pull the posterior part of the eyeball downward and forward, thereby elevating the cornea. Acting about the vertical axis, its medial pull on the posterior part of the eyeball will cause abduction of the corneal pole of the eye. Its medial pull on the inferior aspect of the eyeball will cause extorsion about the anteroposterior axis. This muscle is innervated by the inferior division of the oculomotor nerve.

TABLE 8-2 EXTRAOCULAR MUSCLE FUNCTIONS

UPON THE AT REST (DISTANT VISION) EYE

Muscle Innervation Elevation-Depression Abduction-Adduction Intorsion-Extorsion

Lateral VI ---------- Abduct ---------- Rectus Medial Inf.III ---------- Adduct ---------- Rectus Superior Sup.III Elevate Adduct Intort Rectus Inferior Inf.III Depress Adduct Extort Rectus Superior IV Depress Abduct Intort oblique Inferior Inf.Ill Elevate Abduct Extort Oblique


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Testing the extraocular muscles is an important part of any cranial nerve exam, because it provides a means of evaluating three cranial nerves and the midbrain and pons levels of the brain. Since the lateral and medial rectus muscles are the primary abductors and adductors they can be evaluated by asking the patient to follow the examiner's finger respectively laterally and medially. These abduction-adduction functions will be deficient or absent if these muscles are weak or paralyzed. However, the elevation and depression functions are each carried out by two primary muscles. So if you ask a patient to follow your finger superiorly into elevation you will be testing both the superior rectus and the inferior oblique. Likewise, if you ask a patient to follow your finger inferiorly into depression you are testing both the inferior rectus and superior oblique muscles, which are even innervated by different cranial nerves. So if a patient shows elevation or depression weakness you cannot tell which of the elevators or depressors is weak or paralyzed unless you can isolate and test each one separately by placing it at maximal mechanical advantage (perpendicular to the mediolateral axis or parallel to the visual axis), while placing the other muscle at a mechanical disadvantage (parallel to the mediolateral axis)( see page 10). The inferior oblique can be isolated and tested by first adducting the eye, then elevating it (see Figure 8-1). The adduction puts the superior rectus at a mechanical disadvantage by placing it nearly parallel to the mediolateral (transverse) axis. It also maximizes the elevation capability of the inferior oblique by placing it nearly perpendicular to the mediolateral (transverse) axis (parallel to the visual axis). Elevation from this adducted position will therefore provide a relatively pure test for the inferior oblique. To isolate and test the superior rectus the eye is first abducted and then elevated. The abduction places the inferior oblique nearly parallel with the mediolateral (transverse) axis and the superior rectus nearly perpendicular to the mediolateral (transverse) axis (parallel to the visual axis). Hence, elevation from the abducted position provides a relatively pure test for the superior rectus muscle. To isolate and test the superior oblique the eye is first adducted. This places the inferior rectus nearly parallel to the mediolateral (transverse) axis and the superior oblique nearly perpendicular to the mediolateral (transverse) axis (parallel to the visual axis). So depression from this point will be a relatively pure test for the superior oblique. This is the primary test for trochlear nerve integrity. To isolate and test the inferior rectus the eye is first abducted. This places the superior oblique nearly parallel to the mediolateral (transverse) axis and the inferior rectus nearly perpendicular to the mediolaeral (transverse) axis (parallel to the visual axis). Therefore, depression from this point is a relatively pure test for the inferior rectus.


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Figure 8-1


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To summarize these elevation-depression testing functions note that the oblique muscles are tested in adduction and the rectus muscles are tested in abduction. If the examining physician asks a patient to follow his finger with one eye (the other occluded by being covered) while the examiner traces an H in space, all six extraocular muscles and their three innervating cranial nerves can be tested (see Figure 8-2). If the finger is displaced medially the medial rectus will be tested. From this adducted position, elevation tests the inferior oblique and depression tests the superior oblique (the medial limb of the H). If the examiner's finger is then displaced laterally (the crossbar of the H) the lateral rectus will be tested. From this abducted position, elevation tests the superior rectus and depression tests the inferior rectus (the lateral limb of the H). To speed up bilateral testing, the patient can be asked to follow the examiner's finger through the H with both eyes simultaneously while the examiner watches both eyes to see if one lags behind the other during any part of the movement. In testing both eyes simultaneously the examiner must be aware that opposite muscles are being tested in each eye during each part of the test. Simultaneous testing provides a quick test for 12 muscles and 6 cranial nerves.

NERVES AND VESSELS OF THE ORBIT (Fig. 8-3; N. 87-88, 122-123) While the oculomotor nerve is in the lateral wall of the cavernous sinus it divides into a superior and inferior division. Likewise, the ophthalmic division of the trigeminal nerve, which is entirely sensory, divides into its frontal, lacrimal and nasociliary nerves while it is still in the lateral wall of the cavernous sinus. The major nerves and vessels of the orbit can be organized by relating them to four planes of the orbit. The first or most superior plane is between the periorbita of the orbital roof and the most superior orbital muscles, which from medial to lateral are the superior oblique, levator palpebrae superioris and lateral rec-tus (N. 86; Figure 8-2). There are three nerves in this plane. The trochlear nerve runs along the superior margin of the apical end of the superior oblique muscle and enters it about a cm anterior to the orbital apex (N. 88, 122). The frontal branch of ophthalmic V runs along the superior aspect of the levator palpebrae superioris and gives off a supraorbital branch which courses through the supraorbital foramen or notch. It provides sensory innervation to most of the forehead and anterior scalp. The lacrimal branch

Fig.8-2


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of ophthalmic V runs along the superolateral part of the orbit adjacent to the periorbita to provide sensory innervation to the lacrimal gland and lateral eyelids.

Fig. 8-3 Anterior view of a coronal section through the right orbit posterior to the eyeball

The second plane of the orbit is between the superior rectus and the optic nerve. There are three or four structures in this plane (N. 87-88, 122). The superior division of the oculomotor nerve enters the inferior aspect of the superior rectus muscle about a cm anterior to the orbital apex. It runs through this muscle to also innervate the levator palpebrae superioris. The nasociliary branch of ophthalmic V runs obliquely anteromedially across the superior aspect of the optic nerve about a cm anterior to the orbital apex. It is closely accompanied by the ophthalmic artery, which enters the orbit inferior to the optic nerve in the optic canal and then curves around the lateral aspect of the optic nerve onto its superior aspect. Both the nasociliary nerve and ophthalmic artery run medially toward the interval between the superior oblique and medial rectus muscles where they will give off their ethmoidal branches. The superior ophthalmic vein may also be in this plane, though it is highly variable in location and is sometimes in the first plane. The third plane of the orbit is at the level of the optic nerve, between the optic nerve and the lateral rectus muscle (N. 88, 122). In this plane the abducens nerve runs forward on the medial aspect of the lateral rectus muscle to innervate it. Also in this plane within a cm of the orbital apex the ciliary


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parasympathetic ganglion is situated along with its sensory and motor roots and its short ciliary nerves which extend forward to the eyeball (see oculomotor nerve). The fourth plane of the orbit is between the optic nerve and the inferior rectus muscle where the inferior division of the oculomotor nerve can be located running forward along the superolateral border of the inferior rectus muscle (N. 88, 122). The variable inferior ophthalmic vein may also be situated in this plane. The optic nerve (cranial nerve II) contains the nerve fibers for vision. The nerve cell bodies of origin of these afferent fibers are in the ganglion cell layer of the retina (N. 121). They converge at the optic nerve disc to form the optic nerve which is really a fiber tract of the brain. While the grossly defined optic nerve ends at the optic chiasm the nerve fibers in the optic nerve continue through the chiasm and form the optic tracts which encircle the midbrain to terminate in the diencephalon (CD H&N Section 3). The optic chiasm is related laterally to the internal carotid arteries and as the optic nerves cross the subarachnoid space they have the anterior cerebral arteries situated above them (N. 140). Aneurysms of either of these arteries commonly cause visual defects, as can their occlusion, since they provide arterial supply to the optic chiasm and the intracranial part of the optic nerves. As the optic nerve enters the cranial cavity through the optic canal it is accompanied by the ophthalmic artery which is located inferior to the nerve. A sleeve of each layer of the meninges and the cerebrospinal fluid in the subarachnoid space is carried forward along the nerve to the sclera of the eyeball, where the connective tissue of the meninges becomes continuous with the sclera (N. 85, 89). The central artery of the retina, which has an accompanying vein, arises from the ophthalmic artery in the posterior orbit while the artery is still inferior to the nerve. These central retinal vessels run forward and penetrate the meninges to enter the inferior aspect of the optic nerve about a cm behind the eyeball. In doing so they must cross the subarachnoid space around the optic nerve. One of the theories for the origin of papilledema as a classic sign of increased intracranial pressure holds that the increased pressure can be transmitted forward along the subarachnoid space about the optic nerve. If the pressure is high enough it can obstruct the venous return in the central retinal veins as they cross the subarachnoid space. This in turn, could increase capillary pressure within the optic nerve and cause the swelling of the optic nerve disc. It also could cause an engorgement of retinal veins. Both findings are visible on fundoscopic examination. The oculomotor nerve (cranial nerve III) contains motor fibers to the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus and inferior oblique muscles and parasympathetic preganglionic fibers to the ciliary ganglion, from which postganglionic fibers will distribute to the sphincter pupillae and ciliary smooth muscles of the eyeball (N. 122). The nerve arises from the medial aspect of the cerebral peduncles of the midbrain and enters the interpeduncular fossa (N. 115-116, 122). As it emerges from the midbrain it has the upper basilar artery medial, the superior cerebellar artery inferior and the posterior cerebral artery superior to it (N. 140). Aneurysms of any of these arteries can affect the nerve. It then passes forward through the anterior part of the tentorial notch just inferior to the posterior communicating artery N. 105) where it can be encroached by aneurysms of this artery or by the uncus of the temporal lobe, which may herniate down through the notch during increased supratentorial pressure. It next passes over the posterior clinoid process to enter the lateral wall of the cavernous sinus where it runs forward and divides into superior and inferior divisions before reaching the superior orbital fissure. The superior division innervates the superior rectus and levator palpebrae superioris muscles and the inferior division innervates the medial rectus, inferior rectus and inferior oblique muscles. The parasympathetic preganglionic nerve fibers enter the inferior division of the oculomotor nerve. About a cm anterior to the orbital apex they leave the inferior division over a short stout parasympathetic (oculomotor) root of the ciliary ganglion which enters the ganglion from below (N. 88, 122, 133). These nerve fibers synapse upon the postganglionic parasympathetic neurons within the ciliary ganglion. The nasociliary (sensory) root of the ciliary ganglion comes off the nasociliary nerve within the cavernous sinus. These sensory fibers pass through the ganglion without synapse. There are also sympathetic postganglionic nerve fibers passing through the ciliary ganglion without synapse. These may enter with the sensory or motor root fibers or from adjacent periarterial nerve plexuses. The short ciliary nerves, which course from the ciliary ganglion to the eyeball, will therefore be mixed nerves conveying


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sensory, sympathetic postganglionic, and parasympathetic postganglionic fibers (N. 122, 133). All of these fiber types will run forward within the layers of the eyeball with the parasympathetic postganglionics innervating the ciliary and sphincter pupillae muscles, the sympathetic postganglionics innervating the dilator pupillae muscle and the sensory fibers providing some of the sensory innervation to the cornea and conjunctiva of the eyeball. Therefore, if the oculomotor nerve is injured there will be a ptosis because of the loss of the levator palpebrae superioris muscle. The eye will be held in a fixed abducted position, called external strabismus, because of the unopposed pull of the intact lateral rectus muscle. This will cause the patient to complain of double vision with the dual images side by side (horizontal diplopia). Near vision will be impaired in the involved eye because of the loss of the accommodative processes of the ciliary muscle. In addition the pupil will be dilated because of the unopposed pull of the dilator pupillae muscle. When light is shined in either eye there will be no pupilloconstriction of the involved pupil since the oculomotor parasympathetics are the motor limb of the pupillary light reflex. The optic nerve is the sensory limb of the pupillary light reflex. So the PUPILLARY LIGHT REFLEX PROVIDES A GOOD QUICK TEST FOR THE INTEGRITY OF THE OPTIC AND OCULOMOTOR NERVES WHICH ARE ITS RESPECTIVE SENSORY AND MOTOR LIMBS. Normally when you shine a light in one eye both eyes will show a pupilloconstriction because there are bilateral central projections of the sensory input from each eye. If you get no response from either eye when the light is shined in one eye and a normal bilateral response when it is shined in the other eye there is a sensory limb defect in the abnormal eye. If one eye consistently shows no response and the other shows a consistent normal response when the light is shined in either eye there is a motor limb defect in the abnormal eye. The trochlear nerve (cranial nerve IV) contains only motors fibers to the superior oblique muscle. It is the only cranial nerve to emerge from the dorsal aspect of the brainstem (N. 116, 122) and the only cranial nerve whose root fibers cross completely before exiting the brain so that, e.g., the right trochlear nucleus forms the left trochlear nerve. The trochlear nerve originates in the midbrain and it emerges just below the inferior colliculus. Its root fibers then pass ventrally around the midbrain within the subarachnoid space in the interval between the posterior cerebral artery superiorly and the superior cerebellar artery inferiorly. It enters the lateral wall of the cavernous sinus where it runs forward just inferior to the oculomotor nerve to enter the orbit through the superior orbital fissure. It innervates the superior oblique muscle along its superior border near the orbital apex. If the trochlear nerve is injured the patient will complain of a vertical diplopia (images doubled in the superior-inferior direction), which is particularly alarming during walking down stairs when one sees two of each stair and is not sure which to step on. This occurs because one typically looks down on descending the stairs, and the involved eye cannot be depressed as much as the normal eye. The trochlear nerve is tested by first adducting and then attempting to depress the eye. The abducens nerve (cranial nerve VI) contain only motor fibers to the lateral rectus muscle (the primary abductor). It originates in the low pons (N. 122) and emerges from the inferior pontine sulcus in line with the preolivary sulcus where it has a variable relationship to the anterior inferior cerebellar artery (N. 115, 140). It ascends to midpons levels where it enters the dura over the clivus. It then ascends the clivus to enter the interior of the cavernous sinus (N. 105). It runs forward lateral to the internal carotid artery to enter the orbit through the superior orbital fissure. It innervates the lateral rectus muscle on its medial aspect. The abducens nerve has the longest intracranial and intradural course of any cranial nerve. Further, it is a high hazard course with close arterial, hypophysial, sphenoid sinus, and cavernous sinus relationships within both the posterior and middle cranial fossae. If this nerve is injured the overpull of the intact medial rectus will produce an internal strabismus (an adducted eye which can still be moved vertically) and the patient will complain of horizontal diplopia. The ophthalmic division of the trigeminal nerve contains only SA fibers whose nerve cell bodies are located in the trigeminal ganglion (N. 123). The ganglion has an intimate inferior relationship with the internal carotid artery through the usual dehiscence in the roof of the carotid canal (N. 105). The artery’s pulsations can be a cause of trigeminal neuralgia. The ophthalmic division runs forward in the lateral wall of the cavernous sinus where it divides into its frontal, lacrimal and nasociliary branches prior to entering the superior orbital fissure. The frontal and lacrimal branches were previously described.


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The nasociliary nerve gives off direct long ciliary branches to the eyeball and indirect branches which traverse the ciliary ganglion to enter the short ciliary nerves (N. 88, 133). Both branches provide an important sensory innervation to the cornea and conjunctiva of the eyeball and they serve as the afferent limb of the corneal reflex, wherein the cornea or conjunctiva is stimulated with a piece of facial tissue or by blowing a puff of air on one eye and the normal motor response is eyelid closure of both eyes by the orbicularis oculi muscles. So the CORNEAL (BLINK) REFLEX PROVIDES A GOOD TEST FOR THE INTEGRITY OF THE TRIGEMINAL AND FACIAL NERVES, WHICH PROVIDE ITS RESPECTIVE SENSORY AND MOTOR LIMBS. Normally when you perform a corneal reflex, stimulation of one eye causes both eyes to blink because of bilateral central projections of the sensory input from each eye. If you get no response in either eye when you stimulate one eye and a normal bilateral response when you stimulate the other eye there is a sensory limb defect in the abnormal eye. If one eye consistently does not respond and the other eye consistently does respond to stimulation of either eye then there is a motor limb defect on the abnormal side. Other important branches of the nasociliary nerve are the ethmoidal branches which pass through the ethmoidal canals to innervate ethmoid air cells, dura of the anterior cranial fossa, the upper nasal cavity and the dorsum of the external nose. The sympathetic preganglionic innervation to the orbit originates in the uppermost part of the intermediolateral cell column mostly at T1 or T2 spinal cord levels (N. 133). These fibers exit the cord via these ventral roots, spinal nerves and white rami communicans to enter the stellate ganglion levels of the sympathetic trunk. They ascend the trunk without synapse to the superior cervical sympathetic ganglion. Here they synapse on postganglionic sympathetic neurons whose axons will accompany the internal caro-tid artery through the neck, carotid canal and cavernous sinus as its periarterial sympathetic plexus (N. 131-133). From the cavernous sinus they can enter the orbit by joining any of the orbital nerves in the cavernous sinus or by accompanying the branches of the ophthalmic artery. In the orbit they supply the superior tarsal muscle and the dilator pupillae muscle. If this pathway is interrupted anywhere along its course there will be a fixed constricted pupil (miosis) because of the unopposed constrictor pupillae activity and a ptosis caused by a loss of the superior tarsal muscle. There may also be a loss of sweating over the forehead (anhydrosis) and a flushing or blushing of the forehead (with increased warmth) caused by a loss of secretomotor and vasoconstrictor fibers along the supraorbital branches of the ophthalmic artery or nerve. This combination of findings is the internal carotid-ophthalmic portion of Horner's syndrome caused by an encroachment upon this periarterial sympathetic pathway distal to the superior cervical sympathetic ganglion. The external carotid-facial portion of Horner's syndrome can occur if there is an encroachment upon these periarterial sympathetic pathways. This will cause anhydrosis, flushing and increased warmth of the lower face. LESIONS WHICH INTERRUPT THE SYMPATHETIC INPUT TO THE HEAD AT OR ANYWHERE PROXIMAL TO THE SUPERIOR CERVICAL SYMPATHETIC GANGLION CAN CAUSE A COMPLETE HORNER’S SYNDROME WITH MIOSIS, PTOSIS AND ANHYDROSIS AND FLUSHING OF THE ENTIRE FACE. The ophthalmic artery (N. 87) arises from the internal carotid artery as soon as it emerges from the cavernous sinus. It enters the orbit through the optic canal inferior to the optic nerve. Then it usually curves around the lateral onto the superior aspect of the optic nerve. Its major branches include the previously described central artery of the retina, the posterior ciliary branches which supply the vascular tunic of the eye, the lacrimal branch to the gland, the supraorbital branch to the forehead and anterior scalp, and the ethmoidal branches. The ethmoidal branches, continue beyond the ethmoid air cells to supply the upper nasal cavity, where especially the anterior ethmoidal artery supplies the upper anterior part of the nasal septum, and so may be involved in nosebleed. The ophthalmic veins receive the venous drainage of the eye through both the central retinal veins which drain much of the retina and the four vorticose veins which drain the vascular tunic quadrantically (N. 87 lower, 92-93). The other tributaries of the ophthalmic veins are similar to the ophthalmic artery's branches. They communicate with the angular branch of the facial vein anteriorly, the pterygoid venous plexus through the inferior orbital fissure and the cavernous sinus through the superior orbital fissure. The


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superior ophthalmic vein is usually larger than the inferior ophthalmic vein. They have a variable course through the orbit, but they are commonly situated respectively above and below the optic nerve.

TEMPORAL BONE AND EAR The temporal bone serves a number of significant functions. It contains the vestibulocochlear receptive organs for equilibrium and hearing in the internal ear, the sound conducting mechanisms of the middle ear, and some of the sound collecting mechanisms of the external ear. Its internal aspect serves as part of the floor of both the middle and posterior cranial fossae. Its external aspect serves masticatory functions as part of the temporomandibular articulation and by serving as partial origin for the temporalis and masseter muscles. The temporal bone also transmits the facial nerve from the cranial cavity to its extracranial distribution and serves as the site of origin of a number of important facial nerve branches. Further, three of the four parasympathetic pathways of the head traverse the temporal bone for part of their course. Finally, it serves major vascular transmission functions since the internal carotid artery (and its surrounding sympathetics) enter the cranial cavity through the carotid canal of the temporal bone, and it forms the anterior boundary of the jugular foramen which transmits the major venous drainage of the cranial cavity and cranial nerves IX, X and XI. OSTEOLOGY OF THE TEMPORAL BONE (N. 6, 8, 10-13, 99-100)

The temporal bone has three major parts: petrous, tympanic and squamous. The petrous part of the temporal bone (sometimes called the petrous pyramid) is the anteromedially directed prism-shaped portion of the temporal bone (N. 10-11). Its long axis diverges from both the coronal and midsagittal planes by about 40-50o. It has three major surfaces: anterior, posterior and inferior. It has a base externally where it helps form the mastoid process and an anteromedially directed apex which articulates with the sphenoid body. It is called petrous (=rock) because of its high mineral content that makes it the most radiodense structure on lateral skull radiographs (N. 7; CD Lateral Skull Radiographs 3, 4). The anterior face of the petrous pyramid (N. 11, 13) forms the posterior part of the lateral portion of the middle cranial fossa. Near its apical end there is a shallow depression that just admits a fingertip. This is the impression for the trigeminal ganglion. There is usually a variable-size dehiscence in the roof of the carotid canal which underlies this impression. About 2 cm lateral to this impression there is a foramen called the hiatus for the greater petrosal nerve which transmits this important branch of the facial nerve (N. 11, 13). Just posterolateral to this aperture a visible or palpable elevation marks the position of the underlying superior semicircular canal. Just lateral to this the thin roof of the middle ear cavity is situated. A superior petrosal margin separates the anterior and posterior faces of the petrous pyramid and this is marked by a sulcus for the superior petrosal dural venous sinus. The posterior face of the petrous pyramid (N. 8, 11, 13, 99) contains the internal acoustic meatus. This approximately one cm long cul de sac transmits the facial and vestibulocochlear nerves and the labyrinthine artery into the temporal bone. Laterally, where the posterior surface underlies the mastoid process there is a broad curved sulcus for the sigmoid dural venous sinus. The posterior margin of the petrous pyramid is notched to form the anterior margin of the jugular foramen where the sigmoid and inferior petrosal sinuses enter the internal jugular vein and the glossopharyngeal, vagus and accessory nerves exit the cranial cavity. The lateral part of the inferior face of the petrous pyramid (N. 10, 12, 100) forms the apex of the mastoid process. The stylomastoid foramen and styloid process are situated, in sequence, anteromedial to the mastoid process. The stylomastoid foramen is the external opening of the facial nerve canal. Just medial to the styloid process the temporal bone forms the anterior boundary of the jugular foramen. Immediately anterior to the jugular foramen the external opening of the carotid canal is located. This two to three cm long canal will transmit the internal carotid artery and its pericarotid


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sympathetic plexus superiorly and then anteromedially toward the petrous apex, where they ascend into the cavernous sinus. On the plate of bone which separates the jugular foramen posteriorly from the carotid canal anteriorly there is a small aperture which transmits the tympanic branch of the glossopharyngeal nerve into the middle ear (tympanic) cavity. The tympanic part of the temporal bone is trough-like and forms the anterior wall, floor and part of the posterior wall of the external acoustic meatus (shown but not labeled on N. 6, 10). That part which forms the anterior wall of the external acoustic meatus also forms the flat posterior part of the mandibular fossa. The squamous part of the temporal bone (N. 6, 8, 10-13) forms its curved lateral portion. Internally, it helps form the more lateral part of the middle cranial fossa. Externally it gives off a zygomatic process which articulates with the zygomatic bone. It also forms the highly concave anterior portion of the mandibular fossa. The articular tubercle marks the anterior margin of this fossa. Running mediola-terally through the mandibular fossa is a squamotympanic suture which has a small sliver of the petrous part of the temporal bone insinuated into its medial end. Between this sliver of petrous pyramid and the tympanic part of the temporal bone, the thus defined petrotympanic fissure transmits the chorda tympani from the middle ear cavity into the infratemporal fossa (N. 10). EXTERNAL EAR (N. 94-95) The external ear is composed of the laterally projecting sound collecting auricle (pinna) and the sound transmitting external acoustic meatus. The auricle is a skin-covered elastic cartilage attached to the temporal bone by ligaments. The lateral portion of the external acoustic meatus is formed by a trough-shaped elastic cartilage continuous with the auricle. The deeper portion of the meatus is bony and ends medially at the very obliquely placed tympanic membrane. The meatus is slightly sinusoidal. To provide optimum otoscopic visualization of the tympanic membrane, the meatus can be straightened by applying upward and backward traction on the auricle. Because the obliquely placed tympanic membrane slopes medially as it is traced from its superior to its inferior margins and also as it is followed from its posterior to its anterior margins, the inferior and anterior walls of the meatus are the longest walls and the anteroinferior part of the tympanic membrane is farthest removed from the surface. An appreciation of these geometric asymmetries of the tympanic membrane and external meatus is valuable in searching the external meatus for foreign bodies, removing impacted ear wax and interpreting inflammatory or traumatic disease of the meatus or tympanic membrane. The sensory innervation of the anterior wall of the external acoustic meatus and the anterior part of the external surface of the tympanic membrane is by the auriculotemporal branch of mandibular V (N. 2). In contrast, the sensory innervation of the posterior wall of the external meatus, the posterior part of the out-side of the tympanic membrane, as well as the deeper part of the auricle and the skin over the mastoid process is provided by the auricular branch of the vagus (N. 2). This branch arises from the vagus as it exits the jugular foramen. It runs laterally through the temporal bone to emerge just posterior to the external meatus. On the way through the temporal bone it picks up some fibers of the facial and glossopharyngeal nerves. So the sensory distribution area of the auricular branch of the vagus is shared with the facial and glossopharyngeal nerves, and this is the only cutaneous sensory distribution of these nerves. Hence, in a neuralgia involving any of these nerves pain can be referred to these surface areas. Likewise, in a herpes zoster viral infection (shingles) of these nerves the cutaneous eruption will occur in these surface areas. MIDDLE EAR (N. 94-96) The middle ear is an obliquely placed normally air-filled cavity within the temporal bone which contains the sound conducting ossicular apparatus (N. 94-96). The ossicular mechanism includes the malleus, incus, and stapes, their articulations and the tensor tympani and stapedius muscles. The middle ear communicates anteromedially through the auditory tube with the nasopharynx and posterolaterally with the mastoid antrum and its budded-off mastoid air cells (N. 96). The oblique plane of the middle ear cavity or


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tympanic cavity parallels the obliquity of the tympanic membrane, so its walls diverge substantially from the standard anatomic planes. So while, for simplicity of description, its walls are frequently described as lateral, medial, anterior, posterior, a roof and a floor, they all face very obliquely. The major plane of the tympanic cavity nearly parallels the long axis of the petrous pyramid. The lateral or tympanic wall is largely formed by the tympanic membrane, which faces anteriorly and inferiorly, as well as laterally (N. 94-96). It is shaped like a shallow cone or a circular speaker of an audio system. Its circumference is attached to the inner end of the bony external acoustic meatus and its central apex, the umbo, is directed medially. The malleus (=hammer) is attached to its inner aspect. The malleus has an upper bulbous head, a narrow neck and a long handle, the manubrium. At the upper end of the manubrium there is a small laterally directed lateral process and an anterior process which serves as the axis of rotation of the malleus. The entire length of the manubrium is attached to the inner surface of the tympanic membrane from the lateral process to its inferior end which attaches to the umbo. The lateral process produces a small external elevation near the upper part of the tympanic membrane. From the lateral process small anterior and posterior mallear folds extend to the periphery of the tympanic membrane and divide it into a small upper pars flaccida and a larger lower pars tensa. The pars tensa has a dense fibrous intermediate layer of circular and radial collagen bundles between its outer cutaneous and inner mucosal layers. These structures are all visualized on otoscopic examination of the external surface of the tympanic membrane (N. 95 upper right). The thin roof of the tympanic cavity (=tegmen tympani) is related above to the middle cranial fossa (N. 94). Hence, middle ear infections (otitis media) can produce middle cranial fossa or temporal lobe abscess. The floor or jugular wall of the middle ear cavity is related below to the bulbous upper end of the internal jugular vein within the jugular foramen (N. 94). The posterior or mastoid wall of the tympanic cavity is deficient above where the tympanic cavity communicates posterolaterally with an air-filled space within the mastoid process called the mastoid antrum (N. 96). Many small mastoid air cells bud off of the mastoid antrum. So the inside of the mastoid process has a honeycomb or sponge-like appearance. Middle ear infection can easily spread into the mastoid air cells to cause a mastoiditis. At times this can be difficult to eradicate by antibiotics, because the spongy character of the mastoid process predisposes to residual pockets of infectious material. The lower part of the posterior wall contains the descending portion of the facial canal with its contained facial nerve (N. 95-96). The chorda tympani enters the middle ear cavity through the lower part of its posterior wall (N. 96, 124 (unlabeled), 125). The anterior or carotid wall of the tympanic cavity faces anteromedially. Its lower portion is formed by the carotid canal (N. 96). Above this it is deficient where the bony auditory (pharyngotympanic or Eustachian) tube opens into the tympanic cavity. The upper tympanic end of the auditory tube is bony, while the lower nasopharyngeal end is cartilaginous (N. 94, 100). The cartilaginous auditory tube is not a complete cylinder. Its lateral wall is mostly membranous and it is from this wall that the tensor veli palatini muscle has part of its origin (see Figure 8-4; N. 100). In the adult the lumen of the auditory tube in section is a narrow vertically oriented ellipse with its mucosally lined surfaces in contact (see Figure 8-4; N. 100). So the tube is usually functionally closed. It can be opened by contracting the tensor veli palatini muscle as in swallowing. This muscle will pull the lateral wall of the tube downward and outward to create a more rounded outline and thereby separate the mucosal surfaces. This opens the lumen to allow the pressure in the middle ear cavity to be equilibrated to the external atmospheric pressure. If the mucosa is swollen by a pharyngeal or middle ear inflammatory process this mechanism will be impeded, preventing pressure equalization. This can produce middle ear pain when atmospheric pressure increases or decreases as in skin diving or air travel. In children the auditory tube tends to be more horizontally placed and its ostium normally patent, presumably permitting easier access of infectious organisms into the tympanic cavity from the nasopharynx. This may help explain the greater frequency of middle ear infections in children with upper respiratory infections.


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Figure 8-4

Just above the bony auditory tube there is a hollow bony canal which contains the tensor tympani muscle (N. 95-96). The tendon of the tensor tympani muscle emerges from an aperture at the tympanic end of this canal and runs laterally across the tympanic cavity to attach to the upper end of the manubrium of the malleus. This muscle is innervated by the mandibular division of the trigeminal nerve which exits the foramen ovale in intimate relation to the origin of this muscle. While the current literature questions some of the functions classically ascribed to this muscle, it seems likely that this muscle exerts a medial pull on the manubrium and the attached tympanic membrane. This apparent effect has led to two classical hypotheses about its function. Perhaps its adjusts the tension of the tympanic membrane to enhance the transmission of various sound frequencies. Or it may participate in damping the oscillations of the malleus in the face of high intensity sounds (see stapedial reflex). The medial or labyrinthine wall of the tympanic cavity is mostly formed by the outer aspect of the bony internal ear (bony labyrinth) structures (N. 94, 96-98). High on this wall there is a prominence of the lateral semicircular canal. Just below this is a prominence of the facial canal formed by the posteriorly directed portion of the facial canal. Immediately inferior to this is the oval (vestibular) window, which articulates with the stapes footplate and communicates with the bony vestibule medially. Just inferior to the vestibular window there is a rounded protuberance called the promontory. This is produced by the deeply lying basal turn of the cochlea. Immediately posterior to the promontory is the round (cochlear) window. This is closed by a secondary tympanic (round window) membrane which communicates medially with the basal turn of cochlea. This membrane permits pressure waves produced in the inner ear fluids by movements of the stapes footplate to be dissipated back into the tympanic cavity. The stapes (=stirrup) is shaped like the stirrup of a saddle (N. 94-96). Its oval base or footplate is attached to the oval window by an encircling annular ligament. Extending laterally from each end of the footplate are two curved crura which join at the laterally directed head. The incus (=anvil) is the ossicle which connects the malleus to the stapes. It has an anteriorly facing body which articulates with the head of the malleus in the formation of the synovial incudomallear joint (N. 94-96). It has a posteriorly directed short process which is attached to the floor of the opening into the mastoid antrum. It has an inferiorly directed more slender long process. At its tip the long process bends medially to articulate with the head of the stapes at the synovial incudostapedial joint. As the compressive components of sound waves displace the tympanic membrane inward the manubrium of the malleus is similarly displaced. Since the axis of rotation of the incudomallear joint is a relatively anteroposteriorly directed line through the anterior process of the malleus and the short process of the incus, inward displacement of the manubrium causes outward displacement of both the head of the malleus and the body of the incus, which are situated above this axis. Since the long process of the incus is below this axis it will be displaced inward against the stapes head. The resultant inward displacement of the stapes does not produce a piston-like inward


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displacement of its entire footplate, because the axis about which the stapes footplate moves is a relatively vertical axis through the posterior part of the footplate. Therefore the anterior part of the stapes footplate is displaced inward in a motion similar to that which occurs when one keeps musical time by tapping the ball of the foot against the floor around an axis through the heel. This movement of the stapes footplate generates pressures within the perilymphatic fluid of the internal ear which are, in turn, detected by the hair cells of the cochlea. Posterior to the vestibular window a small stapedius muscle is situated in a tiny bony prominence within the bend which the facial canal makes as it turns from its posteriorly directed course on the medial wall to its inferiorly directed course in the posterior wall of the tympanic cavity (N. 96). The tendon of the stapedius runs forward to attach to the posterior aspect of the head of the stapes. The stapedius is innervated by a branch from the closest available nerve, the facial nerve. When it contracts its posterior pull on the stapes head damps the movements of the anterior part of the stapes footplate into the inner ear. Contraction of the stapedius is typically a reflex response to loud sounds detected by the cochlea and transmitted to the pons through the vestibulocochlear nerve. This stapedial reflex thus has an VIII nerve afferent limb and a VII nerve efferent limb. By damping the oscillations of the end of the ossicular chain it helps protect the sensitive inner ear hair cells from mechanical damage from high intensity sounds. If there is facial nerve injury proximal to its stapedial branch the patient will often complain that moderate intensity sounds, which others find tolerable, are irritating. This is called hyperacusis. If disease processes interfere with the normal movements of the ossicular chain a patient develops a type of deafness referred to as a conductive deafness, since it is usually caused by a diminished ability of the ossicles to conduct sounds from the tympanic membrane to the internal ear. Otosclerosis is a common cause of conductive deafness. It most commonly involves a bony overgrowth which fixes the moveable anterior part of the stapes footplate within the oval window. Conductive deafness can also be caused by diseases which damp the ossicular movement, like fluid accumulating in the middle ear during or after infection and the not uncommon benign soft tissue tumor of the middle ear, called cholesteatoma. A conductive deafness can occasionally be caused by obstruction of the external acoustic meatus. The chorda tympani arises from the facial nerve in the lower part of its descending course not far above the stylomastoid foramen. It enters the tympanic cavity through its posterior wall. It passes anteriorly between the neck of the malleus laterally and the long process of the incus medially to reach the petrotympanic fissure where it exits through the anterior wall of the middle ear (N. 95-96). This nerve can be injured in middle ear disease or during ear surgery with a resultant loss of taste to the anterior two thirds of the tongue and parasympathetic secretomotor innervation to the submandibular and sublingual salivary glands (see infratemporal fossa and suprahyoid region). The sensory innervation of the mucosa of the middle ear cavity, mastoid air cells and upper auditory tube is provided by the tympanic branch of the glossopharyngeal nerve (N. 124, 126). This nerve arises from the glossopharyngeal nerve as the IX nerve exits the jugular foramen. It enters the middle ear cavity through its floor by entering the plate of bone which separates the carotid canal from the jugular foramen. It then ascends onto the surface of the promontory where it provides the major input to the tympanic plexus, which ramifies over the promontory (N. 96). The tympanic branch of IX brings both sensory and parasympathetic preganglionic nerve fibers into the tympanic plexus. Therefore, the pain of otitis media (middle ear infection), mastoiditis and auditory tube disease is predominantly conveyed to the brain over the glossopharyngeal nerve. Its parasympathetic fibers provide secretomotor innervation to the parotid gland (N. 126). THE INTERNAL EAR (N. 94, 97-99) The internal ear is composed of the osseous labyrinth and its contained membranous labyrinth. The osseous labyrinth is formed by a series of interconnected excavations within the bony petrous pyramid. These bony channels contain the interconnected membranous tubes and sacs which form the membranous labyrinth. While many of the structures of the membranous labyrinth are geometrically similar to the surrounding bony labyrinth, they do not fill the bony labyrinth and typically only attach to it


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on one or two aspects. The interval which separates the membranous labyrinth from the remainder of the bony labyrinth is filled with a fluid, called perilymph. The membranous labyrinth itself contains a fluid called endolymph. The long axis of the osseous labyrinth pretty much parallels the oblique axis of the petrous pyramid and is composed of the cochlea, vestibule and three bony semicircular canals (N. 97, 99). The cochlea (=snail shell) is aptly named since it is a coiled bony spiral of two and three quarter turns which resembles a snail shell. It forms the anteromedial part of the osseous labyrinth and it lies on its side so its apex is directed relatively anteriorly and laterally (N. 99). Its basal turn communicates posterolaterally with the relatively round bony vestibule. The ends of the three C-shaped osseous semicircular canals connect with the vestibule and are largely situated posterolateral to it. The plane of each semicircular canal is at right angles to each of the others. The anterior (or superior) semicircular canal is situated at a right angle to the long axis of the petrous pyramid. The posterior semicircular canal is parallel to the long axis of the petrous pyramid. Both of these relatively vertically situated canals are perpendicular to the lateral (or horizontal) semicircular canal. The lateral canal is not exactly in the horizontal plane, since its anterior end is elevated about 30o above its posterior end. But it is still perpendicular to the other canals since all the canals are tilted backward a similar amount. One end of each bony canal is dilated to form an ampulla. The membranous labyrinth is formed of a cochlear duct, a saccule and utricle and three membranous semicircular canals (N. 94, 97-98). The three membranous semicircular canals conform to and are named like their bony semicircular canals. Their dilated ampullated ends contain the hair cells which detect displacements of endolymphatic fluid within them. Since they form a three dimensional coordinate system, angular motion of the head in any direction will be detected by one or more canals and they are the primary organs for detecting rotary acceleration. The bony vestibule contains two membranous sacs, the utricle and saccule. These each contain an otolithic membrane-hair cell apparatus which primarily detects head position and linear motion. The membranous semicircular canals connect to the utricle. The utricle is in turn connected with the saccule by a narrow utriculosaccular duct. The saccule in turn is connected to the cochlear duct by a narrow ductus reuniens. The cochlear duct then follows the turns of the bony cochlea and divides them into two separate perilympathic chambers as it spirals toward the cochlear apex where it ends blindly. The cochlear duct contains the spirally arranged organ of Corti hair cell apparatus which detects sound stimuli that have been transduced into fluid pressure waves by the ossicular apparatus. Since all parts of the membranous labyrinth communicate with each other, the endolymphatic fluid they contain is a continuous but closed compartment (N. 97-98). It is everywhere separated from the perilymphatic fluid compartment by the walls of the membranous labyrinth. Likewise, all parts of the perilymphatic fluid compartment communicate with each other, but the perilympathic fluid also communicates with the cerebrospinal fluid in the subarachnoid space through the cochlear aqueduct (N. 98). This is a minute channel which communicates between the perilymph in the basal turn of the cochlea and the cerebrospinal fluid in the vicinity of the anterior wall of the jugular foramen where it opens. This provides a potential pathway whereby a meningitis could be transmitted to the inner ear to produce a labyrinthitis or vice versa. VESTIBULOCOCHLEAR NERVE The vestibulocochlear or VIII cranial nerve is the nerve for equilibrium and hearing. Its vestibular division innervates the hair cells of the three semicircular canals and the utricle and saccule, and hence conveys information concerning the motion or position of the head in space. The vestibular branches from each of these receptor organs converge at the distal end of the internal acoustic meatus to form the vestibular division of VIII (N. 125). The nerve cell bodies of these sensory neurons are located in a vestibular ganglion situated within the internal acoustic meatus. The cochlear division of the VIII nerve innervates the hair cells of the cochlear duct and therefore conveys hearing. The nerve cell bodies of its sensory fibers are situated within the bony cochlea where they form a cochlear (spiral) ganglion (N.


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125). Their central processes form the cochlear division of VIII which joins the vestibular division within the internal acoustic meatus. The VIII nerve then crosses the subarachnoid space in the cerebellopontine angle to enter the inferior pontine sulcus in line with the postolivary sulcus. At this point it is closely related to the anterior inferior cerebellar artery which usually gives off the labyrinthine artery that accompanies it to the inner ear organs. The VIII nerve not uncommonly develops a vestibular schwannoma within the internal acoustic meatus where it can cause not only VIII nerve hearing and equilibratory deficits, but can also involve the closely related VII nerve. Any lesion of the VIII nerve from the receptor organs to its low pons nuclei of termination can cause a neural (or sensorineural) type of deafness and a host of balance problems. FACIAL NERVE The facial nerve is summarized on pages 246-247. During its temporal bone course it is closely related to the vestibulocochlear nerve only as far as the distal end of the internal acoustic meatus. At this point it enters the facial canal which, at first, has a short laterally directed course that carries it between the cochlea anteriorly and the vestibule posteriorly (N. 99, 125). At the end of its laterally directed course it makes a sharp bend posteriorly. This is called its external genu (in contrast to another knee-like bend its motor fibers make inside the pons). At this point it contains a sensory geniculate ganglion which contains the nerve cell bodies of its general sensory and taste neurons (N. 99, 125-126). It is at the level of the geniculate ganglion that the greater petrosal nerve originates and exits its hiatus. It conveys preganglionic parasympathetic fibers to the pterygopalatine ganglion for the innervation of the palatine, nasal and lacrimal glands (see nasal cavity). From its genu the facial nerve pursues a posteriorly directed course within the facial canal in the medial wall of the middle ear cavity (N. 94, 96, 124-125). It turns inferiorly around the origin of the stapedius muscle where it gives off innervation to this muscle (N. 124). It then descends in the posterior wall of the tympanic cavity and gives off the chorda tympani and its cutaneous branches just before it exits the stylomastoid foramen (N. 96). After exiting the stylomastoid foramen it gives off its branches to the muscles of facial expression and the stylohyoid and posterior belly of the digastric muscles (see pages 246-247; N. 24, 124).

THE ANTERIOR NECK OSTEOLOGY OF THE POSTERIOR PORTION OF THE INFERIOR ASPECT OF THE SKULL Prior to the study of the anterior neck the major features of the posterior part of the inferior aspect of the skull should be identified (N. 10) since it is this region of the skull to which some of the neck musculature gains attachment and through which the cranial nerves and major vessels of the neck emerge from or enter the cranial cavity. The unpaired occipital bone forms much of this aspect of the skull. It surrounds the foramen magnum. Posterior to the foramen magnum the broad curved squamous part of the occipital bone forms much of the posteroinferior aspect of the skull. The convex occipital condyles, which articulate with the atlas, are located adjacent to the anterolateral border of the foramen magnum. A hypoglossal canal is located above the middle of each occipital condyle. It conveys the hypoglossal nerve from the cranial cavity into the upper neck. The occipital bone articulates anterolaterally with the petrous parts (pyramids) of the paired temporal bones. In this petrooccipital fissure the large irregular jugular foramen is located lateral to the occipital condyle. Through this foramen the glossopharyngeal, vagus and accessory cranial nerves emerge from the cranium and the sigmoid and inferior petrosal dural venous sinuses empty into the superior bulb of the internal jugular vein. Just anterior to the jugular foramen a large rounded aperture marks the external opening of the carotid canal, through which the internal carotid artery and its surrounding sympathetic nerve plexus enter the skull. The large mastoid process is located at the


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posterolateral end of the petrous pyramid where it joins the more laterally placed squamous part of the temporal bone. The sternocleidomastoid and some deep back muscles insert into the mastoid process. The tympanic part of the temporal bone forms the anterior wall, floor and part of the posterior wall of the external acoustic meatus. In inferior view it appears largely as a plate of bone forming the posterior part of the mandibular fossa which articulates with the condyle of the mandible. The large pointed process protruding inferiorly from about the middle of the inferior edge of this tympanic plate is the styloid process. The stylomastoid foramen is located between the styloid and mastoid processes. Here the facial nerve emerges from its canal in the temporal bone. The squamous part of the temporal bone forms the larger concave anterior portion of the mandibular fossa and much of the lateral aspect of the temporal bone. Anterior to the foramen magnum the occipital bone articulates with the body of the sphenoid bone. This area forms the roof of both the nasopharynx and the choanae or posterior apertures of the nasal cavity. Paired pterygoid processes of the sphenoid bone protrude inferiorly from the lateral parts of its body (N. 8, 10). Each pterygoid process has a medial lamina which forms the lateral wall of the choanae. At the inferior tip of the medial lamina a small hook-like process, the hamulus, protrudes posteriorly to serve as the upper anterior attachment point of the superior constrictor muscle of the pharynx and a pulley for the tensor veli palatine muscle. The lateral lamina of the pterygoid process forms the medial wall of the infratemporal fossa and serves as origin for the pterygoid masticatory muscles. The petrous part of the temporal bone articulates anterolaterally with the greater wing of the sphenoid bone in a trough-like petrosphenoid fissure. This fissure lodges the cartilaginous portion of the auditory (pharyngotympanic or Eustachian) tube (N. 10, 100). At the posterolateral end of this fissure the external opening of the bony portion of the auditory tube is located. The irregular opening between the anteromedially located petrous apex and the sphenoid bone is the foramen lacerum. In life this is filled with cartilage related to the cartilaginous part of the auditory tube which passes beneath it. This cartilage-filled foramen serves as the floor of the medial end of the carotid canal. The more anteriorly situated bones of the base of the skull are described with the face, infratemporal fossa and the oral cavity. SURFACE ANATOMY OF THE INFRAHYOID PORTION OF THE ANTERIOR CERVICAL TRIANGLE (N. 1, 28-29, 72) If the head is turned to the left, the right sternocleidomastoid muscle is readily palpable from the mastoid process to the sternoclavicular joint (see pages 41 and 230). Turning the head to the opposite side against resistance is one of the good clinical tests for the integrity of this muscle and its innervation, the spinal part of the accessory nerve. The sternocleidomastoid muscle so identified serves to conveniently divide each side of the neck into an anterior and posterior triangle. The posterior triangle is bounded anteriorly by the sternocleidomastoid muscle, posteriorly by the anterior edge of the trapezius muscle and below by the clavicle. The anterior triangle is bounded posteriorly by the sternocleidomastoid muscle, above by the inferior border of the mandible and anteriorly by the midline of the neck. Within the infrahyoid portion of the anterior triangle there are a number of structures of importance to the physical examination of the neck and these are best located by reference to palpable bony, cartilaginous and muscular landmarks. The suprahyoid part of the anterior triangle will be described with the floor of the mouth. Between the sternoclavicular origins of the sternocleidomastoid muscles the jugular notch, which forms the upper border of the sternum, can be visualized and palpated. About 2-3 cm above this notch the transversely oriented ring-like cricoid cartilage is palpable. For purposes of x-ray correlation this part of the airway is at the level of the sixth cervical vertebra (N. 64). Between the cricoid and jugular notch the tubular trachea is palpable deep to both the thin infrahyoid muscles and the thyroid isthmus, which joins the two lobes of this gland together over the front of the second to fourth tracheal rings.


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In the midline about 4 cm above the cricoid the laryngeal prominence of the upper thyroid cartilage can be visualized or palpated and this is located at the level of the fourth cervical vertebra. On either side of this prominence the superior borders and upper portions of the thyroid laminae are palpable. However, the lower lateral parts of the thyroid laminae are partly obscured by the vague fullness of the overlying lobes of the thyroid gland (N. 72, 76). These lobes can be palpated deep to the sternocleidomastoid and infrahyoid muscles where these muscles lie along the lateral aspect of the cricoid cartilage and upper trachea. About 1 cm above the upper border of the thyroid cartilage the body of the hyoid bone can be palpated in the midline at the point where the floor of the mouth meets the anterior neck. On either side of the body the greater horns of the hyoid can be followed posterolaterally by palpation. The hyoid is usually at the level of the third cervical vertebra. The common and internal carotid arteries (N. 72, 137) lie along a line from the sternoclavicular joint to a point midway between the angle of the mandible and the mastoid process. The carotid pulse can be taken by palpating under the anterior border of the sternocleidomastoid muscle at any point along this line. However, to control bleeding the common carotid artery can be best compressed against the prominent anterior tubercle (carotid tubercle) of the transverse process of the sixth cervical vertebra using the cricoid cartilage as a landmark. The thyrohyoid interval is an important landmark, since following the interval laterally to the anterior border of the sternocleidomastoid muscle locates the usual level where the common carotid artery bifurcates into the external and internal carotid arteries (N. 72, 137). At this point a dilation of the arterial wall marks the location of the carotid sinus which serves as a pressure receptor. Unilateral external compression of the carotid sinus at this point can be valuable diagnostically or therapeutically (see page 231). An occasional member of the superficial or deep lymph nodes of the neck, chronically enlarged by past infection, may be palpable in the normal neck (N. 74-75). Superficial cervical nodes should be sought in the occipital, retroauricular and preauricular areas. Some also accompany the external jugular vein vertically across the superficial surface of the sternocleidomastoid muscle and others lie along the anterior jugular veins in the lower anterior neck. The superior deep lateral cervical nodes, which lie along the internal jugular vein, should be sought superiorly along the anterior border of the sternocleidomastoid muscle. The specifically named jugulodigastric node, that drains the posterior tongue and palatine tonsils, is commonly enlarged and can be located at the level of the greater horn of the hyoid bone. Inferior deep lateral cervical nodes should also be sought just above the clavicle and behind the sternocleidomastoid muscle. Here they are sometimes called supraclavicular or scalene nodes, the clinical importance of which is described on page 234. Other more deeply located cervical nodes may be found in the posterior triangle accompanying the accessory nerve or in the anterior triangle in a prelaryngeal or pretracheal position. CERVICAL FASCIA AND COMPARTMENTS (N. 26) The subcutaneous tissue (superficial fascia) of the neck contains the platysma muscle, superficial veins, superficial lymph nodes and the cutaneous branches of the cervical plexus (N. 2, 25, 31, 74). The cervical fascia deep to the subcutaneous tissue is divided into a number of named layers which compartmentalize the structures of the neck and between which lie the major fascial planes of the neck. The investing (superficial) layer of the cervical fascia (N. 26) lies deep to the subcutaneous tissue and forms a cylindrical enclosure for the deeper structures. It splits to enclose the trapezius and sternocleidomastoid muscles and forms a single layer over the posterior and anterior triangles of the neck. The infrahyoid (strap) muscles are surrounded by fascia. The fascia on the deep surface of these muscles is the pretracheal layer. It covers the anterolateral surface of the visceral structures of the neck


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including the larynx, trachea and thyroid gland. Around the sides of these visceral structures the pretracheal fascia is continuous with the buccopharyngeal fascia which covers the lateral and posterior surfaces of the pharynx. With the pretracheal fascia it defines a cylindrical enclosure for the visceral compartment of the neck. The buccopharyngeal fascia forms a continuous membranous external covering over both the buccinator muscles of the cheeks and the pharyngeal musculature (N. 47). The prevertebral layer of the cervical fascia forms a cylindrical investment for the vertebral column and its musculature (N. 26). From posterior midline attachments it encloses the spine and posterior neck muscles. Superiorly, it follows the musculature to the base of the skull. Inferiorly the fascia over the front of the bodies of the cervical vertebrae can be followed down into the posterior mediastinum of the thorax. In the interval between the buccopharyngeal fascia and the prevertebral fascia (or as some authors prefer, between the layers of the prevertebral fascia anterior to the bodies of the cervical vertebrae) an easily separable fascial plane exists called the retropharyngeal "space" (N. 26). This fascial plane serves as a potential highway along which infection can spread from some of the more commonly infected areas of the head down into the mediastinum. The retropharyngeal "space" communicates superiorly around the sides of the pharynx with the fascial planes around the palatine tonsils and in the floor of the mouth, which may become infected respectively by peritonsillar abscess and abscessed teeth. Further, the lymph nodes of the retropharyngeal fascial plane drain the commonly infected areas of the nasal cavity, paranasal sinuses, middle ear and upper pharynx (N. 75). Once infection enters the retropharyngeal "space" along these fascial planes or lymphatic channels it may either form a localized retropharyngeal abscess or spread along the retropharyngeal fascial plane inferiorly into the posterior mediastinum as far as the diaphragm causing a mediastinitis with its serious implications. The internal jugular vein, vagus nerve, and common carotid artery and at higher levels the internal carotid artery are invested in a tubular fascial sheath called the carotid sheath which has continuities with the other fascial layers of the neck. SUPERFICIAL STRUCTURES OF THE ANTERIOR NECK The structures within the superficial fascia of the neck include the platysma muscle, cutaneous branches of the cervical nerves and superficial veins and lymph nodes. In the subcutaneous tissue the platysma muscle (N. 25) extends from the skin over the mandible to an attachment into the upper pectoral skin. This muscle functions to draw the corner of the mouth downward, elevate the upper pectoral skin and widen the neck. Its innervation, by the cervical branch of the facial nerve (N. 24), enters its deep surface from the lower pole of the parotid gland just behind the angle of the mandible. Within the subcutaneous tissues just deep to that part of the platysma muscle which crosses the inferior margin of the mandible, the marginal mandibular branch of the facial nerve (N. 24, 31) typically loops a little below the mandibular margin. It crosses superficial to the facial vessels as they enter the face near a notch in the inferior margin of the mandible. This nerve is easily injured at this point in neck surgery and this causes a disfiguring paralysis of the muscles of the lower lip (see page 240). The platysma is penetrated by the cutaneous branches of the cervical plexus, the main stems of which lie in the subcutaneous tissue deep to the platysma. The cutaneous branches of the cervical plexus (see page 41; N. 2, 31-32) enter the subcutaneous tissue from a key nerve center located at the middle of the posterior border of the sternocleidomastoid muscle. The highest of these is the lesser occipital nerve (C.2,3) which ascends along the posterior border of the sternocleidomastoid muscle to supply the skin behind the mastoid. Next the great auricular nerve (C.2,3) ascends vertically toward the auricle, crossing the sternocleidomastoid, often in company with the external jugular vein. It supplies the skin over the angle of the mandible, lower peripheral auricle and mastoid process. The transverse cervical nerve (C.2,3) passes transversely forward across the sternocleidomastoid to supply much of the skin of the anterolateral neck. Three supraclavicular nerves (C.3,4) descend across the posterior triangle to supply the lower neck and "shoulder pad" area.


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Superficial veins also run within the subcutaneous tissue deep to the platysma (N. 31). The external jugular vein begins above by a variable junction of the posterior auricular and retromandibular veins at the angle of the mandible (N. 3, 31). After descending vertically across the sternocleidomastoid muscle it terminates below in the subclavian vein. This vein is used clinically to evaluate venous pulsations and to estimate venous pressure by gradually tilting the patient upright to see at what height above the heart it collapses. The anterior jugular veins drain from the suprahyoid region toward the jugular notch of the sternum where they may form a venous arch across the midline which may be encountered in tracheotomy (see page 239). There they usually receive a communicating vein from the facial vein which descends along the anterior border of the sternocleidomastoid. The anterior jugular veins then turn laterally deep to the origin of the sternocleidomastoid to empty into the external jugular vein. STERNOCLEIDOMASTOID MUSCLE (N. 27, 29) The sternocleidomastoid muscle arises from the medial clavicle and the sternum and inserts into the mastoid process. This muscle is best tested by turning the chin to the opposite side against the resistance of the examiner's hand on the mandible. It is innervated by the spinal part of the accessory nerve (N. 33, 129). This muscle is removed in radical neck dissection surgery for many types of cancer of the head and neck. Birth trauma causing hemorrhage into the muscle with subsequent scar tissue formation and contracture can cause congenital torticollis where the infant's face will be turned away from the side of the damaged shortened muscle. ACCESSORY (CRANIAL XI) NERVE (N. 32, 128) The accessory nerve is motor to the sternocleidomastoid, trapezius, laryngeal, pharyngeal and palatal muscles. This nerve is formed intracranially by a temporary connection between its cranial and spinal roots. The cranial root (vagal part) arises by rootlets from the postolivary sulcus of the medulla (N. 115, 128). The spinal root (part) emerges by rootlets from the lateral surface of the upper five cervical segments of the spinal cord and ascends the cervical spinal canal to enter the cranial cavity through the foramen magnum and join the cranial root in the posterior cranial fossa near the jugular foramen (N. 128). These roots separate almost immediately, with the cranial root subsequently joining the vagus to distribute to the muscles of the larynx, pharynx and palate. The spinal root exits the cranial cavity through the jugular foramen and descends the upper neck lateral to the nasopharynx. It passes deep to the sternocleidomastoid muscle which it innervates. Then it crosses the posterior triangle of the neck to innervate the trapezius. Testing these muscles evaluates the integrity of the spinal part of the accessory nerve, while testing laryngeal, pharyngeal and palatal muscle function evaluates the cranial root of the accessory nerve as well as the vagus. THE INFRAHYOID (STRAP) MUSCLES (N. 28, 32) The infrahyoid (strap) muscles are four small flat muscles which extend from the rib cage and scapula below to the larynx and hyoid bone above. These muscles collectively function to depress the larynx and, through the hyoid, the floor of the mouth. They are innervated primarily by branches of the ansa cervicalis of the cervical plexus. These muscles are encountered in thyroid surgery, tracheotomy and laryngotomy. THE CERVICAL PLEXUS AND ITS MUSCULAR BRANCHES (N. 32, 130) The cervical plexus is formed by a series of loops between the ventral rami of the first four cervical spinal nerves. The cutaneous branches are described on the previous page. The muscular branches include those to the infrahyoid muscles, diaphragm and some deep neck muscles. The infrahyoid muscles are innervated by branches from the looplike ansa cervicalis which is formed by a superior and inferior root. The superior root arises from the loop between the first and second cervical nerves and runs with the hypoglossal nerve for several centimeters after which this root descends


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anterior to the carotid sheath. The inferior root emerges from the loop between the second and third cervical nerves and descends behind the carotid sheath to form a loop with the superior root. The loop may be formed either lateral or medial to the internal jugular vein at any level from the hyoid to the clavicle. The phrenic nerve arises primarily from the ventral ramus of the fourth cervical nerve with usual contributions from the third and fifth cervical nerves. It descends vertically across the superficial surface of the anterior scalene muscle. It traverses the mediastinum to provide motor innervation to the diaphragm and sensory innervation to its central region (see pages 42, 126). CAROTID SHEATH STRUCTURES AND THE EXTERNAL CAROTID ARTERY The carotid sheath contains the laterally located internal jugular vein, the medially located common carotid artery and its internal carotid artery continuation, and the vagus nerve which is located posterior to and between the artery and vein (N. 26, 31, 71-72). The internal jugular vein (N. 28, 73) begins above at the jugular foramen where the sigmoid and inferior petrosal dural venous sinuses empty into its dilated superior bulb. As it descends through the neck it receives tributaries comparable to many of the branches of the external carotid artery. It terminates below by joining the subclavian vein to form the brachiocephalic vein. As the internal jugular vein descends the neck it comes to lie posterolateral, lateral and then anterolateral to the internal and common carotid arteries. (CD H&N Compare Sections 7 through 18). It shifts relatively anteriorly as it descends in anticipation of the anterior position of the brachiocephalic veins in the superior mediastinum. The common carotid artery (N. 71-76, 139, 141) arises below on the right from the brachiocephalic artery behind the sternoclavicular joint and on the left from the arch of the aorta. It terminates above at the level of the hyothyroid interval by dividing into the internal and external carotid arteries. The internal carotid artery supplies the upper brain structures, orbit and forehead regions, while the external carotid artery supplies most of the cranial meninges, most of the extracranial regions of the head and the upper cervical structures. At its origin the internal carotid artery is posterolateral to the external carotid artery (CD H&N Sections 14, 15), but as it ascends toward the carotid canal it assumes an internal or medial position. Occlusion of the common or internal carotid arteries, which provide a major blood supply to the brain, can produce signs and symptoms similar to those of intracranial cerebral vascular accidents (stroke). The patency of the common carotid system can be visualized by arteriogram, ultrasound or magnetic resonance angiography. The bifurcation of the common carotid artery and the initial portion of the internal carotid artery are dilated to form the carotid sinus (N. 71-72, 126, 131, 137). This serves as a pressure receptor and receives afferent innervation from the carotid sinus branch of the glossopharyngeal nerve (N. 126). When the arterial pressure is increased a carotid sinus reflex is generated which will produce a blood pressure reduction and a slowing of heart rate with the vagus nerve serving as the efferent limb of this reflex. This occurs normally and also when carotid sinus compression is performed by applying direct pressure unilaterally to a carotid sinus by compressing the artery at the level of the hyothyroid interval. Carotid sinus compression can be used diagnostically to test the integrity of the afferent and efferent limbs of the reflex and their medulla connections, or therapeutically to terminate episodes of paroxysmal atrial tachycardia. If the blood pressure falls physiologically, the efferent limb of this reflex will become sym-pathetic with a resultant increase in heart rate and blood pressure. The carotid body (N. 71, 126, 131, 137) is a small flat chemoreceptor only a few mm in diameter located on the posteromedial aspect of the common carotid artery's bifurcation. It is connected to this artery by small vessels. Its sensory innervation, like that of the carotid sinus, is primarily from the glossopharyngeal nerve, but the vagus usually also provides some innervation (N. 126-127, 131). The


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stimulus is typically hypoxia and the reflex response involves an increase in the rate and depth of respiration. The external carotid artery gives off eight main branches the distribution of which will be considered with the subregions they supply (N. 34, 72, 137, 139). Arising early are superior thyroid artery which courses anteriorly to descend to the thyroid gland and the ascending pharyngeal artery which passes medially to ascend along the pharynx. The lingual and facial arteries arise next from the anterior aspect of the artery to course anteriorly through the suprahyoid region to the tongue and face. The occipital and posterior auricular arteries course posteriorly to their respective regions. The external carotid artery terminates behind the neck of the mandible by dividing into a maxillary artery which passes deep to the neck of the mandible to enter the infratemporal fossa and the superficial temporal artery which distributes over the temporal scalp area. VAGUS NERVE (N. 127) The vagus nerve contains five functional nerve fiber types: somatic afferent from the skin over the mastoid, deep central part of the auricle, the posterior wall of the external acoustic meatus, and the posterior part of the outer surface of the tympanic membrane; visceral afferent from the carotid body and visceral afferent and efferent innervating the heart, larynx, trachea, lung and gastrointestinal system and its accessory organs down to the level of the left side of the transverse colon; taste fibers from the epiglottis; and motor innervation to most of the voluntary muscles of the pharynx, upper esophagus, palate and larynx. As the vagus nerve exits from the jugular foramen it demonstrates superior and inferior ganglia which house respectively the nerve cell bodies for the somatic and visceral afferent functional fibers of this nerve. At this point it gives off a recurrent meningeal branch to the dura of the posterior cranial fossa. In the jugular foramen it also gives off an auricular branch which passes through the temporal bone to emerge behind the external auditory meatus. It conveys the somatic afferent fibers of the vagus from the posterior wall of the external auditory meatus, posterior half of the outer surface of the tympanic membrane, deep part of the auricle, and skin over the mastoid. On the way through the temporal bone this nerve is commonly joined by the somatic afferent fibers from the facial and glossopharyngeal nerves which supply these same areas. The significance of this small cutaneous distribution of these three cranial nerves lies in the fact that a neuralgia involving any of these nerves may be referred to these areas or a herpes zoster infection (shingles) of any of these nerves may cause a cutaneous eruption in these areas. At the level of the inferior ganglion pharyngeal branches arise which provide motor innervation to the pharynx and palate and some sensory innervation to the carotid body. The superior laryngeal nerve also arises at this point and then courses downward to divide into an external branch that is motor to the cricothyroid muscle and an internal branch that provides sensory innervation to the interior of the supraglottic portion of the larynx. Several cervical cardiac branches arise from the vagus as it descends through the neck. They may join similar branches of the cervical sympathetic trunk as they descend to the cardiac plexuses about the arch of the aorta. They provide some parasympathetic and visceral afferent innervations to the heart. The recurrent laryngeal nerves of the two sides arise differently in that the right one loops under the right subclavian artery (N. 33, 76-77) and the left one loops under the arch of the aorta (N. 76-77, 82, 127). Both of these ascend near the tracheoesophageal groove and provide sensory and motor innervation to the trachea and upper esophagus and to all the muscles of the larynx except the cricothyroid muscle. In its course through the thorax the vagus gives off parasympathetic and visceral afferent innervation to the heart, lungs, and esophagus and in the abdomen innervates the gastrointestinal tract and its


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accessory organs as far as the level of the left side of the transverse colon (described fully with the thorax and abdomen). THE SUBCLAVIAN ARTERY AND ITS BRANCHES (N. 33, 137, 139) The right subclavian artery arises from the brachiocephalic artery posterior to the right sternoclavicular joint, while the left subclavian artery arises from the aortic arch. Both subclavian arteries arch up into the base of the neck to pass superior to the first rib in the scalene interval between the anterior and middle scalene muscles (see scalene interval syndrome on page 45). They end at the outer border of the first rib where they become continuous with the axillary arteries. Each subclavian artery is divided into three parts based on its relationship to the anterior scalene muscle. The first part is medial, the second part posterior and the third part lateral to the anterior scalene muscle. Most of the usual four branches of the subclavian artery arise from the first part. These are the internal thoracic, vertebral, thyrocervical and costocervical arteries. The internal thoracic (mammary) artery descends into the mediastinum. The vertebral artery ascends to enter the transverse process of the sixth cervical vertebra (N. 33, 137). It then ascends through all of the transverse foramina of the upper cervical vertebrae. As it exits the transverse foramen of the atlas it courses posteriorly and medially along the superior aspect of its posterior arch to enter the lateral aspect of the foramen magnum, where its course is described with the cranial cavity (pages 203-204). As it ascends the cervical spine it gives off important radicular arteries which accompany the cervical nerve roots to contribute to the arterial input of the spinal arteries at cervical levels. The thyrocervical trunk (artery) arises from the superior aspect of the subclavian artery lateral to the vertebral arteries. Its inferior thyroid branch ascends on the anterior aspect of the anterior scalene muscle to the level of the cricoid cartilage (C6) where it loops medially posterior to the carotid sheath structures and descends to the posterior aspect of the inferior pole of the thyroid gland. The thyrocervical trunk usually gives off other branches to adjacent neck and shoulder structures. A minor costocervical trunk (artery) arises from the deep aspect of the subclavian artery and gives off some branches to the upper intercostal spaces and deep posterior cervical muscles. CERVICAL SYMPATHETIC TRUNK (N. 131-133) The cervical sympathetic trunk lies immediately in front of the transverse processes of the cervical vertebrae and posterior to the carotid sheath. It typically has three ganglia. The inferior cervical ganglion is located at the level of the seventh cervical vertebra just posterior to the vertebral artery. It is usually fused with the first thoracic ganglion to form the cervicothoracic (stellate) ganglion. The middle cervical ganglion may be located in a high position at the level of the sixth cervical vertebra adjacent to the medially looping inferior thyroid artery or in a low position on the anterior surface of the vertebral artery. The large elongated superior cervical ganglion is located at the level of the second cervical vertebra. Only the cervicothoracic ganglion receives direct white ramus communicans preganglionic input from its closely related first thoracic nerve. The preganglionic fibers destined for the middle and superior cervical sympathetic ganglia enter the sympathetic trunk at upper thoracic levels and must ascend to reach the postganglionic nerve cell bodies in these cervical ganglia. All of the cervical ganglia provide gray rami communicans to adjacent cervical spinal nerves with the superior commonly providing those to the first through fourth, the middle to the fifth and sixth, and the cervicothoracic to the seventh and eighth cervical and first thoracic nerves. All the cervical ganglia provide cervical cardiac sympathetic branches to the cardiac plexuses. The cervicothoracic ganglion also gives off postganglionic nerves to the vertebral plexuses which follow these vessels to the brain. The superior ganglion also gives off postganglionic branches that form the external and internal carotid artery plexuses which distribute with all the branches of these vessels to provide the primary


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pathway for sympathetic innervation to the head. The superior ganglion further provides some communications to the last four cranial nerves and some direct branches to the larynx and pharynx. If the sympathetic pathway to the head is interrupted at any point including the upper thoracic spinal cord segments and nerves, the cervical sympathetic trunk, or the plexuses about the carotid arteries a Horner's syndrome develops. This causes ipsilateral ptosis (drooping upper eyelid), miosis (constricted pupil), facial anhydrosis (loss of sweating) and facial cutaneous vasodilation (flushing and increased warmth). CERVICAL LYMPHATICS AND SUMMARY OF THE LYMPHATIC DRAINAGE OF THE HEAD AND NECK AND GENERAL BODY (N. 74-75, 235, 261) The lymph nodes and vessels of the cervical region are divided into superficial and deep groups. The superficial cervical nodes drain the scalp, face, and superficial neck regions. These areas drain into occipital, retroauricular, preauricular, external jugular and anterior jugular groups which follow the superficial veins into the deep cervical group of nodes. The deep lateral cervical nodes lie along the internal jugular vein. They drain all of the ipsilateral head and neck structures. The inferior deep lateral cervical nodes are sometimes called the supraclavicular or scalene nodes because of their position above the clavicle and upon the surface of the scalene muscles. These nodes may also interrupt any of the major lymphatic channels draining into the great veins at the base of the neck. From the inferior deep lateral cervical nodes a jugular lymphatic trunk is formed containing the lymphatic drainage of the ipsilateral head and neck. On the right side this may variably join with the subclavian lymphatic trunk draining the right upper limb and the bronchomediastinal trunk draining the right side of the thorax to sometimes form a right lymphatic duct (N. 261). These lymphatics empty into the junction of the right subclavian and internal jugular veins as they form the right brachiocephalic vein. On the left the jugular, subclavian and bronchomediastinal trunks commonly join the thoracic duct (N. 261). This duct ascends out of the thorax and loops up behind the carotid sheath structures before emptying into the junction of the left subclavian and internal jugular veins as these form the left brachiocephalic vein. The thoracic duct drains lymph from the lower half of the body, i.e., below the diaphragm. Therefore, lymph from the right upper quadrant of the body, i.e., the right side of the head and thorax and the right upper limb, drains into the great veins at the right side of the base of the neck. However, lymph from the lower half of the body and the left upper quadrant drains into the great veins on the left side of the base of the neck. Valves are present in the terminal parts of these lymphatic vessels to prevent venous reflux. Since the inferior deep lateral cervical nodes can interrupt these major lymphatic channels prior to their termination in the venous system, they may serve as a final filter of tumor cells spreading through the lymphatic system from near or distant primary sites. These nodes are sometimes also called sentinel nodes, since their enlargement on the right side should send the physician looking for a primary tumor site in the right upper quadrant of the body and their enlargement on the left indicates a primary site in the lower half or left upper quadrant of the body. "Scalene node biopsy" can be performed on these nodes to try to microscopically establish the site of the primary tumor. The head and neck areas drained by the deep lateral cervical lymph nodes are prone to infection and carcinoma, and therefore these nodes have therapeutic as well as diagnostic importance. Therapeutically the surgical procedure of radical neck dissection for head and neck carcinoma is designed to remove all of these nodes in a "block" which also includes the sternocleidomastoid muscle, the submandibular gland, and the major veins to insure as complete a lymph node extirpation as possible.


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THE THYROID AND PARATHYROID GLANDS (N. 76-78) The thyroid gland is an endocrine gland composed of a right and left lobe, the lower portions of which are connected across the midline by the thyroid isthmus. A narrow pyramidal lobe extending upward from the isthmus region is sometimes present. An appreciation of the location and major relationships of the gland is essential to its clinical evaluation by palpation. The isthmus is located anterior to approximately the second to fourth tracheal rings. The wide lower parts of the lobes lie lateral to the upper trachea. From this point the lobes narrow as they extend up along the side of the cricoid and thyroid cartilages of the larynx as high as the oblique line of the thyroid cartilage, along which they taper to an apex (N. 72-73; CD H&N Sections 18 to 16). The infrahyoid muscles lie superficial to the gland (N. 27-28). The lower part of the lobes are additionally covered by the origin of the sternocleidomastoid muscle. These somewhat obscuring overlying muscles are responsible for the difficulty one has in precisely delimiting the normal thyroid gland by palpation, though most focal or general enlargements are detectable by palpation. In enlargement of the thyroid gland its upward expansion is generally limited by the attachment of an infrahyoid muscle to the oblique line of the thyroid cartilage, but the gland can expand inferiorly behind the sternum into the superior mediastinum and also encroach on the trachea which it surrounds on three sides. The rich blood supply of the thyroid gland (N. 76-78) and its close neural relationships are of importance in the surgical approach to this gland and in interpreting some of the occasional complications of thyroid gland surgery. The superior thyroid artery is usually the first branch of the external carotid artery. It courses downward and medially to descend along the superomedial border of the gland. This artery also gives off the superior laryngeal artery which penetrates the thyrohyoid interval to supply the upper larynx. The inferior thyroid artery is a branch of the thyrocervical trunk. The inferior thyroid artery ascends along the anterior scalene muscle. It then loops medially and downward behind the carotid sheath to approach the posterior surface of the lower part of the thyroid lobe where it divides into a number of terminal branches. When the thyroid arteries are ligated in thyroidectomy the closely adjacent nerves must be avoided. Near its origin the superior thyroid artery may be closely related to the entire superior laryngeal nerve, but for most of its course it is accompanied by the external branch of this nerve which supplies the cricothyroid muscle of the larynx. The recurrent laryngeal nerve, the principal motor innervation to the laryngeal musculature, lies in close juxtaposition to the terminal branches of the inferior thyroid artery. The complications of inadvertent injury to these nerves during thyroid surgery are described on page 239. An occasionally present thyroid ima artery may be a branch of the aorta, brachiocephalic or common carotid arteries and when present ascends to the thyroid gland anterior to the trachea. The thyroid gland is drained by superior thyroid veins which ascend to the internal jugular veins, middle thyroid veins that pass transversely to the internal jugular veins, and inferior thyroid veins which descend on the front of the trachea to enter the left brachiocephaIic vein. The parathyroid glands (N. 77-78) produce parathormone which regulates blood calcium levels. They may vary from two to six in number and are commonly only several mm in diameter and flattened. They usually lie in close juxtaposition to the posterior surface of the thyroid lobes. Their superior-inferior position is highly variable. Because of the variable position of these glands, the difficulty in precisely identifying them and the desirability of preserving their function when thyroidectomy is performed, a thin posterior shell of thyroid gland is commonly left intact to spare the parathyroids as well as to preserve some thyroid function.


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THE LARYNX The larynx serves as the part of the airway connecting the pharynx to the trachea. Its vocal folds function to produce sound and by their valvular mechanism participate in respiration, swallowing, defecation, urination and any other functions which require increased intraabdominal or intrathoracic pressures. The walls of the larynx are formed by a cartilaginous and fibrous framework, the intrinsic laryngeal muscles and a mucous membrane lining. LARYNGEAL CARTILAGES (N. 79; CD H&N Sections 15 to 17) The major cartilages of the larynx include the unpaired thyroid, cricoid and epiglottic cartilages and the paired arytenoids. The thyroid, cricoid and base of the arytenoid cartilages are hyaline cartilage, which is subject to normal calcification later in life. Therefore, these may become visible radiographically. The rest of the cartilages are elastic cartilage. By their rigidity these cartilages serve to maintain the patency of this part of the airway. They also participate in vocal fold mechanisms and serve as points of attachment of laryngeal, pharyngeal and infrahyoid musculature. The thyroid (=shield) cartilage is formed by two laminae which join anteriorly in the midline at an angle of about 90o to form the laryngeal prominence. The superior border is deeply notched in the midline to form the superior thyroid notch. A superior horn (cornu) ascends from the junction of the superior and posterior borders of each lamina. From the junction of the inferior and posterior borders of each lamina an inferior horn (cornu) descends to articulate with the cricoid cartilage. The outer surface of each lamina is marked by an oblique line along which some infrahyoid and the inferior pharyngeal constrictor muscles attach. The cricoid (=signet ring) cartilage is shaped like a signet ring with a narrow arch anterolaterally and a broad flat lamina posteriorly. On each side the lateral portion of the upper border of the cricoid lamina presents an obliquely situated cylindrically shaped articular surface for each arytenoid. At the point where the lamina and arch meet on each side there is an articular surface which articulates with a similar facet on the medial surface of each inferior horn of the thyroid cartilage to form the synovial cricothyroid joint. The two cricothyroid joints act together to permit rotation around a transverse axis that passes through both joints and this will result in tension or relaxation of the vocal folds (N. 81). In this rotation the anterior part of the thyroid cartilage and anterior arch of the cricoid are approximated to tense the vocal folds. The reverse motion relaxes the vocal folds. Each arytenoid cartilage is triangular in shape and presents three named angles and four surfaces. The pointed superior angle is the apex. The thick posterior angle is the muscular process for the insertion of the posterior and lateral cricoarytenoid muscles. The long slender anterior process is the vocal process. This forms the cartilaginous part of the vocal folds and serves for attachment of the vocal ligament. The medial surface is smooth and flat. The anterolateral surface is irregular and serves for insertion of the thyroarytenoid muscle. The posterior surface is deeply concave for attachment of the arytenoid muscles. The inferior surface of each arytenoid is concave for articulation with the cylindrical articular surfaces of the cricoid lamina in the formation of the synovial cricoarytenoid joints. Two major types of motion of the arytenoid upon the cricoid can contribute to the adduction-abduction motions of the vocal folds which occur at these joints (N. 81). First, the arytenoids can glide medially up or laterally down the inclined cricoid articular surfaces. Second, the arytenoids can also rotate around a vertical axis through the joint so that when the muscular process is pulled laterally the vocal process moves medially and vice versa. The epiglottic cartilage is a leaf-shaped cartilage with its narrow inferior stem attached to the upper inner aspect of the junction of the thyroid laminae. The epiglottic cartilage and its covering mucous membrane form the epiglottis. The epiglottis folds down across the laryngeal inlet (opening of the larynx to the pharynx) during swallowing to help close it.


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MEMBRANOUS AND LIGAMENTOUS FRAMEWORK OF THE LARYNX (N. 79-80) The superior border and horns of the thyroid cartilage are joined to the hyoid bone above by the thyrohyoid membrane. The upper border of the cricoid cartilage and lower border of the thyroid cartilage are joined by the conus elasticus which is an elastic membrane shaped like a cone. The conus elasticus continues internal to the thyroid cartilage to become continuous at its upper edge with the vocal ligaments (CD H&N Section 16). These extend from the vocal processes of the arytenoids forward to attach to the inner surface of the thyroid laminae near their median junction point. The thickened anterior portion of the conus elasticus attaches above to the inferior margin of the junction of the thyroid lamina as the median cricothyroid ligament. Another fibrous membrane, extends from the arytenoid cartilages forward and upward to the sides of the epiglottis. The mucosally covered free superior edge of this membrane forms the aryepiglottic fold (N. 66, 78). Inferiorly this membrane ends in the vestibular ligaments, which extend from the upper part of the anterior border of the arytenoid cartilages to the inner surface of the thyroid laminae near their junction. The vestibular ligaments lie above and parallel to the vocal ligaments. INTERIOR OF THE LARYNX (N. 64, 82) The nearly vertically positioned opening from the pharynx into the larynx is the laryngeal inlet (aditus) (N. 64, 66). It is bounded anteriorly by the epiglottis, posteriorly by the arytenoid cartilages and laterally by the aryepiglottic folds. The aditus leads into the laryngeal vestibule which is the part of the laryngeal cavity above the vocal folds. Each vocal fold (vocal cord or true vocal cord) is formed by the mucosally covered vocal ligament and vocal process of the arytenoid cartilage. These form, respectively, its membranous and cartilaginous parts. The part of the laryngeal cavity between the vocal folds is called the rima (=slit) glottidis. The region of the larynx formed by the vocal folds and the intervening rima glottidis is called the glottis. This is the narrowest level of the airway in terms of total cross-sectional area. Laryngeal lesions are frequently related to the glottis, being described as glottic, supraglottic and infraglottic in position. The vestibular ligaments and their mucosal covering form the vestibular folds (false vocal cords). On laryngoscopic examination of the larynx the vestibular folds partly obscure the vocal folds. This relationship can be visualized in a superior view of an anatomic specimen (N. 59, 76). The concavity between the vestibular and vocal folds of each side is called the laryngeal ventricle. At its anterior end a tubular extension, the laryngeal saccule extends upward lateral to the vestibular folds and this may become pathologically dilated to form an outpouching of the laryngeal wall into the soft tissues of the neck called a laryngocele. The air within the larynx provides a natural contrast medium that allows many of its interior features to be visualized radiographically. CT and MRI enhance these features. MUSCLES OF THE LARYNX (N. 80-81) Each cricothyroid muscle is located on the lateral surface of the larynx. It attaches above to the inferior border and horn of the thyroid lamina and below to the lateral aspect of the cricoid arch. When it contracts it approximates the anterior part of the thyroid cartilage to the cricoid arch. This both tenses the vocal folds and simultaneously adducts them. Each posterior cricoarytenoid muscle arises from the posterior surface of the cricoid lamina. Its fibers pass superiorly and laterally to insert on the muscular process of the arytenoid cartilage. On contraction this muscle pulls the muscular process medially and backward thereby swinging the vocal processes laterally to abduct the vocal folds. Each lateral cricoarytenoid muscle arises from the upper border of the lateral part of the cricoid arch and passes posteriorly to insert on the arytenoid's muscular process. When this muscle contracts it pulls


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the muscular process forward and laterally thereby swinging the vocal process medially to adduct the vocal folds. The arytenoid muscle connects the posterior surfaces of the two arytenoid cartilages. Some of the fibers are transverse and some are oblique. The oblique fibers pass from the muscular process of one arytenoid to the apex of the other and even beyond into the aryepiglottic folds where they may function to partially close the laryngeal inlet in swallowing. The major action of this muscle is to approximate the arytenoid cartilages and thereby contribute to adduction of the vocal folds. Each thyroarytenoid muscle (CD H&N Section 16) arises from the interior of the thyroid lamina just lateral to the attachment of the vocal ligament. It lies lateral to the vocal ligament and inserts into the anterolateral surface of the arytenoid cartilage. Contraction of this muscle pulls the arytenoid forward thereby relaxing the vocal fold as a whole. An inner part of this muscle attaches to the vocal fold and has been called the vocalis muscle (N. 80). The vocalis muscles have been described as being able to modify the length or thickness of the vibratile segment of the vocal folds. A few fibers of the thyroarytenoid pass into the aryepiglottic folds and may help to partially close the laryngeal inlet in swallowing. Since the glottis is the narrowest level of the airway in terms of total cross-sectional area, it limits the volume of air which can be exchanged. Therefore, in a deep inspiration the vocal folds must be widely abducted and this is a function of the only abductor, the posterior cricoarytenoid muscle. When the mucous membrane of the airway becomes markedly swollen, as in an acute laryngitis, the airway may become obstructed at its narrowest point, the glottis, necessitating a laryngotomy or tracheotomy (see page 232). The adductors of the vocal folds include the lateral cricoarytenoid, arytenoid and cricothyroid muscles. The vocal folds are tightly adducted to close the glottis in breath holding, during swallowing to prevent aspiration of food, and during defecation and urination to permit the contraction of the abdominopelvic muscles to increase intrabdominal and intrathoracic pressure (a Valsalva maneuver) without a glottic "leak". In speech the vocal folds are adducted to a paramedian position, narrowing the rima glottidis to a thin slit through which air expressed from the lungs sets the folds into vibration. The pitch or frequency of vibration is then dependent upon the length, thickness, and tension of the vibratile segment of the vocal fold, each of which can be controlled by the laryngeal muscles. Therefore, the vocal folds function like a string instrument. LARYNGEAL INNERVATION AND BLOOD SUPPLY (N. 71-73, 82) The superior laryngeal nerve arises from the inferior ganglion of the vagus nerve and descends in the neck to join the superior thyroid artery, where this nerve divides into an internal and external branch. The internal branch of the superior laryngeal nerve enters the larynx through the thyrohyoid membrane in company with the superior laryngeal artery branch of the superior thyroid artery. It carries sensory innervation from the mucous membrane of the supraglottic larynx which is a major stimulus site for the cough reflex. Hence, this nerve commonly serves as the afferent limb of the cough reflex. The efferent limb of this reflex is complex and includes sequential vocal fold adduction, abdominal muscle contraction to increase intraabdominal and intrathoracic pressure and then sudden vocal fold abduction to express a blast of air. The external branch of the superior laryngeal nerve continues in close relationship with the superior thyroid artery to innervate the cricothyroid muscle and the lower portion of the inferior pharyngeal constrictor muscle. The recurrent laryngeal nerve branches from the vagus on the right at the level of the subclavian artery. On the left it arises at the level of the arch of the aorta at which point it has a critical relationship with the left lung root, where it can be involved by cancer of the left lung root. After looping under these vessels each nerve ascends near the tracheoesophageal groove providing motor and sensory innervation to both trachea and esophagus (N. 82). As it passes behind the thyroid gland it comes into a close relationship with the terminal branches of the inferior thyroid artery (N. 77-78). Then the terminal inferior laryngeal branches of this nerve enter the larynx behind the cricothyroid joint accompanied by the


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inferior laryngeal artery, a branch of the inferior thyroid artery. THE RECURRENT LARYNGEAL NERVE INNERVATES ALL OF THE INTRINSIC LARYNGEAL MUSCLES EXCEPT THE CRICOTHYROID MUSCLE (INNERVATED BY THE EXTERNAL BRANCH OF THE SUPERIOR LARYNGEAL NERVE) AND PROVIDES SENSORY INNERVATION TO THE INFRAGLOTTIC LARYNX. Injury to the internal branch of the superior laryngeal nerve can be hazardous because it abolishes ipsilaterally a major afferent limb of the cough reflex. Injury to the external branch of the superior laryngeal nerve can reduce the pitch of the voice by paralyzing the tensor of the vocal folds and leaving that fold slackened with a "bowed" edge during phonation. Injury to the recurrent laryngeal nerve during thyroid surgery or by bronchogenic carcinoma of the left lung can paralyze all of the ipsilateral laryngeal muscles except the cricothyroid. This leaves the vocal fold fixed in a tensed adducted position by the unopposed action of the cricothyroid muscle. Intracranial or jugular foramen injury to the vagus nerve and the accompanying cranial root of the accessory nerve removes all ipsilateral motor and sensory innervation to the larynx and causes the vocal folds to assume a fixed intermediate or "cadaveric" position. Paralysis of one vocal fold is often well compensated over time by the ability of the other fold to cross the midline and narrow the glottis sufficiently for normal speech. However, if both recurrent laryngeal nerves are injured or if there is a central nervous system lesion causing spasticity in all of the laryngeal muscles bilaterally (three pairs of which are adductors), the vocal folds assume an adducted position thereby obstructing the airway, causing laryngeal stridor and necessitating laryngotomy or tracheotomy for survival. To establish an airway on an elective or emergency basis laryngotomy or tracheotomy may be performed. Laryngotomy (=laryngostomy) is typically performed in the cricothyroid interval where the important overlying structures that may be encountered include the anterior jugular veins, strap muscles and a pyramidal lobe of the thyroid gland. A tracheotomy (=tracheostomy) is performed through the upper tracheal rings where the important overlying structures that may be encountered from superficial to deep include the anterior jugular veins and jugular venous arch, strap muscles, thyroid isthmus, inferior thyroid veins and thyroid ima artery.

THE PHARYNX (N. 47, 61, 64-68, 70)

The pharynx functions dually as part of the respiratory and digestive tracts in that it connects the nasal cavity to the larynx and the oral cavity to the esophagus. It has a posterior relationship to the cervical vertebrae and anterior relationships to the nasal cavity, oral cavity and larynx (N. 64, 66). On the basis of these anterior relations the pharynx can be divided into a nasopharynx, oropharynx and laryngopharynx. The nasopharynx extends from the mucous membrane covering the inferior surface of the sphenooccipital junction region of the skull down to the level of the soft palate (N. 64, 66; CD H&N Sections 7,8). Its anterior wall is deficient where it communicates widely with the nasal cavity through openings called the choanae. The auditory (pharyngotympanic or Eustachian) tube opens into the lateral wall to establish a communication between the airway and the middle ear cavity (N. 42, 68). The opening of the auditory tube is delimited behind by the torus tubarius, a prominent elevation formed by the posterior lip of the cartilaginous part of the tube. Behind and above the torus tubarius the pharyngeal wall outpouches to form the pharyngeal recess. On the posterosuperior wall of the nasopharynx an aggregation of lymphatic tissue forms the pharyngeal tonsils or adenoids which may become visibly or palpably enlarged in children, but are typically atrophied in adults. The oropharynx extends from the soft palate to the level of the upper epiglottis and is widely open anteriorly to the oral cavity (N. 64, 66; CD H&N Sections 9 to 13). The back of the dorsum of the tongue forms part of its anterior wall. Between the back of the tongue and the front of the epiglottis two valleculae (N. 68) or valleys are located, separated by a median glossoepiglottic fold. Since they are


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gravitationally dependent in the upright position, the valleculae are a potential site for lodgement of a foreign body, e.g., a fishbone. The lateral walls of the junctional region between oral cavity and oral pharynx are the fauces. The fauces include the palatine tonsils and are bounded anteriorly and posteriorly by the palatoglossal and palatopharyngeal arches, which cover muscles of the same name. Both the fauces and soft palate are described with the oral cavity. The laryngopharynx or hypopharynx extends from the upper epiglottis above to the laryngotracheal junction below where it narrows to become the esophagus (N. 64, 66; CD H&N Sections 14 to 17). Above it communicates anteriorly with the larynx through its inlet. Below the inlet the mucosa of the anterior wall of this part of the pharynx covers the back of the arytenoid and cricoid cartilages and their overlying arytenoid and posterior cricoarytenoid muscles as well as the posterior border and internal surface of the back of the thyroid laminae. The piriform fossa or recess is the depression between the mucous membrane over the interior of the posterior part of each thyroid lamina and thyrohyoid membrane laterally and the mucous membrane over the external aspect of the epiglottis, aryepiglottic fold, and arytenoid and cricoid cartilages medially. This fossa serves as the primary path for passage of food, and especially liquids, through the laryngeal pharynx. The laryngeal nerves ramify under the mucous membrane of this fossa to form a plexus (N. 67) where topical local anesthetic can be applied to produce laryngeal anesthesia. It is another potential site for foreign bodies to lodge. The air within the pharynx down to the laryngeal aditus provides a natural contrast medium that allows many of its internal features to be visualized radiographically. Barium swallow permits visualization of the interior of the lower pharynx and provides a means of functionally evaluating pharyngeal swallowing mechanisms which are summarized with the description of the oral cavity. The cervical part of the esophagus begins as the pharynx narrows at the level of the lower border of the cricoid cartilage (C6 vertebral level). So the pharyngoesophageal and laryngotracheal junctions are at the same level. The entire esophagus is described with the posterior mediastinum in the section on the thorax. PHARYNGEAL MUSCULATURE (N. 67, 70) The pharyngeal wall is formed by an inner mucosa, a submucosa of well developed fascia, musculature, and an outer buccopharyngeal fascial layer. The pharyngeal musculature is skeletal in type and made up of three pharyngeal constrictor muscles and two significant vertical muscles. The constrictor muscles have their fibers largely transversely oriented and the upper part of each muscle overlaps the external aspect of the lower part of the next highest muscle like a stack of upright paper cups. The superior pharyngeal constrictor muscle arises from the medial pterygoid plate of the sphenoid bone, the mylohyoid line of the mandible and the connective tissue pterygomandibular raphe between these bony attachments. The middle pharyngeal constrictor muscle arises mostly from the greater and lesser horns of the hyoid bone. The inferior pharyngeal constrictor muscle arises from the oblique line of the thyroid cartilage and the side of the cricoid cartilage. All three constrictors insert posteriorly in a midline pharyngeal raphe which attaches above to the base of the skull (N. 67). The major vertical muscles include the stylopharyngeus and palatopharyngeus. The stylopharyngeus arises from the styloid process, enters the pharynx between the superior and middle constrictors and inserts into the posterior border of the thyroid lamina and the pharyngeal mucosa. The palatopharyngeus arises from the palate and descends under the palatopharyngeal arch to also insert into the thyroid cartilage and pharynx. The vertical muscles elevate the pharynx and larynx, while the constrictor muscles contract sequentially from above downward during swallowing. See swallowing function summary on pags 258-259.


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PHARYNGEAL INNERVATION (Fig. 8-5, N. 61, 71, 78, 121-122, 126-127) The pharyngeal plexus is a network of nerves lying on the posterior surface of the middle constrictor muscle. It receives pharyngeal branches from the superior cervical sympathetic ganglion that innervate the blood vessels of the pharynx. It also receives pharyngeal branches from the vagus nerve which provide the motor innervation of all of the pharyngeal muscles except the stylopharyngeus. As the glossopharyngeal nerve swings forward around posterolateral aspect of the stylopharyngeus muscle to course to the tongue, it provides motor innervation to this muscle and branches to the pharyngeal plexus which supply sensory innervation to most of the pharyngeal mucosa. A convenient way to clinically evaluate both the pharyngeal sensory functions of the glossopharyngeal nerve and the pharyngeal motor functions of the vagus is to elicit the gag reflex. Normally a sensory stimulus, e.g., a tongue blade, applied to each side of the oropharynx will elicit a symmetrical gag response with the posterior pharyngeal wall moving straight forward because of an equal constrictor contraction bilaterally (N. 47 top; Fig. 8-5A). If the gag reflex is absent or asymmetrical it indicates that its afferent limb, the glossopharyngeal nerve, or its efferent limb, the vagus nerve, or their central connections in the medulla are damaged. If the glossopharyngeal nerve is damaged the patient will not sense the touch and hence no gag response will occur when that side is touched. However, if the vagus is damaged the pharynx will demonstrate a curtain-like deviation toward the normal side, because the unopposed pull of the constrictor muscles on the normal side (which are fixed anterolaterally to bone, cartilage or ligament) will pull the posterior pharyngeal wall in that direction (Fig. 8-5B). GLOSSOPHARYNGEAL NERVE (N. 126) The glossopharyngeal nerve contains five functional fiber types: somatic afferent from the skin of the posterior wall of the external auditory meatus, posterior part of the outer surface of the tympanic membrane, deep central auricular area, and the mastoid region; visceral afferent from the pharynx, middle ear, auditory tube, soft palate, fauces, posterior one third of the tongue, parotid gland, and carotid sinus and body; taste from the posterior one third of the tongue; parasympathetic to the parotid gland; and motor to the stylopharyngeus muscle. After emerging from the postolivary sulcus of the medulla the glossopharyngeal nerve traverses the posterior cranial fossa where it presents a superior ganglion which contains the nerve cell bodies of its somatic afferent fibers. Just outside the jugular foramen an inferior ganglion contains the nerve cell bodies of its visceral afferent and taste fibers.

Immediately after it exits from the jugular foramen the glossopharyngeal nerve gives off a tympanic branch which ascends through the floor of the middle ear cavity to enter the tympanic plexus over the promontory on the medial wall of the middle ear. This provides the sensory innervation of the middle ear, mastoid air cells and auditory tube. This is the pathway for the pain of a middle ear infection. Sensory fibers from the tympanic plexus join the auricular branch of the vagus to help supply the skin over the mastoid, deep auricle, posterior wall of the external auditory meatus and outer surface of the tympanic membrane. The tympanic nerve also contains parasympathetic preganglionic neurons which will provide some secretomotor control of the parotid gland over pathways which will not be described since their unilateral interruption usually produces no significant deficit.


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Figure 8-5 - Transverse section of oropharnyx

The carotid sinus nerve, carotid body branches and the pharyngeal and stylopharyngeus branches are given off the glossopharyngeal nerve as it descends along the posterior border and loops around the lateral surface of the stylopharyngeus muscle. Then the glossopharyngeal nerve passes downward and forward just deep to the lower part of the tonsillar fossa (N. 68 lower) to which it gives sensory tonsillar branches and enters the tongue to provide general sensory and taste innervation to its posterior one third via its lingual branches (see oral cavity).

FACE ANTERIOR ASPECT OF THE SKULL AND ITS SURFACE ANATOMY (N. 1, 4-6) The squamous portion of the frontal bone forms the forehead region and terminates below in the superior margin of the orbit. There is a supraorbital notch or foramen at about the junction of the medial and intermediate thirds of the superior orbital margin where the supraorbital nerves and vessels emerge from the orbit onto the forehead. The notch is usually palpable in the living subject and can be used to locate and locally anesthetize this nerve which innervates most of the forehead and anterior scalp. Sometimes the notch is converted into a foramen. The zygomatic bone forms most of the lateral orbital margin and the maxilla forms the bulk of the inferior and medial orbital margins. About a cm below the inferior orbital margin at the junction of its medial and intermediate thirds, the infraorbital foramen can be located. This transmits the infraorbital nerves and vessels and it is palpable by palpating deeply just below this part of the orbital margin. It is on the same vertical line as the supraorbital notch. Lateral to the orbital margin the zygomatic bone forms the anterior portion of the zygomatic arch, which as the outcropping cheek bone buttress of the face is susceptible to fracture. The most posterior part of the zygomatic arch is formed by the zygomatic process of the temporal bone. Medial to the orbit the small paired nasal bones articulate with the frontal bone above, the maxilla laterally, and each other medially to form the bridge of the nose. These are also subject to frequent fracture. Below the nasal bones the large pear-shaped apertures bounded by the maxillae laterally and inferiorly are the anterior nasal (piriform) apertures. These are filled in life by the nasal cartilages which form the lower part of the external nose. The bodies of the two maxillae occupy the interval from the floor of the orbit to the roof of the mouth. Along their U-shaped lower margins the superior alveolar processes support the 16 permanent maxillary teeth within their dental alveoli.


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The mandible (N. 1, 4-6, 15-18) is formed by a horizontally oriented body and paired vertical rami. The posterior border of each ramus meets the inferior border of the body at the angle. The mentum forms the point of the chin where the two halves of the mandibular bodies are joined in the midline. The superior margin of the mandibular body is raised to form the U-shaped inferior alveolar process containing alveoli for the 16 mandibular teeth. The same vertical line through the supraorbital notch that fell through the infraorbital foramen pretty much passes through a mental foramen on the external surface of the mandible. This foramen is located about a cm or two below that part of the alveolar process that accommodates the mandibular premolar teeth. From it the mental branches of the inferior alveolar nerve and vessels emerge onto the face. As each ramus of the mandible is followed superiorly its anterior margin ends above in a somewhat pointed coronoid process, while its posterior border ends above in a bulbous condylar process. The coronoid and condylar processes are separated by the mandibular notch. All of these structures are palpable from the surface, particularly the bulbous upper end of the condylar process, which is called the head or condyle of the mandible. This is located just anterior and inferior to the external auditory meatus where its movement is easily palpable on opening and closing the jaw. Below this the condylar process narrows to a mandibular neck. The condyle contains a semicy-lindrical articular surface on its superior aspect which articulates through an articular disc with the mandibular fossa of the temporal bone above to form the temporomandibular joint. FACIAL VESSELS (N. 3) The arteries of the face are derived from many sources including the facial, superficial temporal and maxillary arteries from the external carotid artery system and the ophthalmic artery from the internal carotid system. On the face these vessels anastomose widely with each other and across the midline with the vessels of the opposite side. Hence, in the event one of these vessels must be ligated the collateral circulation is good. Likewise, in occlusion of the internal carotid or external carotid arteries these anastomoses can serve as very important collateral channels. After a course through the suprahyoid region the facial artery and vein enter the face near a shallow notch in the inferior border of the mandible some two to three cms anterior to the angle of the mandible. These vessels ascend the face, mostly deep to the facial muscles, angling obliquely medially toward the medial orbital margin. As they ascend they give off a branch to the upper lip which will anastomose with some of the arteries of nosebleed on the nasal septum. The facial artery ends at the medial angle of the eye as the angular artery which anastomoses above with branches of the ophthalmic artery and below with the infraorbital branches of the maxillary artery which emerge from the infraorbital foramen. The angular branch of the facial vein anastomoses with the ophthalmic veins of the orbit which in turn communicate with the cavernous sinus. This provides the pathway for spreading infection from the face through the ophthalmic emissary veins into the cavernous sinus to cause a possible cavernous sinus thrombosis. The facial area which can potentially drain into the cavernous sinus is the triangular area defined by the midline and by lines drawn from the external auditory meatus to the lateral corners of the eye and mouth. In the preantibiotic era this was somewhat dramatically called death's triangle, since picking pimples in this area could cause infectious material to enter the venous system and cause a cavernous sinus thrombosis with its resultant high incidence of fatality. The superficial temporal vessels emerge from the upper border of the parotid gland and run superficial to the zygomatic arch. They ascend just anterior to the auricle to divide into branches which supply the scalp. Some of these branches anastomose with the supraorbital branches of the ophthalmic artery. Branches of the infraorbital artery, which is a terminal branch of the maxillary artery, also anastomose with these branches of the ophthalmic artery. The periorbital anastomoses of the branches of the ophthalmic artery with the angular branch of the facial artery, the superficial temporal artery, and the infraorbital terminal of the maxillary artery provide multiple opportunities for communications between the internal and external carotid arterial systems. In the very common occlusion of the internal carotid artery at its origin there will be collateral flow from these external carotid branches into the internal carotid artery via the ophthalmic artery and this can be identified and quantified by Doppler ultrasound.


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CUTANEOUS NERVES OF THE FACE (N. 2) The sensory innervation of the face is supplied by several terminal branches from each of the three major divisions of the trigeminal nerve. Unlike spinal nerves, the dermatomal distribution of the divisions of the trigeminal nerve shows little overlap. The ophthalmic division of the trigeminal nerve (ophthalmic nerve, V1) supplies the skin of the forehead and the anterior scalp to the interauricular line (the line drawn over the top of the scalp between the external auditory meatus of each side). The ophthalmic dermatome also extends down the dorsum (= anterior border, which is only dorsal in a quadriped) of the nose to its tip. The major terminal branches of the ophthalmic division of the trigeminal nerve which supply this area include the supraorbital nerve and a terminal branch of the ethmoidal nerves (see orbit description of ophthalmic division branches). The largest of these is the supraorbital nerve which emerges onto the face through the supraorbital notch or foramen where it breaks up into many branches which will ascend the forehead to supply much of the anterior scalp as far posteriorly as the interauricular line. A terminal branch of the ethmoidal nerves emerges between the nasal bone and nasal cartilages to supply the dorsum of the nose. Finally, a number of small terminals of other branches of the ophthalmic division end in the upper eyelids and forehead. The maxillary division of the trigeminal nerve (maxillary nerve, V2) has a dermatomal distribution shaped like an inverted comma. The head of the comma is the area between the eye and the mouth from the lateral aspect of the nose to the anterior cheek. This is supplied by the infraorbital nerve terminal of the maxillary division, which on emerging from the infraorbital foramen divides into multiple branches. These ascend to the lower eyelids, descend to the upper lip and extend medially to the lateral aspect of the external nose. The tail of the comma-shaped dermatome ascends lateral to the orbit over the zygomatic bone and into the anterior part of the temporal fossa where zygomatic branches of the maxillary division distribute. The mandibular division of the trigeminal nerve (mandibular nerve, V3) supplies a dermatomal area shaped like a large bent U or horseshoe. This is the area over the mandible from the chin, along the mandibular body and ramus, and into the intermediate temporal fossa area. This does not include the angle of the mandible which is supplied by the cervical plexus and is part of the C2 dermatome. Three branches of the mandibular division are involved in this distribution. The mental nerve terminal of the inferior alveolar nerve emerges from the mental foramen and divides into multiple branches to the full thickness of the lower lip, the chin and the skin over the anterior body of the mandible. The buccal nerve branch of the mandibular division emerges onto the face through the buccal fat pad, part of which extends deep to the ramus of the mandible. After emerging from deep to the ramus of the mandible along its anterior border, it gives off branches which supply the skin over the posterior body and ramus of the mandible and the full thickness of the cheek from its mucous membrane to the skin. The auriculotemporal nerve branch of the mandibular division of the trigeminal nerve emerges through the upper part of the parotid gland behind the neck of the mandible. It ascends superficial to the zygomatic arch just anterior to the tragus of the auricle (the small cartilaginous elevation immediately anterior to the external auditory meatus). Here it gives off branches to the temporomandibular joint, the anterior wall of the external auditory meatus and the external surface of the anterior half of the tympanic membrane. Then the auriculotemporal nerve ascends into the temporal region just anterior to the auricle, in close company with the superficial temporal vessels. Here it supplies the skin over the intermediate part of the temporal fossa from the maxillary division dermatome anteriorly to the interauricular line posteriorly. Trigeminal neuralgia (tic douloureux) is the most common of the cranial nerve neuralgias. It has multiple etiologies including viral involvement of the nerve cell bodies of the trigeminal ganglion or irritation from adjacent arteries, but in many cases the cause remains undetermined. It tends to be episodic with pain of such severe character that it is often completely disabling. The neuralgia may involve one or more divisions or branches of the trigeminal nerve and its superficial pain distribution is in the characteristic dermatome of the involved nerve. The pain typically also has a deep component that may involve the


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distribution of the ophthalmic division to the orbit and frontal and ethmoid air sinuses; maxillary division to the upper jaw and upper teeth, palate, and maxillary air sinus; or mandibular division to the lower jaw and teeth, tongue and floor of the mouth, and infratemporal fossa. If the divisions of the trigeminal nerve are involved by destructive rather than irritative lesions the anesthetic areas typically fit the dermatomal distribution well, since there is very little dermatomal overlap between the divisions. MUSCLES OF FACIAL EXPRESSION (N. 25, 48) The muscles of facial expression are innervated by branches of the facial nerve. They are largely aggregated about the facial apertures. Those about the nose and ear have minimal physiological or clinical significance and will not be considered. However those about the eye and mouth are of considerable importance. Both of these apertures are guarded by sphincteric muscles which have other muscles blending into them and into the skin about the apertures. The orbicularis oculi muscle is the sphincteric muscle of the eye. It has an origin medially from the medial palpebral ligament and the adjacent lacrimal crest (see orbit). From here the muscle fibers encircle the orbit. Some of them run in the eyelids and others spread out into a thin flat sheet for several cms beyond the orbital margin. While gravity is a partial natural depressor of the upper eyelid in the upright position, tight closure, especially with force, is achieved by the contraction of the orbicularis oculi muscle. The frontal belly (frontalis muscle) of the occipitofrontalis or epicranius muscle attaches into the portion of the orbicularis oculi that underlies the eyebrow. Since the other end of this frontalis attaches to the galea aponeurotica, if the galea is fixed by the contraction of the other muscles which attach to it, a contraction of the frontalis will elevate the eyebrows producing transverse wrinkles in the skin of the forehead. The orbicularis oris muscle is the sphincteric muscle of the mouth. Its fibers encircle the mouth in the upper and lower lips and they have continuities with the buccinator muscles laterally and the elevator and depressor muscles of the upper and lower lips and the angles of the mouth. Contraction of appropriate parts of this muscle can purse the lips or pucker the lips. The elevator of the upper lip descends from the maxilla and elevates the upper lip as in showing the upper teeth. The elevators of the angle of the mouth (zygomatic muscles) descend from the zygoma to elevate the corners of the mouth as in a smile. The depressor of the lower lip ascends from the mandible to depress the lower lip as in showing the lower teeth. The depressor of the angle of the mouth ascends from the mandible to turn down the corner of the mouth as in a frown. The buccinator muscle attaches posteriorly to the pterygomandibular raphe where the superior pharyngeal constrictor partly arises (N. 70). It runs forward within the cheek and blends into the orbicularis oris muscle (N. 25, 47-48). On contraction its fibers tend to compress the contents of the oral cavity, whether it be food or air as in blowing up a balloon. It is used in mastication to squeeze food out of the troughs or vestibules between the cheeks and gums and to help control the placement of food between the teeth. THE PAROTID GLAND The parotid gland is a salivary gland which partly enwraps the posterior border of the ramus of the mandible and its associated muscles (N. 24, 46-47; CD H&N Sections 7 to 11). It is related anteriorly to the ramus of the mandible and its attaching masseter and medial pterygoid muscles and posteriorly to the mastoid process and upper end of the sternocleidomastoid muscle. Its superficial portion overlies the posterior part of the masseter muscle, while the deep part extends medially into contact with the styloid process and its attaching muscles and the posterior belly of the digastric muscle. The upper pole of the parotid gland extends into the posterior part of the mandibular fossa of the temporal bone where it is wedged into the narrow interval between the temporomandibular joint anteriorly and the anterior wall of the external auditory meatus posteriorly. Therefore, if the parotid becomes swollen by virtue of an inflammatory response (parotitis, e.g., mumps) or because of an obstruction of its duct by a calculus, pain can occur on chewing and this can be referred to either the temporomandibular joint or the external


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auditory meatus. The parotid gland receives its parasympathetic secretomotor innervation from the glossopharyngeal nerve via its tympanic branch. The details of this pathway will not be described since it lacks major clinical import. The parotid duct emerges from the anterior border of the gland and runs forward across the masseter to the anterior margin of the masseter and mandibular ramus. Here it turns deeply into the buccal fat pad which overlies the buccinator muscle and after penetrating the buccinator enters the oral cavity opposite the second maxillary molar tooth. As it runs forward superficial to the masseter muscle the parotid duct is located about a finger's breadth below the inferior margin of the zygomatic arch. It is palpable at this point, particularly as it turns deeply around the anterior margin of the muscle. The duct commonly has some accessory parotid lobules extending along it. A number of structures course through the substance of the parotid gland including the facial nerve and its branches, the external carotid artery and its terminal maxillary and superficial temporal branches and the retromandibular vein. The external carotid artery ascends through the deep part of the parotid gland (N. 47 top) to the level of the neck of the mandible, where it ends by dividing into maxillary and superficial temporal branches (N. 34, 72). The maxillary artery runs forward into the infratemporal fossa deep to the mandibular neck. The superficial temporal artery ascends superficial to the zygomatic arch just anterior to the tragus in close company with the auriculotemporal nerve. The retromandibular vein is formed behind the neck of the mandible by the junction of the superficial temporal and maxillary veins (N. 73). It descends within the substance of the parotid gland where it is typically located just superficial to the external carotid artery (N. 47 top, 73). Near the lower pole of the parotid gland the retromandibular vein is joined by the posterior auricular vein to form the external jugular vein (N. 3). FACIAL NERVE (N. 124) The facial nerve (VII cranial nerve) contains motor fibers which innervate the muscles of facial expression, stapedius, stylohyoid and the posterior belly of the digastric. It contains VE fibers which provide the parasympathetic innervation to the submandibular, sublingual, palatine, nasal and lacrimal glands through relays in ganglia in the floor of the mouth and the pterygopalatine ganglion. It also provides SA fibers to the skin of the posterior wall of the external auditory meatus, posterior half of the outside of the tympanic membrane, deep auricle, and mastoid region. Finally, it provides taste innervation to the anterior two-thirds of the tongue. The facial nerve emerges from the inferior pontine sulcus as two roots, a large motor root containing its motor fibers and a small nervus intermedius containing the rest of its functional fiber types (N. 115). It closely accompanies the vestibulocochlear nerve across the subarachnoid space into the internal acoustic meatus, where it can be compressed by vestibular schwannomas developing within the VIII cranial nerve. Its course through the temporal bone is described on page 226. The geniculate ganglion is located at its external genu. It contains the nerve cell bodies of its general sensory and taste neurons. At this genu the greater petrosal nerve arises. This nerve contains the preganglionic parasympathetic fibers to the pterygopalatine ganglion which will provide secretomotor innervation to the palatine, nasal and lacrimal glands. At its second bend in the temporal bone as it passes from the medial to the posterior wall of the tympanic cavity a branch to the stapedius muscle arises. Just before it emerges from the stylomastoid foramen it gives off the chorda tympani nerve which conveys taste fibers to the anterior two-thirds of the tongue and parasympathetic preganglionic neurons which will provide secretomotor innervation to the submandibular and sublingual glands. After the facial nerve emerges from the temporal bone through the stylomastoid foramen it turns laterally to enter the parotid gland where it commonly divides into five major branches some of which may be doubled (N. 24, 124). As these branches run through the parotid gland they typically course superficial to the external carotid artery and retromandibular vein (N. 47 top). These branches then emerge from the anterior border of the parotid gland to radiate toward the muscles of facial expression which they innervate.


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The temporal branch runs along a line from the lower border of the tragus to just above the eyebrow where it innervates the frontalis muscle. If it is injured the ipsilateral forehead wrinkles are usually flattened by the gravitational sag of the forehead. The temporal branch can be tested by asking a patient to raise the eyebrow or wrinkle the forehead. The zygomatic branch projects along a line from the lower tragus to the outer corner at the eye where it provides much of the innervation to the orbicularis oculi muscle. If this nerve is injured the paralysis of the lid portion of the orbicularis oculi causes the lower eyelid to gravitationally sag and evert, thereby losing the lower eyelid wall that normally retains the tears. So tearing across the cheek may occur. This branch is tested by asking a patient to close the eyes tightly, which will not be possible when this branch is injured. The buccal branches are usually doubled and accompany the parotid duct to innervate the buccinator muscle, the elevator of the angle of the mouth, the elevator of the upper lip, and the upper lip part of the orbicularis oris. Loss of these muscles will cause a decreased prominence of the nasolabial fold by the gravitational sag of the cheek. These branches can be tested by asking the patient to show the teeth, smile, pucker up the lips, whistle, or blow out the cheeks. There will be inability to show the upper teeth on that side by loss of the ipsilateral elevator of the upper lip. The ipsilateral smile will be lost by paralysis of the elevator of the angle of the mouth. A symmetrical pucker of the lips or a whistle will be impossible because of loss of the ipsilateral upper quadrant of the orbicularis oris muscle. Likewise, the patient won't be able to blow out the cheeks because they won't be able to keep the lips tightly approximated with a quadrantic loss of the orbicularis oris; so there will be an air leak through the lips. The marginal mandibular branch of the facial nerve usually dips just below the lower margin of the mandible as it crosses the superficial aspect of the facial vessels. Then it ascends to innervate the depressor of the angle of the mouth, the depressor of the lower lip and the ipsilateral lower lip quadrant of the orbicularis oris muscle. The paralysis of the lower lip part of the orbicularis oris will cause the ipsilateral lower lip to sag gravitationally so that the lower lip wall that normally retains the saliva will be lost and saliva will tend to drool down the chin. The loss of the lower quadrant of the orbicularis oris will also cause an asymmetrical pucker and an inability to whistle or blow out the cheeks. Likewise, on attempting to show the teeth the loss of the ipsilateral depressor of the lower lip will prevent full disclosure of the lower teeth on that side. So showing the teeth provides a good quick test for both buccal and mandibular branches of the facial nerve. The cervical branch of the facial nerve descends into the neck from the lower pole of the parotid gland. While its innervation to the platysma is often not checked clinically, because of its functional insignificance, it can be checked by asking the patient to flare out the skin of the neck or pull up the skin of their upper chest, usually with the examiner demonstrating what is requested. If the facial nerve is injured proximal to its branching there will likely be paralysis of the entire side of the face with all of the above described postural and movement deficits. This is called a facial (Bell's) palsy. If individual nerve branches are injured, as by a parotid gland tumor, the deficits will be more localized and will only be demonstrated by testing all of the branches.

INFRATEMPORAL FOSSA OSTEOLOGY OF THE INFRATEMPORAL FOSSA (N. 6, 10, 16) The infratemporal fossa is that region of the head bounded laterally by the ramus of the mandible and zygomatic arch; medially by the pterygoid process of the sphenoid bone and the lateral wall of the nasopharynx and oropharynx, including the cartilaginous auditory tube; anteriorly by the posterior surface of the maxilla; posteriorly by the mastoid and styloid processes and the muscles arising therefrom; and superiorly by the inferior aspect of the greater wing of the sphenoid bone and the squamous and tympanic portions of the temporal bone.


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The roof of the infratemporal fossa is formed anteriorly by the inferior aspect of the greater wing of the sphenoid bone as far laterally as the point where an anteroposteriorly running ridgeline, the infratemporal crest, separates the temporal fossa above from the infratemporal fossa below. The foramen ovale is situated at the medial end of the greater wing of the sphenoid. It transmits the mandibular division of the trigeminal nerve from the middle cranial fossa into the infratemporal fossa. Posterolateral to foramen ovale the foramen spinosum transmits the middle meningeal artery from the infratemporal fossa into the middle cranial fossa. Posterior to the foramen spinosum there is a small inferiorly directed sphenoid spine which gives this foramen its name and serves as the superior attachment of the sphenomandibular ligament. Just posteromedial to these apertures the greater wing of the sphenoid articulates with the petrous pyramid in a deep trough-like fissure, where the cartilaginous part of the auditory tube is lodged (N. 10, 100). Posterior to the greater wing of the sphenoid the squamous part of the temporal bone forms the roof of the infratemporal fossa. This forms the prominent articular tubercle that demarcates the anterior end of the mandibular fossa. A transverse fissure crosses the depths of the mandibular fossa. The chorda tympani emerges from the middle ear cavity at the medial end of this fissure to join the lingual nerve within the infratemporal fossa. The lateral wall of the infratemporal fossa is formed largely by the internal aspect of the ramus of the mandible (N. 17). Just above the middle of the internal surface of the ramus, the mandibular foramen transmits the inferior alveolar nerve and vessels into the mandibular canal. Anterior to the mandibular foramen the broad tongue-like lingula points superiorly, from which direction it receives the attachment of the sphenomandibular ligament. The bony medial wall of the infratemporal fossa is the pterygoid process of the sphenoid bone (N. 6, 10, 16). This is composed of a medial and lateral pterygoid laminae or plates which are separated by the pterygoid fossa. The pterygoid plates give origin to the pterygoid muscles. The anterior wall of the bony infratemporal fossa is formed by the posterior aspect of the maxilla. This contains some apertures which transmit the posterior superior alveolar nerves into the maxilla (see nasal cavity, pages 255). Where the medial border of the maxilla abuts the pterygoid process of the sphenoid bone a pterygomaxillary fissure transmits the maxillary artery from the infratemporal fossa into the pterygopalatine fossa. Where the superior part of the posterior surface of the maxilla is adjacent to the anterior margin of the greater wing of the sphenoid an inferior orbital fissure is located. The inferior orbital and pterygomaxillary fissures meet at a right angle and just medial to this junction point is where the pterygopalatine fossa is located. TEMPOROMANDIBULAR JOINT (N. 18) The temporomandibular joint is a synovial joint between the mandibular fossa of the temporal bone and the condyle of the mandible. It is divided into two synovial cavities by the presence of a fibrocartilaginous articular disc. The upper surface of this disc is convex behind and concave in front as it is molded, respectively, by the concavity of the mandibular fossa and the convexity of the articular tubercle. The lower surface of the disc is concave by being molded over the convexity of the mandibular condyle. The capsule of this joint is especially strengthened medially and laterally by the presence of reinforcing ligaments. There are two major extracapsular ligaments which help suspend the mandible from the skull like guy cables. These include the sphenomandibular ligament which runs downward and forward from the sphenoid spine to the lingula and the stylomandibular ligament which runs from the styloid process downward and forward to the lower posterior border of the mandibular ramus. These suspending ligaments transfer the transverse axis of rotation of the mandible for jaw opening and closing from an expected position through the two condyles down to a position closer to the middle of the ramus. This can be demonstrated by palpating the mandibular condyle during jaw opening and closing. If an index finger is placed on the mandibular condyle (located just anterior and inferior to the external auditory meatus) and the thumb is placed on the angle of mandible, when the jaw is opened the mandibular condyle is felt to move downward and forward while the angle moves downward and backward. This clearly demonstrates that the transverse axis is somewhere within the mandibular ramus between these two points. On jaw opening the mandibular condyle and the articular disc move downward and forward on the anterior sloping


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surface of the mandibular fossa, bringing them precariously close to the apex of the articular tubercle (N. 18, lower right illustration). In a dislocated jaw the condyle slips anterior to the articular tubercle and gets trapped there by the resulting spasm of the jaw closing muscles. This can occur unilaterally or bilaterally. If one attempts to reduce the dislocation by simply pushing the condyle backward it will impact the articular tubercle, and with enough force this attempt can fracture the mandibular neck. So to reduce the dislocation it is imperative that the mandible be displaced downward before it is displaced backward so that the condyle will be fully cleared below the articular tubercle. Another major motion permissible at this joint is lateral deviation of the jaw to the left or to the right. This motion is necessary for food grinding activities. If one palpates the two mandibular condyles and then deviates the jaw laterally to the left the right condyle moves forward and downward while the left condyle just seems to rotate in place. So the right condyle essentially rotates around a vertical axis through the left condyle. Other less important jaw motions will not be considered. MUSCLES OF MASTICATION (N. 48-49) There are four major muscles of mastication and a number of accessory masticatory muscles related to the tongue, cheek and suprahyoid or floor of the mouth regions. The accessory muscles are described with their respective regions. The four major muscles are the masseter, temporalis, lateral pterygoid and medial pterygoid muscles. The masseter muscle descends from the lower and inner aspect of the zygomatic arch and inserts into most of the external surface of the ramus of the mandible (N. 48). Its major function is jaw closing. It is innervated by a branch of the mandibular division of the trigeminal nerve which reaches its deep surface by passing laterally through the mandibular notch (N. 48 lower). The temporalis muscle (N. 48) arises from the broad expanse of the temporal fossa on the lateral aspect of the skull (bounded above by curved temporal lines). Its descending fibers converge upon the coronoid process of the mandible. This muscle will elevate the coronoid process and hence it is a jaw closer. It is innervated by the branches of the mandibular division of the trigeminal nerve (N. 50, 71). The pterygoid muscles (N. 49) are situated deep to the mandibular ramus within the infratemporal fossa. The more superficially situated lateral pterygoid muscle has an upper head arising from the bony boundaries of the inferior orbital fissure and a lower head arising from the lateral aspect of the lateral pterygoid plate. From here the muscle fibers pass transversely and laterally to insert on the neck of the mandible and the capsule and articular disc of the temporomandibular joint. It tends to pull the mandibular condyle and the articular disc forward and medially. So it is the major active jaw opener. When the muscle of one side is operating independently it will tend to deviate the chin to the opposite side; i.e., the left lateral pterygoid muscle will deviate the chin to the right (See Figure 8-6). When both muscles contract simultaneously they cause straight jaw opening, since their laterally directed vectors will cancel. It is innervated by a branch of the mandibular division of the trigeminal nerve (N. 50, 71).


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Figure 8-6

The medial pterygoid muscle arises from medial aspect of the lateral pterygoid plate and from the pterygoid fossa between the medial and lateral pterygoid plates (N. 49). It descends with a lateral inclination to attach to the internal aspect of the lower mandibular ramus. It is a jaw closer and may aid in deviating the jaw to the opposite side. It is innervated by a branch of the mandibular division of the trigeminal nerve. Three of the four muscles of mastication are primary jaw closers, which explains why if these muscles are become hyperactive, as in tetanus, the jaw assumes a closed position. The normal opener of the jaw is gravity, with an assist from the lateral pterygoid muscle (and other accessory muscles) when resistance is offered. If injury of the mandibular division of the trigeminal nerve is suspected the integrity of the masseter and temporalis muscles can be evaluated by asking the patient to bite down so that contraction of these muscles can be visualized or palpated. If this nerve is injured the chin will tend to point toward the side of the lesion by the unopposed pull of the normal opposite lateral pterygoid muscle; i.e., if the right mandibular division is injured the unopposed pull of the left lateral pterygoid muscle will cause the chin to deviate to the right in the at rest position (See Figure 8-6B). To exaggerate this deviation the patient can be asked to open the mouth against the resistance of the examiner's hand under the chin. This will maximize the activity of the intact lateral pterygoid and cause the chin to deviate dramatically toward the side of the lesion. The loss of the ipsilateral lateral pterygoid muscle can also be demonstrated by asking the patient to attempt to deviate the jaw away from the suspected side of the lesion, which activity will not be able to be carried out. SO, IN A LESION OF THE MANDIBULAR DIVISION OF THE TRIGEMINAL NERVE THE RULE IS THE CHIN POINTS TO THE SIDE OF THE LESION AT REST AND ON ACTIVELY ATTEMPTING TO OPEN THE JAW AGAINST RESISTANCE. MANDIBULAR DIVISION OF THE TRIGEMINAL NERVE (MANDIBULAR NERVE, V3) (N. 2, 50, 71) The mandibular division of the trigeminal nerve contains two functional fiber types: SA fibers to its dermatomal distribution, the mandibular teeth, the anterior two-thirds of the tongue and the mucous membrane of the floor of the mouth including the lingual aspect of the gums of the lower jaw; and motor fibers to the four major muscles of mastication, and the tensor veli palatini, tensor tympani, mylohyoid and anterior belly of the digastric muscle. After emerging from the foramen ovale the mandibular division of


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the trigeminal nerve divides into its branches in the plane between the lateral pterygoid muscle laterally and the medial pterygoid, tensor veli palatini and tensor tympani muscles medially. The mandibular division of the trigeminal nerve gives off four branches that are primarily sensory. Its posteriorly directed auriculotemporal nerve branch usually arises by two roots that surround the middle meningeal artery as the artery approaches the foramen spinosum. Posterior to the artery these roots unite and accompany the maxillary vessels deep to the neck of the mandible to pass onto the face through the upper pole of the parotid gland as previously described with the face. Fractures of the mandibular neck can injure this nerve and the maxillary vessels. Both the inferior alveolar and lingual nerve branches of the mandibular division descend on the superficial surface of the medial pterygoid muscle with the inferior alveolar nerve posterior to the lingual nerve. The inferior alveolar nerve enters the mandibular canal to innervate all of the mandibular teeth and send a mental branch through the mental foramen to innervate the lower lip. Just before the inferior alveolar nerve enters the mandibular foramen it gives off a small branch which will provide motor innervation to the mylohyoid muscle and the anterior belly of the digastric muscle in the floor of the mouth. High in the infratemporal fossa the lingual nerve is joined by the chorda tympani which originates from the facial nerve within the temporal bone. The chorda tympani emerges from the middle ear cavity through the medial part of the fissure in the mandibular fossa. Then it runs inferiorly and anteriorly deep to the inferior alveolar nerve to join the lingual nerve from behind. The chorda tympani contains taste fibers to the anterior two-thirds of the tongue and preganglionic parasympathetic fibers to parasympathetic ganglia in the floor of the mouth which will innervate the submandibular and sublingual salivary glands. The further course of the lingual nerve is fully described with the floor of the mouth (page 254). As it arises from the mandibular division the lingual nerve contains only SA fibers for general sensation from the anterior two-thirds of the tongue, the mucous membrane of the floor of the mouth and some of the lingual aspect of the gingiva (gums). The chorda tympani fibers only accompany the lingual nerve and are not considered part of its functional components since they leave the brain with the facial nerve. The buccal nerve runs inferiorly and forward deep to or through the insertion of the temporalis muscle to innervate the full thickness of the cheek. THE MOTOR BRANCHES OF THE MANDIBULAR DIVISION OF THE TRIGEMINAL NERVE INNERVATE THE FOUR MAJOR MUSCLES OF MASTICATION AND FOUR MORE MUSCLES, TWO OF WHICH ARE TENSORS, THE TENSOR VELI PALATINI AND TENSOR TYMPANI, AND THE OTHER TWO OF WHICH ARE THE MYLOHYOID AND ANTERIOR BELLY OF THE DIGASTRIC MUSCLE. The branches to the lateral pterygoid, temporalis and masseter muscles course laterally to innervate their respective muscles. The muscular branches to the medial pterygoid, tensor veli palatini and tensor tympani muscles arise from the deep surface of the mandibular division to innervate these medially situated muscles. The tensor veli palatini and tensor tympani muscles are described with the palate and temporal bone, respectively. VESSELS OF THE INFRATEMPORAL FOSSA The maxillary artery (N. 51, 72) courses forward deep to the mandibular neck to enter the infratemporal fossa. As it traverses the fossa on the way to its exit from the fossa via the pterygomaxillary fissure, it may be situated either superficial or deep to the lateral pterygoid muscle. Within the infratemporal fossa it supplies branches to all the muscles of mastication and gives off two other important branches, the middle meningeal artery and the inferior alveolar artery. Both of these vessels arise from the maxillary artery while it is deep to the mandibular neck or just after it emerges anterior to the neck. The middle meningeal artery ascends deep to the lateral pterygoid muscle to enter the foramen spinosum. As it approaches the base of the skull it commonly has the auriculotemporal nerve's roots splitting around it. The inferior alveolar artery descends in company with the inferior alveolar nerve to supply the same structures the nerve supplies. The important branches of the terminal part of the maxillary artery within the pterygopalatine fossa are described with the nasal cavity. There is a plexus of veins about the pterygoid muscles (N. 73) which receives the venous drainage of the structures supplied by the maxillary artery and communicates widely with facial, orbital, and cranial


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veins. This plexus drains into the maxillary veins which pass posteriorly deep to the mandibular neck to enter the retromandibular vein. There is also a lymphatic network in the infratemporal fossa which receives some of the lymphatic drainage of the nasopharynx, oropharynx, palatine tonsils, paranasal sinuses, upper teeth and the floor of the mouth. It transmits its drainage to both the superior deep lateral cervical and retropharyngeal lymph nodes (N. 75).

ORAL CAVITY OSTEOLOGY OF THE ORAL CAVITY Most of the mandible is described with the face and infratemporal fossa. The internal aspect of the body of the mandible serves as attachment for muscles of the floor of the mouth and tongue (N. 17). Immediately adjacent to the junction of the two halves of the body of the mandible the small mental (genial) spines are situated. This area serves for the mandibular attachment of the important genioglossus muscles. The bony palate serves as both the roof of the oral cavity and the floor of the nasal cavity (N. 10, 37). It is formed by the horizontally situated paired palatine processes of the maxilla anteriorly and the horizontal portion of the palatine bones posteriorly. The paired palatine bones (N. 10, 37) are L-shaped bones with a vertical portion contributing to the lateral walls of the nasal cavity and a horizontal portion forming the posterior part of the hard palate. Where the horizontal part of the palatine bone abuts the posterior part of the alveolar process of the maxilla, there is a large greater (anterior) palatine foramen which transmits the greater (anterior) palatine vessels and nerves onto and forward along the hard palate. Posterior to this foramen there are one or two smaller lesser (posterior) palatine foramina which are bounded by the pterygoid process of the sphenoid bone posteriorly. These convey the lesser (posterior) palatine nerves and vessels posteriorly to the soft palate. Where the alveolar process of the maxilla meets its palatine process a sulcus is present which transmits the greater palatine nerve and vessels forward and toward the midline. Just internal to the midline junction of the alveolar processes of the two maxillae an incisive canal permits communication between the greater palatine vessels and nerves below and the vessels and nerves of the nasal septum above. Just posterior to the posterior end of the maxillary alveolar process a hook-like hamulus protrudes posterolaterally from the inferior margin of the medial plate of the pterygoid process of the sphenoid bone. This serves as a pulley about which the tendon of the tensor veli palatini changes its pull from a vertical to lateral direction (see soft palate). SUPRAHYOID (FLOOR OF THE MOUTH) REGION (N. 27-29) The portion of the anterior triangle of the neck above the hyoid bone can be divided into two triangular areas by the two bellies of the digastric muscle (N. 27-29, 58). The submental triangle is bounded medially by the anterior midline of the neck, inferiorly by the body of the hyoid bone and laterally by the anterior belly of the digastric. The submandibular triangle is bounded anteriorly by the anterior belly of the digastric, posteriorly by the posterior belly of the digastric, and superiorly by the inferior margin of the body of the mandible. The hyoid bone (N. 15, 73) is a U-shaped bone. Above, the hyoid serves for attachment of the suprahyoid muscles and some of the tongue muscles in the floor of the mouth. Below, it is connected to the thyroid cartilage of the larynx by the thyrohyoid membrane and serves as attachment for some infrahyoid muscles. The major suprahyoid muscles include the digastric, stylohyoid and mylohyoid muscles, but, the hyoglossus and other tongue muscles are intimately related to this region.


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The digastric muscle (N. 27-29, 58) is composed of two bellies separated by an intermediate tendon which is bound down to the hyoid bone by a fascial sling. The posterior belly attaches above just medial to the mastoid process of the temporal bone, while the anterior belly attaches anteriorly to the lower part of the inner aspect of the mandible near the midline. The intermediate tendon passes through a split in the adjacent stylohyoid muscle. The posterior belly of the digastric is innervated by the closely adjacent facial nerve as this nerve emerges from the stylomastoid foramen. Its anterior belly is innervated by the branch of the mandibular division of the trigeminal nerve which innervates the mylohyoid muscle. Acting from fixed upper attachments the muscle can elevate the hyoid bone thereby elevating the floor of the mouth and with it, the base of the tongue during swallowing. By elevating the hyoid it can also help to elevate the larynx during swallowing and phonation. Acting from a fixed point below, the digastric can aid in forcibly depressing the mandible. The stylohyoid muscle (N. 27-29, 58) arises from the styloid process of the temporal bone and descends in front of the posterior belly of the digastric muscle, which passes through a split in its lower part. The stylohyoid inserts into the hyoid bone. It is innervated by the facial nerve as the nerve emerges from the stylomastoid foramen. It functions, like the digastric, to elevate the hyoid and larynx in swallowing and phonation. The flat mylohyoid muscle (N. 27-29, 46-47, 58) arises on the inner aspect of the body of the mandible and inserts below into the hyoid and anteriorly into a midline raphe with its fellow of the opposite side. It is innervated by the mylohyoid branch of the mandibular division of the trigeminal nerve which arises from the inferior alveolar nerve, just before it enters the mandibular canal. This muscle functions like the digastric to elevate the floor of the mouth, hyoid and larynx. The flat hyoglossus muscle (N. 47, 58-59) arises from the greater cornu and lateral part of the body of the hyoid. Its fibers run upward and slightly forward so that as the anterior part of the muscle inserts into the tongue it passes deep to the mylohyoid. The posterior part of the muscle extends posteriorly beyond the mylohyoid muscle. It is innervated, like most of the tongue muscles, by the hypoglossal nerve and functions to depress the dorsum of the tongue. The non-muscular structures of the suprahyoid region can be related to the two major planar muscles in this region, the mylohyoid and hyoglossus muscles. These two muscles separate three layers or planes of structures in the floor of the mouth. In the first plane superficial to the mylohyoid muscle the most significant structures in the submental triangle are the submental lymph nodes (N. 74-75). These nodes are important in draining the medial part of the chin and lower lip as well as the medial portion of the anterior tongue. In the first plane superficial to the mylohyoid muscle the submandibular triangle (N. 31, 46-47, 72) contains the large superficial portion of the submandibular salivary gland with the facial vein coursing over its superficial surface (N. 73) and the facial artery related to its deep and superior aspects (N. 72-73). At the point where the facial vessels turn up onto the face the marginal mandibular branch of the facial nerve typically loops a little below the margin of the mandible to cross the superficial aspect of these vessels in the upper part of the submandibular triangle (N. 24, 31). This important nerve innervates the lower part of the orbicularis oris and adjacent lower lip muscles (see anterior neck and face). The submandibular lymph nodes (N. 74-75) may be located along any surface of the submandibular gland. These nodes receive lymphatics which accompany the facial vessels and drain the lower face and the lateral part of the anterior tongue. The submandibular nodes also receive some of the drainage from the submental nodes and in turn drain into the superior deep lateral cervical nodes. In the second plane between the mylohyoid muscle and the hyoglossus muscle (N. 47 lower, 58, 71-72) the deep part of the submandibular gland and the submandibular duct turn forward around the posterior border of the mylohyoid muscle. It is also in this plane that the hypoglossal nerve passes forward about a cm above the hyoid bone (N. 34, 47 lower, 59, 71-72). At a higher level in this same interval the lingual nerve curves forward into the tongue making a loop around the anterior portion of the


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submandibular duct as the nerve passes lateral, then inferior, and finally medial to the duct (N. 46-47, 59). It provides branches to the anterior two-thirds of the tongue for its somatic afferent innervation. The lingual nerve receives taste afferents as well as parasympathetic preganglionic fibers from the facial nerve via the chorda tympani branch of the facial nerve which joins the lingual nerve high in the infratemporal fossa (N. 123-124). The taste afferents accompany the lingual nerve to mediate taste sensation from the anterior two-thirds of the tongue. The parasympathetic preganglionic fibers provide secretomotor innervation to the submandibular and sublingual glands after synapse in postganglionic neurons near or within these glands. The flat sublingual salivary gland is situated in the anterior part of this plane (N. 46-47, 56, 58). Its upper border forms the sublingual fold of the floor of the mouth which is visualized when the tongue is elevated (see interior of the oral cavity). Its multiple ducts open into the mouth along this fold. In the third plane deep to the hyoglossus muscle (N. 47, 59) the lingual artery runs forward into the tongue. This artery can usually be visualized in the posterior part of the submandibular triangle before it runs deep to the hyoglossus muscle. The glossopharyngeal nerve also courses forward into the tongue in the plane deep to the hyoglossus muscle but at a higher level than the lingual artery (N. 71-72). It, too, may be visualized high in the posterior part of the submandibular triangle before it passes deep to the hyoglossus. MUSCLES OF THE TONGUE (N. 59, 64) The tongue has a dorsum which is named from the quadripedal position. It is the tongue's superior aspect in the bipedal position. It also has a root or base where it is attached to the posterior part of the floor of the mouth. Its apex is its anterior tip. The tongue contains both extrinsic and intrinsic muscles. The extrinsic muscles of the tongue tend to be the major movers of the tongue and the intrinsic muscles mostly change its shape. The intrinsic muscles are small bundles of skeletal muscle that run anteroposteriorly, mediolaterally and vertically within the mass of the tongue. They function to change the shape or the intrinsic tone of various portions of the tongue. The extrinsic tongue muscles arise outside the tongue and insert into the tongue. In the order of their clinical testing importance they include the genioglossus, styloglossus, palatoglossus and hyoglossus. ALL OF THE EXTRINSIC AND INTRINSIC MUSCLES OF THE TONGUE ARE INNERVATED BY THE HYPOGLOSSAL NERVE, EXCEPT THE PALATOGLOSSUS WHICH IS ALSO A PALATAL MUSCLE AND THEREFORE IS INNERVATED BY THE VAGUS NERVE (and accompanying cranial root fibers of the accessory nerve), which provides the primary motor innervation of the palatal muscles. The genioglossus muscles (N. 59, 64) have a nearly midline origin from the area of the mental spines of the mandible. From this point their fibers radiate toward the dorsum of the tongue as far posteriorly as its root. From a clinical testing point of view it is imperative to recognize that as these fibers course posteriorly toward the root of the tongue they diverge laterally away from the midline (See CD H&N Section 11). The functions of this muscle are to protrude the tongue and deviate the tongue laterally to the opposite side (See Figure 8-7). In tongue protrusion the intrinsic muscles of the anterior tongue contract to make the anterior portion of the tongue a rigid pillar. Then contraction of the genioglossus fibers which attach to the root of the tongue will pull the posterior part of the tongue forward and protrude the firmed-up tip. If the right and left genioglossus muscles contract equally the tongue can be protruded straight forward, since their lateral obliquities of pull will cancel out. If one muscle is paralyzed the normal opposite muscle will protrude the tongue and deviate it toward the paralyzed side (See Figure 8-7).


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Figure 8-7 - Superior view of tongue and mandible

SO THE RULE IS THAT THE PROTRUDED TONGUE POINTS “TOWARD" THE SIDE OF THE PARALYZED GENIOGLOSSUS MUSCLE OR THE SIDE OF THE INJURED HYPOGLOSSAL NERVE. The integrity of a genioglossus muscle can also be tested by pushing the tongue against the opposite cheek, e.g., to test the left genioglossus the tongue pushes out the right cheek. Strength can be tested by the examiner using his fingers to apply a counterforce to the tongue through the cheek. The styloglossus muscles (N. 59) originate from the relatively laterally placed styloid processes and run anteriorly, inferiorly and medially into the tongue. On contraction these muscles will retract the tongue (pull it back into the mouth). If one muscle is paralyzed the remaining normal muscle will retract the tongue and pull it toward the normal muscle's side. So when the tongue is at rest in the mouth or being retracted into the mouth it will tend to deviate "away from" the side of a paralyzed styloglossus muscle or injured hypoglossal nerve (See Figure 8-7). The styloglossus muscle will also simultaneously retract and elevate the tongue on swallowing to help deliver a bolus of food from the oral cavity into the oropharynx. The palatoglossus muscle (N. 57, 59, 68) descends from the soft palate to the lateral aspect of the posterior part of the tongue with a slight forward inclination. Anterior to the fossa of the palatine tonsil it raises a mucosal fold called either the palatoglossal arch or the anterior pillar of the fauces. On contraction it elevates and retracts the tongue and, therefore helps deliver a bolus of food from the oral cavity to the oropharynx in swallowing. It is the only tongue muscle not innervated by the hypoglossal nerve. Its innervation is by the vagus nerve's palatal branches which run through the pharyngeal plexus.

The hyoglossus muscle is described with the suprahyoid region. HYPOGLOSSAL NERVE (N. 129) The hypoglossal nerve (cranial nerve XII) is predominantly a motor nerve. It innervates all of the tongue muscles except the palatoglossus muscle. Its multiple rootlets arise from the preolivary sulcus of the medulla where it may be involved with vascular lesions or tumors involving the medulla (N. 115). The vertebral artery lies immediately anterior to its rootlets and hence can involve the nerve in an aneurysm (N. 140). As the roots cross the subarachnoid space to the hypoglossal canal they are stretched across the foramen magnum where they can be involved in herniations of the cerebellum down through the foramen magnum in increased intracranial pressure (N. 105, 129). After emerging from the hypoglossal canal the nerve descends lateral to the nasopharynx and oropharynx where it can be involved in tumor or abscess of these organs (N. 71, 73, 129). It then swings forward between the internal jugular vein and the carotid arteries (N. 32, 34) to enter the floor of the mouth in the plane between the mylohyoid and hyoglossus muscles, where it breaks up into multiple branches to


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all tongue muscles except the palatoglossus. WHEN THE HYPOGLOSSAL NERVE IS INJURED THE CLASSIC SIGNS ARE THAT THE PROTRUDED TONGUE POINTS TO THE SIDE OF THE INJURY BY THE UNOPPOSED PULL OF THE NORMAL GENIOGLOSSUS MUSCLE, AND THE RETRACTED TONGUE DEVIATES AWAY FROM THE SIDE OF THE INJURY BY THE UNOPPOSED PULL OF THE NORMAL STYLOGLOSSUS MUSCLE (see muscles of the tongue). Tongue movements will be compromised during speech and swallowing. Speech may become thick and slurred and there can be swallowing difficulty, particularly with solid foods (see swallowing functions, pages 258-259). THE FAUCES (N. 47, 56, 68) The fauces is the narrowed region between the oral cavity and the oropharynx bounded superiorly by the soft palate, inferiorly by the posterior portion of the tongue and laterally by the pillars of the fauces and the intervening tonsillar fossa. The anterior pillar of the fauces is called the palatoglossal arch (fold) since it extends from the palate to the tongue and is produced by the palatoglossus muscle (N. 68). The posterior pillar of the fauces is called the palatopharyngeal arch (fold) since it extends from the soft palate into the oropharynx and is formed by the palatopharyngeus muscle. The tonsillar fossa is situated between the palatoglossal and palatopharyngeal arches. In the older adult the palatine tonsils have typically atrophied or been removed so the fossa is simply the depression between the folds. Prepubertal children (and some adults) have large palatine tonsils which protrude medially between the palatoglossal and palatopharyngeal arches (N. 47, 56-57, 68). Underlying the mucosa the floor of the tonsillar fossa is formed mostly by the superior constrictor muscle. The palatine tonsils are richly vascularized by branches of the palatine, lingual, facial and pharyngeal vessels which approach it from all sides (N. 68). Hence, tonsillectomy can be accompanied by substantial bleeding. The glossopharyngeal nerve swings forward into the tongue just a few mms lateral to the lower part of the tonsillar fossa where it can be injured directly by tonsillectomy or compromised by the subsequent swelling (N. 68). Since this nerve provides general sensory and taste innervation to the posterior one-third of the tongue these functions may be compromised following tonsillectomy. THE PALATE (N. 56-57, 65, 67-68) The bony hard palate is covered by a mucosa containing glands. The greater (anterior) palatine nerve and vessels emerge from the greater palatine foramen just medial to the last maxillary molar and run forward in the palatal sulcus along the junction of the palatine and alveolar processes of the maxilla to supply most of the hard palate (N. 39-40, 51). These nerves are sensory branches of the maxillary division of the trigeminal nerve and the vessels are branches of the maxillary vessels which arise in the pterygopalatine fossa. The soft palate is a fibromuscular septum which is attached to the posterior margin of the hard palate. It descends obliquely posteriorly toward the oropharynx to form a palatine velum (=veil) (N. 56-57, 64, 68). The finger-like uvula descends in the midline from the free posteroinferior margin of the soft palate. The major muscles of the soft palate include the tensor veli palatini, levator veli palatini, palatoglossus and palatopharyngeus. ALL OF THE MUSCLES OF THE PALATE ARE INNERVATED BY THE VAGUS NERVE THROUGH ITS PHARYNGEAL BRANCHES TO THE PHARYNGEAL PLEXUS, EXCEPT THE TENSOR VELI PALATINI, WHICH IS INNERVATED BY THE MANDIBULAR DIVISION OF THE TRIGEMINAL NERVE. The palatoglossus and palatopharyngeus muscles have been described with the tongue and pharynx. They are dual function muscles since they can respectively elevate the tongue and pharynx and also depress the palate. The levator veli palatini muscle largely arises from the inferior aspect of the petrous portion of the temporal bone. Its fibers descend into the palate with a substantial medial inclination (N. 57). In a midsagittal view of the nasopharynx they appear to enter the palate along the inferior aspect of the pharyngeal orifice of the auditory tube, just anterior to the torus tubarius (N. 68, 65). When this muscle contracts it will not only elevate the palate but will tend to pull the palate to its own side (See Figure 8-8)


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Figure 8-8

SO IF THE LEVATOR VELI PALATINI IS PARALYZED BY A LESION OF THE VAGUS NERVE THE PALATE WILL TEND TO BE PULLED "AWAY FROM" THE SIDE OF THE LESION (TOWARD THE NORMAL SIDE) BY THE UNOPPOSED PULL OF THE NORMAL MUSCLE. This at rest deviation can be exaggerated by asking the patient to say “ahhh” which will actively recruit the normal levator veli palatini and cause marked palatal deviation away from the side of the lesion. Palatal elevation is an important component of swallowing and speech. During swallowing the palate is elevated against the posterior pharyngeal wall to close off the nasopharynx to prevent fluids or food from going up into the nasopharynx when the pharyngeal constrictors contract. Likewise, during speech the palate is elevated to close off the nasopharynx and nasal cavity in the production of oral sounds like P and T. If the palatal muscles are paralyzed the patient may complain of reflux of fluids into the nasal cavity during swallowing. In addition, speech will have a nasal quality, because of the leakage of air up into the nasal cavity during the production of oral sounds. The tensor veli palatini muscle has a bony origin from the pterygoid fossa of the sphenoid bone just lateral to where the medial pterygoid plate attaches to the sphenoid body (N. 10). It also has an origin from the membranous lateral wall of the auditory tube (N. 100). Its fibers descend vertically toward the hamulus of the medial pterygoid plate (N. 57, 65). They make a right angle bend about the hamulus to change to a medial direction and they therefore insert into the palate in a horizontal plane. During contraction the pulley-like effect of the hamulus causes the muscle to exert a lateral pull upon the palate. Since the relaxed palate has an arched configuration (N. 56-57), simultaneous contraction of the two tensor veli palatini muscles will tense the palate (See Figure 8-8). IF ONE TENSOR VELI PALATINI MUSCLE IS PARALYZED THE UNOPPOSED PULL OF THE OPPOSITE MUSCLE WILL CAUSE THE UVULA TO BE DEVIATED "AWAY FROM" THE SIDE OF THE LESION (TOWARD THE NORMAL SIDE). All of the same palatal dysfunctions and tests described for the levator veli palatini will apply. Since the tensor veli palatini is innervated by the mandibular division of the trigeminal nerve, palatal asymmetry and its swallowing and speech dysfunctions can be a sign of either vagal or mandibular trigeminal nerve injuries. To determine which nerve is involved the other functions of the vagus (gag reflex, vocal cord paralysis) and the mandibular division of the trigeminal nerve (muscles of mastication and dermatomal distribution) must be evaluated. THE INTERIOR OF THE ORAL CAVITY (N. 56) On examining the interior of the oral cavity, particular attention should be paid to the soft palate, faucial region, tongue, floor of the mouth, openings of the salivary glands and teeth. The sensory innervation of the hard palate is by the greater palatine nerves, while that of the soft palate is via the lesser palatine nerves (N. 41, 57, 61). The soft palate should be inspected for symmetry of the uvula and the palatal


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arches, at rest and on saying “ahhh”. The palatoglossal and palatopharyngeal arches can be well visualized along with the palatine tonsil and tonsillar fossa between them (N. 56-57). The dorsum of the tongue is seen to be relatively smooth on its anterior two-thirds and irregular over its posterior one-third where the lingual tonsillar tissue raises multiple elevations (N. 60). Between the anterior two-thirds and posterior one-third of the tongue a V-shaped row of large circumvallate papillae can be visualized. At the apex of the V a blind pit, the foramen cecum, marks the point of origin of the thyroid gland primordium of the embryo. The anterior two-thirds of the tongue receives general sensory innervation by the lingual nerve branch of mandibular V (N. 61). Its taste innervation is via the chorda tympani fibers that originate from the facial nerve and run with the lingual nerve. The posterior one-third of the tongue receives both its general sensory and taste innervation from the glossopharyngeal nerve (N. 61). If the tip of the tongue is elevated the floor of the mouth can be inspected (N. 56). This reveals the midline fold, the frenulum which attaches the undersurface of the tongue to the floor of the mouth. On either side of the frenulum's attachment to the floor of the mouth two small elevations, the sublingual caruncles, are located. These contain the oral openings of the ducts of the submandibular glands. Since this is a naturally narrow part of the duct, calculi which develop in the duct frequently impact at this point. On either side of the sublingual caruncle the sublingual folds can be seen diverging like the limbs of a V as they are traced posteriorly in the floor of the mouth on either side of the tongue. These represent the superior borders of the sublingual glands and the multiple ducts of each sublingual gland open along these folds. The sensory innervation of the floor of the mouth is by the lingual nerve. The opening of the parotid duct can usually be visualized on the mucosal aspect of the cheek opposite the second maxillary molar (N. 56). Parotid calculi can be retained at this point. Inflammation of any of the salivary glands, e.g., mumps, can cause inflammation of the openings of their ducts into the oral cavity. The buccal mucosa is innervated by the buccal branch of mandibular V (N. 61). In the adult there are typically 16 maxillary and 16 mandibular permanent teeth (N. 62-63). In each set there are paired central and lateral incisors, canines, two premolars (bicuspids) and three molars. The mandibular teeth are provided sensory innervation by the inferior alveolar branch of mandibular V. The maxillary teeth are provided sensory innervation by multiple superior alveolar branches of the maxillary division of the trigeminal nerve. SUMMARY OF SWALLOWING MECHANISMS Normal swallowing requires the integrated sequential activity of muscles of the tongue, floor of the mouth, soft palate, larynx, pharynx and esophagus. To deliver the bolus of masticated food from the oral cavity into the oropharynx the palatoglossus and styloglossus muscles are activated to elevate and retract the tongue. This is contributed to by the contraction of floor of the mouth muscles like the digastric and mylohyoid, which by elevating the floor of the mouth also help elevate the tongue. As the bolus of food is delivered into the oropharynx, the nasopharynx and larynx must be closed to prevent the bolus from going into the nasopharynx or larynx. The levator and tensor veli palatini muscles contract to elevate the tensed palate so that its posterior aspect contacts the posterior pharyngeal wall. The nasopharynx is further sealed off from the oropharynx by the contraction of the vertical muscles of the pharynx which will create a transverse fold in the posterior pharyngeal wall opposite the soft palate. Food is prevented from entering the larynx partly by closure of the inlet by the epiglottis and partly by adduction of the vocal folds. The epiglottis is folded down over the laryngeal inlet by multiple mechanisms. The small aryepiglottic and thyroepiglottic muscles help to actively depress it. It may be indirectly depressed by the retraction of the tongue which pushes the epiglottis ahead of it. A further mechanism of closure of the laryngeal inlet is by the laryngeal elevation produced by the suprahyoid and


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vertical pharyngeal muscles which bring the rest of the laryngeal inlet up to meet the depressed epiglottis. The vocal folds are tightly adducted by the action of their adductors to serve as a second line of defense to prevent any food from descending the airway. After the nasopharynx and larynx are sealed by these mechanisms and the oral cavity itself is sealed by the retraction and elevation of the tongue, the pharyngeal constrictors above the bolus of food contract while those below the bolus relax. This constrains the bolus to descend the pharynx. Most liquids are reflected along the sides of the epiglottic baffle and descend the laryngeal pharynx within the piriform recesses. As the bolus enters the esophagus a similar contraction of the musculature above the bolus and relaxation below the bolus causes the food to descend the esophagus. The upper musculature of the esophagus is skeletal and therefore voluntary, while its lower musculature becomes smooth muscle which is involuntary.

NASAL CAVITY OSTEOLOGY OF THE NASAL CAVITY (N. 4, 8, 37-38) The ethmoid bone (N. 4, 8, 37, 38 bottom, 43) is a major component of both the upper nasal septum and the lateral nasal walls. It is composed of a sagittally oriented perpendicular plate and two large laterally situated labyrinths which are joined to the perpendicular plate by the cribriform plates. The labyrinths contain the bulk of the ethmoid air cells, though during development these commonly encroach into adjacent articulating bones. The labyrinths also form the superior and middle conchae. The nasal septum (N. 8, 38) is formed above by the perpendicular plate of the ethmoid, posteroinferiorly by the triangular vomer bone and anteroinferiorly by the septal cartilage. It is not uncommonly deviated to one side and this can cause obstruction of that side of the nasal cavity. The lateral nasal wall (N. 8, 37) is usually marked by three elongate scroll-like conchae (or turbinates) named superior, middle and inferior conchae. In coronal section they have a curved appearance not unlike the conch shells for which they are named. The ethmoid labyrinth forms the upper part of the lateral nasal wall including the superior and middle conchae and a sometimes present supreme concha. The maxilla forms much of the lower part of the lateral nasal wall, and the inferior concha, a separate bone, attaches to it. The nasal and lacrimal bones contribute to the anterosuperior part of the lateral nasal wall. The perpendicular part of the palatine bone and medial pterygoid plate of the sphenoid bone form the posterior portion of the lateral nasal wall. The roof of the nasal cavity is formed from front to back by the nasal bone, cribriform plate of the ethmoid bone and body of the sphenoid. The floor of the nasal cavity is formed by the palatine process of the maxilla and the horizontal portion of the palatine bone. A sphenopalatine foramen is situated at the posterior end of the middle concha near the point where the anterior and inferior walls of the sphenoid body meet. This foramen will transmit the important sphenopalatine artery and nasal nerves from the pterygopalatine fossa into the nasal cavity. It marks the locus of the pterygopalatine fossa on the lateral nasal wall. The incisive foramen (canal) is located near the anterior junction of the palatine processes of the two maxillae. It permits communication between palatine and nasal nerves and vessels. The apertures of the paranasal sinuses will be described with the interior of the nasal cavity. The external nose (N. 35) has a dorsum, which is its anterosuperior border in the bipedal position. The upper part of the nose, which is situated between the orbits, is the root or bridge of the nose. It is this part which is bony, and the paired nasal bones are major contributors. The larger lower portion of the external nose is formed by a number of cartilages some of which are continuous deeply with the septal cartilage.


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INTERIOR OF THE NASAL CAVITY (N. 36) The nasal cavity communicates anteriorly with the external world through the nares and posteriorly with the nasopharynx through the choanae. The nasal cavity is lined by a glandular mucosa which is so richly vascularized that it resembles erectile tissue. Its function is to help humidify and warm the inspired air. The conchae projecting into the cavity from its lateral wall facilitate these functions by increasing the surface area and creating a turbulent flow which brings more of the inspired air into contact with the mucosa. The mucosa also contains a motile ciliated epithelium which moves the mucous coat and its adherent particulate matter toward either the external nares or the choanae. Beneath each concha there is a recess called a meatus which is named for its overlying concha. Each meatus is normally open inferiorly to the nasal cavity. Since at least the lower conchae and meatuses are visualizable with a nasal speculum or the larger otoscopic attachments of the ophthalmoscope, it is important to be aware of what normally opens into each meatus. The inferior meatus receives the nasolacrimal duct. The middle meatus has a large rounded ethmoidal bulla protruding from its lateral wall. There is a semilunar hiatus curving around the anterior and inferior aspects of the bulla. The ostia of the frontal sinus, maxillary sinus and the anterior ethmoid air cells commonly open into the middle meatus through the semilunar hiatus. The multiple eth-moid air cells are divided into anterior, middle and posterior groups on the basis of where they open into the nasal cavity. The anterior ethmoid air cells open into the semilunar hiatus. The middle ethmoid air cells produce the bulla ethmoidalis and open onto its apex. The posterior ethmoid air cells typically open into the superior (and occasionally a supreme) meatus. The cribriform plate of the ethmoid bone and the anterior face of the sphenoid body form a recess in the roof of the nasal cavity called the sphenoethmoidal recess. The ostium of the sphenoid sinus opens into this recess through the anterior face of the sphenoid body. PARANASAL SINUSES (N. 5, 7, 36, 42-45; CD Normal Skull 1 to 4) The paired paranasal sinuses develop as outgrowths of the nasal cavity. The ethmoid and maxillary sinuses usually show some early development by birth while the sphenoid and frontal sinuses typically begin to show some development during the first two years of life. They all show gradual enlargement up to adulthood, but their degree of development and symmetry can vary substantially even amongst adults. Their normal functions are relatively trivial, serving to lighten the head and acting as resonating chambers for the voice. The small ostia prohibit much participation in warming and humidifying the inspired air. Their importance is more clinical than physiological, since they are frequently infected and occasionally the sites of tumors. Their geometry and relationships are important to both diagnosis and treatment. The frontal sinus develops in both the vertical squamous and horizontal orbital portions of the frontal bone. Its degree of development in either direction is highly variable as is its right-left symmetry. It is related anteriorly to the forehead, inferiorly to the orbit, and posteriorly and superiorly to the anterior cranial fossa, all of which can be encroached by frontal sinus disease. Its drainage canal is in its inferior wall so it drains well gravitationally in the upright position if the canal is patent. Its aeration can be evaluated by placing the patient in a dark room and transilluminating the sinus by inserting an upwardly directed penlight against the orbital roof between the superior orbital margin and the eyeball. If it is air-filled it will produce a pink glow over the forehead. If it is fluid- or tumor-filled it will remain dark. The pain of frontal sinusitis is typically referred to the forehead since the sinus is innervated by the supraorbital nerves. The maxillary sinus (antrum) occupies the body of the maxilla. It is related superiorly to the orbit, anterolaterally to the face, posteriorly to the infratemporal fossa and inferiorly to the palate and upper teeth (N. 42-44). Hence, maxillary sinus disease can encroach on any of these areas. It can be transilluminated


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in a dark room by placing a downwardly directed penlight between the eyeball and the inferior orbital margin and looking for a pink glow on the face below the orbit indicating its normal aeration. Fluid or tumor within the sinus will compromise transillumination. It is innervated by the infraorbital nerve and by the superior alveolar nerves which descend to the upper teeth beneath its mucous membrane. Hence, the pain of maxillary sinusitis can be referred to the anterior face or upper teeth. Its ostium is located very high on its medial wall, so it will not drain well gravitationally in the upright position until it is nearly full of fluid (N. 55, 43-44). The multiple ethmoid air cells are related laterally to the medial wall of the orbit and superiorly to the anterior cranial fossa, into which ethmoid air cell disease can spread (N. 43). They are innervated by the ethmoidal branches of the ophthalmic division of V, so their pain can be referred to either the orbital or nasal distribution areas of these nerves. They are too deep to be transilluminated, so their evaluation must be largely roentgenographic. Their ostia are usually near their inferior walls. So they drain pretty well in the upright position. The paired sphenoid sinuses occupy the body of the sphenoid bone and are frequently very asymmetrical. Because of their depth they can only be evaluated roentgenographically. Their ostium is located about two-thirds of the distance up their anterior wall (N. 36-37, 44). So in the upright position they must be two-thirds full of fluid before they will drain well when infected. A knowledge of the geometry of the sinuses and the location of their ostia is important in the treatment of sinus infection. When the sinuses are infected the accompanying mucosal swelling can block their narrow ostia causing fluid to accumulate in the sinus with consequent pain production. Treatment typically involves the application of mucosal shrinking medication to the ostium of the involved sinus to open it. If nosedrops are simply applied by tilting the head backward they will just run down the floor of the nasal cavity into the nasopharynx and not reach any of the ostia. If a frontal, ethmoidal or maxillary sinusitis is to be treated the drops should be applied with the patient lying on their side with the involved side of the head down and the top of the head dependent by allowing it to hang over the edge of the bed. This causes the lateral nasal wall to assume an inclined plane which will direct the drops into the middle meatus where they will puddle and be able to treat the involved ostia (***try tilting the N. 43 top illustration into the above described position to confirm this). Once opened, the frontal and ethmoid sinuses will drain well when the patient returns to the upright position. However, drainage of the maxillary sinus through its superomedially located ostium requires that the patient lie with the normal side of the head down and the top of the head made most dependent by allowing it to hang over the edge of the bed.This creates the appropriate inclined plane on the medial wall of the involved sinus for the fluid to run superiorly to its ostium (***try tilting the N. 43 top illustration into the above described position to confirm this). To get nose drops to the ostium of the sphenoid sinus it is necessary for the patient to lie supine with the occipital region of the head hanging over the edge of the bed to place the sphenoethmoidal recess in the most dependent position (***try tilting the N. 36 illustration into the above described position to confirm this). Applied nosedrops will then puddle in this recess to shrink the mucosa. Following treatment, drainage is accomplished by reversing position and having the patient lie prone with the frontal region of the head hanging over the edge of the bed. This creates the necessary incline of the anterior wall of the sphenoid sinus for fluids to drain superiorly to its ostium. (***try tilting the N. 36 illustration into the above described position to confirm this). NERVES AND VESSELS OF THE NASAL CAVITY (N. 39-41, 51-53) The olfactory nerve (Cranial Nerve I) provides nerve fibers for smell to the mucosa over the superior concha and a similar area of the opposing nasal septum (N. 39, 41). The multiple rootlets of the olfactory nerve reach the nasal cavity from their olfactory bulb origin from the brain by passing through the foramina of the cribriform plate of the ethmoid bone.


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While the nasal cavity receives its general sensory innervation and blood supply from many different sources, the maxillary division of the trigeminal nerve and the maxillary artery provide the major supply. The focal point for their branching and distribution is the pterygopalatine fossa. The pterygopalatine fossa (N. 6, 16, 39, 41, 52-54) can be located at the junction of the inferior orbital fissure with the pterygomaxillary fissure. Its posterior wall is formed by the sphenoid bone where the pterygoid process attaches to its body. The anterior wall is formed by the upper part of the posterior aspect of the maxilla. Its medial wall is formed by the upper part of the perpendicular portion of the palatine bone. The pterygopalatine fossa's major openings include two fissures, two foramina and two sets of canals. The pterygomaxillary fissure transmits the maxillary artery into the fossa where it will give off its posterior superior alveolar, palatine, sphenopalatine and infraorbital branches (N. 40, 51). Corresponding veins accompany the branches of the maxillary artery. The foramen rotundum conveys the maxillary division of the trigeminal nerve into the fossa where it will divide into its pterygopalatine, posterior superior alveolar, greater and lesser palatine, nasal, zygomatic and infraorbital branches (N. 39, 41, 52-54). On emerging from the fossa these nerves will be mixed, containing not only the SA input from maxillary V, but also continuations of the autonomics which entered the fossa through the nerve of the pterygoid canal. The nerve of the pterygoid canal (N. 39, 41, 52-54) enters the pterygopalatine fossa through the pterygoid canal which is located in the attachment of the pterygoid process to the sphenoid body. It is a mixed parasympathetic preganglionic and sympathetic postganglionic nerve. Its parasympathetic preganglionic fibers emerge from the brain with the facial nerve, and run with the facial nerve to its genu within the facial canal of the temporal bone (see facial nerve). Here they enter the greater petrosal nerve, which exits the anterior face of the petrous pyramid through the hiatus for the greater petrosal nerve. This nerve runs anteromedially across the middle cranial fossa along the anterior face of the petrous pyramid to pass into the interval between the trigeminal ganglion superiorly and the dehiscence in the roof of the carotid canal inferiorly. As it runs past the internal carotid artery it picks up some sympathetic postganglionic fibers from its periarterial plexus. Then it enters the pterygoid canal located in the anterior wall of the foramen lacerum. At this point the greater petrosal nerve becomes a mixed autonomic nerve known as the nerve of the pterygoid canal. It runs forward through the pterygoid canal to enter the posterior wall of the pterygopalatine fossa. Here its parasympathetic preganglionic fibers enter a small pterygopalatine ganglion to synapse on the contained postganglionic parasympathetic neurons (N. 53, 134). The pterygopalatine ganglion is suspended in the fossa by the pterygopalatine branches of maxillary V. All nerves leaving the pterygopalatine fossa will receive some sensory fibers from maxillary V, sympathetic postganglionic fibers to blood vessels from the nerve of the pterygoid canal, and parasympathetic postganglionic fibers to glands from the pterygopalatine ganglion. The inferior orbital fissure transmits the zygomatic and infraorbital branches of maxillary V (now mixed nerves) and the infraorbital vessels out of the pterygopalatine fossa into the orbit (N. 51-52). Here the zygomatic nerve and its branches will ascend the lateral orbital wall where its parasympathetic postganglionic fibers will be transferred to the lacrimal branch of ophthalmic V, just before this nerve enters the lacrimal gland (N. 52, 134). This is clinically the most important parasympathetic distribution of the pterygopalatine ganglion, since interruption of this pathway will remove the secretomotor stimulus to the lacrimal gland. This can diminish lacrimal secretion to the point where the cornea will dry and opacify to produce vision loss. A lesion causing this type of a dry eye can be caused by an interruption of either the preganglionic or postganglionic part of the parasympathetic pathway. Such a lesion could be located anywhere along this complex path from the pontine origin of the facial nerve to the orbit. As the infraorbital nerves and vessels course through the infraorbital canal they give off middle and anterior superior alveolar nerves and vessels which will descend the interior of the maxillary sinus to supply it on the way to the more anterior maxillary teeth (N. 51-52).


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There is a sphenopalatine foramen in the medial wall of the pterygopalatine fossa (N. 37). This transmits the mixed nasal nerves and the sphenopalatine artery into the nasal cavity (N. 39-41, 51). These supply much of the posterior part of both the septal and lateral nasal walls. The nerves provide sensory innervation to its mucosa, parasympathetic innervation to the mucosal glands and sympathetic innervation to the blood vessels. Greater and lesser palatine canals descend out of the pterygopalatine fossa onto the palate. They convey greater and lesser palatine nerves and vessels (N. 39-41, 51). The greater and lesser palatine nerves will provide sensory, glandular and vascular innervation to the mucosa of the hard and soft palate respectively. At its anterior end the greater palatine artery gives off a branch which ascends the incisive foramen to supply the anteroinferior part of the nasal septum (N. 51). Posterior superior alveolar nerves and vessels emerge from the pterygopalatine fossa through the pterygomaxillary fissure and enter foramina on the posterior aspect of the maxilla (N. 51-52). Here they descend the interior of the maxillary sinus to supply it and the maxillary molar teeth. In general, the pterygopalatine ganglion provides parasympathetic secretomotor innervation to the lacrimal gland and the glands of the nasal cavity, palate and some paranasal sinuses, with its most clinically significant distribution to the lacrimal gland. Branches of other nerves and vessels also supply the nasal cavity. The most important of these are the ethmoidal nerves and vessels which supply the anterosuperior part of both the septal and lateral nasal walls (N. 39-41, 51). The anterior inferior portion of the nasal septum is the most common site of nosebleed. This is called Kiesselbach's area and it receives arterial supply from four major sources (See Figure 8-9; N. 40, 51). The sphenopalatine artery's septal branch descends the nasal septum diagonally to approach Kiesselbach's area from its posterosuperior aspect. Nosebleed from this artery is the most serious to control, since it is so difficult to reach. Control of a life-threatening nosebleed from this vessel may require a transmaxillary sinus approach to the pterygopalatine fossa to ligate the end of the maxillary artery. The ethmoidal arteries send branches into this area from above. In addition, the greater palatine artery sends branches into this area from below through the incisive foramen. Finally, the facial artery through its branch to the upper lip supplies the area from its inferior aspect. So nosebleed may originate from branches of the maxillary artery (sphenopalatine and greater palatine) or from the facial artery of the external carotid artery system or from the ethmoidal branches of the ophthalmic artery from the internal carotid artery system. A knowledge of the location and potential source of a nosebleed is critical in the treatment of nosebleed by pressure points, chemical or electrical cautery or surgery.


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Figure 8-9

SOME RULES SUMMARIZING THE INNERVATION OF THE SKELETAL MUSCLES OF THE HEAD AND NECK

Four Rules of One Exception 1. All of the muscles of the larynx are innervated by the recurrent laryngeal branch of the vagus except

one, the cricothyroid, which is innervated by the external branch of the superior laryngeal branch of the vagus.

2. All of the muscles of the pharynx are innervated by the pharyngeal branches of the vagus except one,

the stylopharyngeus, which is innervated by the glossopharyngeal nerve. 3. All of the muscles of the palate are innervated by the palatal branches of the vagus except one, the

tensor veli palatini, which is innervated by the mandibular division of the trigeminal nerve. 4. All of the muscles of the tongue are innervated by the hypoglossal nerve except one, the palatoglossus

which is innervated by the vagus. Four Other Rules for Simplifying the Innervation of Muscles of the Head and Neck 1. The mandibular division of the trigeminal nerve innervates the four muscles of mastication and four

more, two of which are tensors (tensor veli palatini and tensor tympani) and two more, the mylohyoid and anterior belly of the digastric.

2. The facial nerve innervates all the muscles of facial expression and three more: stapedius, stylohyoid

and posterior belly of the digastric. 3. The trochlear nerve innervates the superior oblique, the abducens innervates the lateral rectus and the

oculomotor supplies all of the other skeletal muscles of the orbit. 4. The spinal root or part of the accessory nerve innervates sternocleidomastoid and trapezius.

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