120 Exercise 14

._ Cerebellum

Figure 14.7 Photo of lateral aspect of the human brain.

nolssectlon:

mThe Sheep Brain Obtain a sheep brain, protective skin cream or disposable gloves, dissecting tray, and instruments, and bring them to your laboratory bench.

I . Tum your sheep brain so that you are viewing its left lat­eral aspect. Compare the various area:; of the sheep brain (cerebrum, brain stem, cerebellum) to the photo of the human brain in Figure 14.7. Relatively speaking, which of these structures is obviously much larger in humans?

2. Place the ventral surface of the sheep brain down on the dissecting tray and observe the fragments of the dura mater. Feel its consistency and notice its toughness. Cut through the dura mater along the line of the longitudinal fissure. Gently force the cerebral hemispheres apart laterally to expose the corpus callosum, the huge fiber tract deep to the longitudinal fissure.

3. Examine the superior surface of the brain. Notice that. like the human brain. its surface is thrown into convolutions (fissures and gyri). Identify the arachnoid mater, which ap­pears on the brain surface as a del icate "cottony" material spanning the fissures.

Ventral Structures Figure 14.8a and b shows the important features of the ven­tral surface of the brain. Tum the brain over so that its ventral surface is up.

I. Look for the clublike olfactory bulbs on the inferior sur­face of the frontal lobes of the cerebral hemispheres.

How does the size of these olfactory bulbs compare with those of humans?

ls the sense of smell more important as a protective and a food-getting sense in sheep or in humans?

2. The optic nerve (II) carries sensory impulses concerned with vision from the retina of the eye. Identify the optic nerves, the optic chiasma (the point where some of the fibers of each optic nerve cross over to the opposite side), and the optic trncts, which continue from the optic chiasma.

3. Posterior to the optic chiasma, identify the stalk of the pituitary gland and then the mammillary body. Notice that the sheep's mammillary body is a single rounded eminence. In humans it is a double structure.

4. Identify the cerebral peduncles on the ventral aspect of the midbrain, just posterior to the mammillary body. Also identify the large oculomotor nerves (Ill), which arise from the ventral midbrain surface, and the tiny trochlear nerves (IV). seen at the midbrain-pons junction. These cranial nerves provide motor fibers to extrinsic muscles of the eyeball.

5. Moving posteriorly from the rnidbrain, identify first the pons and then the medulla oblongata.

6. Return to the junction of the pons and midbrain and pro­ceed posteriorly to identify the following cranial nerves, all arising from the pons:

• Trigeminal nerves (V), which are involved in chewing and sensations of the head and face

e Abducens nerves (VI), which abduct the eye (and thus work in conjunction with cranial nerves III and IV)

• Facial nerves (V[l), large nerves involved in taste sensa­tion, gland function (salivary and l:.tcrimal glands), and facial expressions

i Continue posteriorly to identify the following:

• Vestibulocochlear nerves (Ylll), purely sensory nerves that are involved with hearing and equilibrium

• Glossopharyngeal nerves (IX), which contain motor fibers innervating throat structures and sensory fibers trans­mitting taste stimuli

* Vagus nerves (X), often called ·•wanderers," which serve many organs of the head, thorax. and abdominal cavity

• Accessory nerves (XI), which serve muscles of the neck. larynx, and shoulder; notice that the accessory nerves arise from both the medulla and the spinal cord

• Hypoglossal nerves (XII), which stimulate tongue and neck muscles

Donal Structures I. Refer to Figure 14.8b as a guide in identifying the fol­lowing structures. Reidentify the cerebral hemispheres. How does the depth of the fissures in the sheep's cerebral hemi­spheres compare to that in the human brain?

120 Exercise 14

_-~ Cerebrum

Brain siem 4

“= Cerebellum

Figure 14.7. Photo of iateral aspect of the human brain.

Dissection:

hy The Sheep Brain

Obtain a sheep brain, protective skin cream or disposable gloves, dissecting tray, and instruments, and bring them to your laboratory bench.

1. Turn your sheep brain so that you are viewing its left lat- eral aspect. Compare the various areas of the sheep brain (cerebrum, brain stem, cerebellum) to the photo of the human brain in Figure 14.7. Relatively speaking, which of these structures is obviously much larger in humans?

2. Place the ventral surface of the sheep brain down on the dissecting tray and observe the fragments of the dura mater. Feel its consistency and notice its toughness. Cut through the dura mater along the line of the longitudinal fissure. Gently force the cerebral hemispheres apart laterally to expose the corpus callosum, the huge fiber tract deep to the longitudinal fissure.

3. Examine the superior surface of the brain. Notice that, like the human brain, its surface is thrown into convolutions (fissures and gyri). Identify the arachnoid mater, which ap- pears on the brain surface as a delicate “cottony” materia! spanning the fissures.

Ventral Structures

Figure 14.8a and b shows the important features of the ven- tral surface of the brain. Turn the brain over so that its ventral surface is up.

1. Look for the chiblike olfactory bulbs on the inferior sur- face of the frontal lobes of the cerebral hemispheres.

How does the size of these olfactory bulbs compare with those of humans?

Is the sense of smell more important as a protective and a food-getting sense in sheep or in humans?

2. The optic nerve (ID carries sensory impulses concerned with vision from the retina of the eye. Identify the optic

nerves, the optic chiasma (the point where some of the fibers of each optic nerve cross over to the opposite side), and the optic tracts, which continue from the optic chiasma.

3. Posterior to the optic chiasma, identify the stalk of the pituitary gland and then the mammillary body. Notice that the sheep’s mammillary body is a single rounded eminence. In humans it is a double structure.

4, Identify the cerebral peduncles on the ventral aspect of the midbrain, just posterior to the mammillary body. Also identify the large oculomotor nerves (IJ), which arise from the ventral midbrain surface, and the tiny trochlear nerves

(IV), seen at the midbrain-pons junction. These cranial nerves provide motor fibers to extrinsic muscles of the eyeball.

5. Moving posteriorly from the midbrain, identify first the pons and then the medulia oblongata.

6. Return to the junction of the pons and midbrain and pro- ceed posteriorly to identify the following cranial nerves, all arising from the pons:

e Trigeminal nerves (V), which are involved in chewing and sensations of the head and face

e Abducens nerves (V1), which abduct the eye (and thas

work in conjunction with cranial nerves HT and IV)

e Facial nerves (Vfl), large nerves involved in taste sensa-

tion, gland function (salivary and lacrimal glands), and facial expressions

7, Continue posteriorly to identify the following:

e Vestibulocochlear nerves (VI), purely sensory nerves that are involved with hearing and equilibrium

e Glossopharyngeal nerves (IX), which contain motor fibers innervating throat structures and sensory fibers trans-

mitting taste stimuli

» Vagus nerves (X)}, often called “wanderers,” which serve many organs of the head, thorax, and abdominal cavity

« Accessory nerves (XI), which serve muscles of the neck, larynx, and shoulder; notice that the accessory nerves arise from both the medulla and the spinal cord

e Hypoglossal nerves (XII), which stimulate tongue and neck muscles

Dorsal Structures

|. Refer to Figure 14.8b as a guide in identifying the fol- lowing structures. Reidentify the cerebral hemispheres. How does the depth of the fissures in the sheep’s cerebral hemi-

spheres compare to that in the human brain?

Gro>S Anatomy of the Bram and Cranial erves 121

Stal-Y-_ c t

C~ erebrc1l

(a)

~ ~

'.t

~ ~ . ~

· ~ -

~

~ Fig ure 14 .8 Intact 5.heep 'brain. (a) Dic1yra rn mat1C vf:' t: tial view. (b ) Ph,Jtog;anh:: showing venrr.i l c1 nd dorsa l views

4444455 ddd

ISI

IAA

AA AAAAIAAIIIAI

SESS Gross Anatomy of the Brain and Craniai Nerves 121

4 |

\, ——+,-_— Cerebrum

Stalk of otuilary ciena +, a é \\ N — Optic nerve

i 7 a, a

\ Sy" a Optic chiasma ae cg | é ope ‘ Or fio tract

ae al pi ae a

ry aad ‘ i WO VIGEGE 4

i

Mam

ary boay

j

Cerebral peduncie

Te ih Wik cs

accessory nérve (A!

{a)

Ventral Ral gst |

Mamimiltary

body a-— Cerenrurn

Cerebral peduncie

Pans ———

= a 4 Cerebellum

ead lis Meculle (by

Figure 14.8 Intact sheep brain. (a) Diagrammatic ventral view, (b) Photographs showing ventral and dorsal VIEWS,

~--=--=~- --=~~---

122 Exercise 14

--•--Occipital lobe of cerebral hemisphere

Pineal body

Superior colliculi of corpora quadrigemina

lnfetior collicuii of corpora quadrigemina

Cerebeilum

Figure 14.9 Means of exposing the dorsal midbrain structures of the sheep brain.

2. Carefully examine the cerebellum. Notice that, in con­trast to the human cerebellum, it is not divided longitudinally, and that its fissures are oriented differently.

3. To expose the dorsal surface of the midbrain. gently force the cerebrum and cerebellum apart, as shown in Figure 14.9. Identify the corpora quadrigemina, four rounded promi­nences on the dorsal midbrain surface. What is the function of the corpora quadrigemina?

Also locate the pineal body, which appears as a small oval protrusion in the midline just anterior to the corpora quadrigemina.

Internal Structures 1. The internal structure of the brain can only be examined after further dissection. Position the brain ventral side down and make a cut completely through it in a superior to inferior di­rection. Cut through the longitudinal fissure, corpus callosum: and mid line of the cerebellum. Refer to Figure l 4.10 as you work.

2. A thin nervous tissue membrane immediately ventral to the corpus callosum separates the lateral ventricles from each other. Pierce this membrane and probe the cavity of the lat­eral ventricle.

3. Identify the thalamus, which forms the walls of the third ventricle. The intermediate mass spanning t~e ventricular cavity appears as a round protrusion of the thalamus wall.

4. The hypothalamus forms the floor of the third ventricle. Identify the optic chiasma, stalk of the pituitary, and mam­millary body on its exterior surface. You can see the pineal bo<ly at the posterior end of the third ventricle.

5. Locate the midbrain by identifying the corpora quadrigemina that form its dorsal roof. Follow the cerebral aqueduct through the midbrain tissue to the fourth ventricle. Identify the cerebral peduncles, which form its anterior walls.

6. Identify the pons and medulla. anterior to the fourth ven­tricle. The medulla continues into the spinal cord without any obvious anatomical change, but the point at which the fourth ventricle narrows to a small canal is generally accepted as the beginning of the spinal cord.

7. Identify the cerebellum posterior to the fourth ventricle and notice the internal treelike arrangement of its white matter called the arbor vitae.

. 8. Check with your instructor to determine if cow spinal cord sections (preserved) are available for the spinal cord studies in Exercise 15. If not, save the small portion of the spinal cord from your brain specimen. Otherwise, dispose of all organic debris in the appropriate laboratory containers and clean the dissecting instruments and tray before leaving the laboratory. •

ess nee senien tony

122 Exercise 14

Occipital lobe of

cerebral hemisphere

Pineal body

Superior collicuti of corpora quadrigemina

inferior collicuii

of corpora quadrigemina

Cerebellum

Figure 14.9 Means of exposing the dorsal midbrain structures of the sheep brain.

2. Carefully examine the cerebellum. Notice that, in con- trast to the human cerebellum, it is not divided longitudinally, and that its fissures are oriented differently,

3. To expose the dorsal surface of the midbrain. gently force the cerebrum and cerebellum apart, as shown in Figure 14.9. Identify the corpora quadrigemina, four rounded promi- nences on the dorsal midbrain surface. What is the function of the corpora quadrigemina?

Also locate the pineal body, which appears as a small oval protrusion in the midline just anterior to the corpora quadrigemina.

Internal Structures

1. The internal structure of the brain can only be examined after further dissection. Position the brain ventral side down and make a cut completely through it in a superior to inferior di- rection. Cut through the longitudinal fissure, corpus callosum; and midline of the cerebellum. Refer to Figure 14.10 as you work.

2. A thin nervous tissue membrane immediately ventral to the corpus callosum separates the lateral ventricles from each

other. Pierce this membrane and probe the cavity of the lat- eral ventricle,

3. Identify the thalamus, which forms the walls of the third ventricle. The intermediate mass spanning the ventricular cavity appears as a round protrusion of the thalamus wall.

4. The hypothalamus forms the floor of the third ventricle. Identify the optic chiasma, stalk of the pituitary, and mam- millary body on its exterior surface. You can see the pineal body at the posterior end of the third ventricle.

5. Locate the midbrain by identifying the corpora guadrigemina that form its dorsal! roof. Follow the cerebral aqueduct through the midbrain tissue to the fourth ventricle.

Identify the cerebral peduncles, which form its anterior walls.

6. Identify the pons and medulla, anterior to the fourth ven- tricle. The medulla continues into the spinal cord without any obvious anatomical change, but the point at which the fourth ventricle narrows to a sinall canal is generally accepted as the beginning of the spinal cord.

7. Identify the cerebellum posterior to the fourth ventricle and notice the internal treelike arrangement of its white matter called the arbor vitae.

.8. Check with your instructor to determine if cow spinal cord sections (preserved) are available for the spinal cord studies in Exercise 15. If not, save the small portion of the spinal cord from your brain specimen. Otherwise, dispose of all organic debris in the appropriate laboratory containers and clean the dissecting instruments and tray before leaving the laboratory.

Gross Anatomy of the Brain and Cranial Nerves 123

Cerebr a.I hernisphereCorpus callosum-·---,. I Pineal body

i

(a)

----+--- r- Corpora quadrigemina I (midbrain)

Fourth ventncle

Spinal cord

Cerebral ---­hemisphere

(b)

Cerebeliurn

----.-,--..,....,..--:==:~--::---,.~~,:::!-- Corpora quadrigemina

- Medulla

Figure 14.10 Sagittal section of the sheep brain showing internal structures. (a) Diagrammatic view. (b) Photograph.

Gross Anatomy of the Brain and Cranial Nerves 123

Cerebral hemisphere Corpus callosum—-—

Pineal body

Corpora quadrigemina

nt (midbrain) yon K

= —_- Cerebellum Kt

ne

intermediate mass

of the thalamus a

i a ° y f GE

We Olfactory bulb — ;

Third ventricle —

Fourth veniricie

Spinal cord

—— Medulla oblongata

raat . Cerebral aqueduct Optic chiasma

Pons Hypothalamus —~ Pituitary gland— Cerebral peduncle

(a) Mammillary body

Cerebral hemisphere

Cerebellum Corpus callosurn

Pineal body

Corpora

Cerebral peduncle quadrigemina

Optic chiasma Medulla

Pons (b)

Figure 14.10 Sagittal section of the sheep brain showing internal structures. (a) Diagrammatic view. (b) Photograph.