Divided into 3 chambers: Anterior and posterior (containing Aqueous Humor); vitreous chamber (vitreous body that is made up of vitreous humor) The eyeball proper has three layers; however, there is an additional loose connective tissue layer that surrounds the eyeball, allowing its movement within the orbit. The loose connective tissue layer is composed posteriorly of bulbar fascia, which forms the true socket for the eyeball, and anteriorly of bulbar conjunctiva. The three layers of the eyeball are the: Fibrous layer (outer coat), consisting of the sclera and cornea. Vascular layer (middle coat), consisting of the choroid, ciliary body, and iris. Inner layer (inner coat), consisting of the retina that has both optic and non-visual parts.
Fibrous Layer of the Eyeball1. Sclera: tough opaque part of the fibrous layer (coat) that covers the posterior five sixths of the eyeball Functions as a fibrous skeleton, providing shape and resistance and attachment for both the extrinsic and the intrinsic muscles of the eye
2. Cornea The transparent part of the fibrous coat covering the anterior one sixth of the eyeball. It differs with sclera primarily in terms of the regularity of the arrangement of the collagen fibers and the degree of hydration of each.
Vascular Layer of the EyeballThe vascular layer of the eyeball (also called the uvea or uveal tract) consists of the choroid, ciliary body, and iris. 1. The choroid A dark reddish brown layer between the sclera and the retina Forms the largest part of the vascular layer of the eyeball and lines most of the sclera. Within, larger vessels of the vascular lamina are located externally (near the sclera). The finest vessels (the capillary lamina of the choroid or choriocapillaris, an extensive capillary bed) are innermost, adjacent to the avascular light-sensitive layer of the retina, which it supplies with oxygen and nutrients. Engorged with blood in life, this layer is responsible for the red eye reflection that occurs in flash photography. The choroid is continuous anteriorly with the ciliary body. The choroid attaches firmly to the pigment layer of the retina, but it can easily be stripped from the sclera.
2. The Ciliary Body Is muscular as well as vascular, connects the choroid with the circumference of the iris. It provides attachment for the lens; contraction and relaxation of the smooth muscle of the ciliary body controls thickness (and therefore the focus) of the lens. Folds on the internal surface of the ciliary body, the ciliary processes, secrete aqueous humor, which fills the anterior and posterior chambers of the eye The anterior chamber of the eye is the space between the cornea anteriorly and the iris/pupil posteriorly. The posterior chamber of the eye is between the iris/pupil anteriorly and the lens and ciliary body posteriorly.
3. The Iris The iris, which lies on the anterior surface of the lens, is a thin contractile diaphragm with a central aperture, the pupil, for transmitting light. When a person is awake, the size of the pupil varies continually to regulate the amount of light entering the eye. Two involuntary muscles control the size of the pupil: the parasympathetically stimulated sphincter pupillae closes the pupil, and the sympathetically stimulated dilator pupillae opens it.
Inner Layer of the Eyeball1. Retina Consists of two functional parts with distinct locations: an optic part and a non-visual retina. Optic part sensitive to visual light rays and has two layers: a neural layer and pigment cell layer. The neural layer is light receptive. The pigment cell layer consists of a single layer of cells that reinforces the light-absorbing property of the choroid in reducing the scattering of light in the eyeball. Non-visual retina an anterior continuation of the pigment cell layer and a layer of supporting cells over the ciliary body (ciliary part of the retina) and the posterior surface of the iris (iridial part of the retina), respectively. The functional optic part of the retina terminates anteriorly along the ora serrata (L. serrated edge), an irregular border slightly posterior to the ciliary body. The ora serrata marks the anterior termination of the light-receptive part of the retina. The retina is supplied by the central artery of the retina, except for the cones and rods of the outer neural layer which receive nutrients from the capillary lamina of the choroid, or choriocapillaris. It has the finest vessels of the inner surface of the choroid, against which the retina is pressed. A corresponding system of retinal veins unites to form the central vein of the retina.
2. Fundus The fundus is the posterior part of the eyeball. It has a circular depressed area called the optic disc (optic papilla) and just lateral to that is the macula lutea (yellow spot) Optic dis is where the sensory fibers and vessels conveyed by the optic nerve enter the eyeball. Because it contains no photoreceptors, the optic disc is insensitive to light. Consequently, this part of the retina is commonly called the blind spot. The macula lutea is a small oval area of the retina with special photoreceptor cones that is specialized for acuity of vision. It is not normally observed with an ophthalmoscope. At the center of the macula lutea is a depression, the fovea centralis (L. central pit), the area of most acute vision. The fovea is approximately 1.5 mm in diameter; its center, the foveola, does not have the capillary network visible elsewhere deep to the retina.
>Refractive Media of the Eyeball On their way to the retina, lightwaves pass through the refractive media of the eyeball: cornea, aqueous humor, lens, and vitreous humor.1. Cornea Largely responsible for refraction of the light that enters the eye. It is transparent, owing to the extremely regular arrangement of its collagen fibers and its dehydrated state. The cornea is sensitive to touch; its innervation is provided by the ophthalmic nerve (CN V1). It is avascular. Its nourishment is derived from the capillary beds at its periphery, the aqueous humor, and lacrimal fluid. The latter also provides oxygen absorbed from the air.
2. Aqueous Humor In the anterior and posterior chambers of the eye is produced in the posterior chamber by the ciliary processes of the ciliary body. Provides nutrients for the avascular cornea and lens. After passing through the pupil into the anterior chamber, the aqueous humor drains into the scleral venous sinus (L. sinus venosus sclerae, canal of Schlemm) at the iridocorneal angle. The humor is removed by the limbal plexus, a network of scleral veins close to the limbus, which drain in turn into both tributaries of the vorticose and the anterior ciliary veins (Fig. 7.34B).
3. Lens The lens is posterior to the iris and anterior to the vitreous humor of the vitreous body. It is a transparent, biconvex structure enclosed in a capsule. The highly elastic capsule of the lens is anchored by the zonular fibers (suspensory ligament of the lens) to the ciliary body and encircled by the ciliary processes; an isolated unattached lens assumes a nearly spherical shape. Most refraction is produced by the cornea, the convexity of the lens, particularly its anterior surface, constantly varies to fine-tune the focus of near or distant objects on the retina. The ciliary muscle in the ciliary body changes the shape of the lens To accommodate far vision, the attachments around its periphery pull the lens relatively flat (sympathetic) To accommodate near vision, the smooth muscle of the circular ciliary body contract. The circle, like a sphincter, becomes smaller in size and the tension on the lens is reduced, allowing the lens to round up (parasympathetic)
4. Vitreous Humor A transparent jelly-like substance in the posterior four fifths of the eyeball posterior to the lens. It is formed during embryonic life and consists of mostly water plus collagen fibers and hyaluronic acid. The vitreous body also contains phagocytic cells that remove debris. In addition to transmitting light, the vitreous humor holds the retina in place and supports the lens. The hyaloid canal is a narrow channel that is inconspicuous in adults and runs through the vitreous body from the optic disc to the posterior aspect of the lens. In the fetus, it is occupied by the hyaloid artery.
>Vasculature of the EyeballArtery Mainly from the ophthalmic artery, branching from the internal carotid artery The infraorbital artery which branches from external carotid artery which supplies the orbital floor The retina is supplied internally by the central artery which branches from the ophthalmic artery, running within the dural sheath, and externally by the capillary lamina of the choroid (choriocapillaris) which is made up from ciliary arteries which are branches of the ophthalmic artery
Vein Drainage to the superior and inferior ophthalmic veins, which pass through the superior orbital fissure and enter the cavernous sinus. The central vein of the retina usually enters the cavernous sinus directly, but it may join one of the ophthalmic veins. The vortex or vorticose veins from the vascular layer of the eyeball drain into the inferior ophthalmic vein. The scleral venous sinus is a vascular structure encircling the anterior chamber of the eyeball through which the aqueous humor is returned to the blood circulation.
>Innervation CN II: the optic nerve, providing visual sensory signals to the brain CN V: sensory innervation of the cornea and conjunctiva