The Eyeball Figure 1110

a. Shape. In the main, the eyeball is a spherical bulb-like structure. Its anterior surface, transparent and more curved, is known as the cornea of the eyeball.

b. Wall of the Eyeball. The eyeball is a hollow structure. Its wall is made up of three layers known as coats or tunics.

(1) Sclera. The outermost layer is white and very dense FCT (fibrous connective tissue). It is known as the sclera, scleral coat, or fibrous tunic. Its anterior portion is called the cornea. As already mentioned, the cornea is transparent and more curved than the rest of the sclera. The fixed curvature of the cornea enables it to serve as the major focusing device for the eyeball.

(2) Choroid. The middle layer of the wall of the eyeball is known as the choroid, the choroid coat, or the vascular tunic. This layer is richly supplied with blood vessels. It is also pigmented with a black material. The black color absorbs light rays and prevents them from reflecting at random.

Figure 11-10. A horizontal section of the eyeball.

(3) Retina. The inner layer of the wall of the eyeball is known as the retina, retinal coat, or internal tunic. The actual photoreceptor elements are located in the retina at the back and sides of the eyeball. These elements are the rods and cones. They constitute the nervous portion of the retina. In the anterior part of the eyeball, the retina continues as a nonnervous portion.

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