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11-27. THE EYEBALL

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.

(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.

c. Internal Structures of the Eyeball.

(1) The nervous retina.

(a) The photoreceptors of the nervous portion of the retina (figure 11-11) contain chemicals known as visual pigments (rhodopsin). The cones are more concentrated in the center at the back of the eyeball. The cones can register colors and are used for acute vision. However, cones require more intense light than do rods. The rods are distributed more toward the sides of the nervous retina. Although the rods are capable of registering less intense light, rods perceive only black and white.

(b) If you look directly at an object, light from the object will fall in a small depression of the retina called the fovea centralis. The fovea centralis is at the posterior end of the eyeball, exactly opposite the centers of the cornea, pupil, and lens. The fovea centralis is found in a small yellow area of the retina called the macula lutea. The macula lutea is the area of the retina where vision is sharpest.

FOVEA = small depression

CENTRALIS = center

MACULA = spot

LUTEA = yellow

(c) Associated with the rods and cones are the beginnings of neurons of the optic nerve. These neurons pass out of the eyeball at the posterior end (in a point medial and superior to the fovea centralis). At the point of exit, there are no rods or cones. Therefore, it is called the blind spot (optic disc).

(2) Ciliary body. The anterior end of the choroid layer thickens to form a circular "picture frame" around the lens of the eyeball. This is also near the margin of the base of the cornea. The framelike structure is called the ciliary body. It includes mostly radial muscle fibers, which form the ciliary muscle.

(3) Ligaments. The lens is suspended in place by ligaments (fibers of the ciliary zonule). These ligaments connect the margin (equator) of the lens with the ciliary body.

(4) Lens. The lens is located in the center of the anterior of the eyeball, just behind the cornea.

(a) The lens is biconvex. This means that it has two outwardly curved surfaces. The anterior surface is flatter (less curved) than the posterior surface.

(b) The lens is transparent and elastic. (As one grows older, the lens becomes less and less elastic.) The ligaments maintain a tension upon the lens. This tension keeps the lens flatter and allows the lens to focus on distant objects. When the ciliary muscle contracts, the tension on the lens is decreased. The decreased tension allows the lens to thicken. The greater thickness increases the anterior curvature and allows close objects to be seen clearly.

(c) The process of focusing the lens for viewing close objects clearly is called accommodation. The process of accommodation is accompanied by a reduction in the pupil size as well as a convergence of the two central lines of sight (axes of eyeball).

(5) Iris. Another structure formed from the anterior portion of the choroid layer is the iris. The iris is located between the lens and the cornea.

(a) The pupil is the hole in the middle of the iris. The size of the pupil is controlled by radial and circular muscles in the iris. The radial muscles are dilators. The circular muscles are constrictors. By changing the size of the pupil, the iris controls the amount of light entering the eyeball.

(b) The iris may have many different colors. The actual color is determined by multiple genes.

(6) Chambers. The space between the cornea and the lens is called the anterior cavity. The space between the cornea and the iris is called the anterior chamber. The space between the iris and the lens is called the posterior chamber (see fig 11-10). Both chambers of the anterior cavity are filled with a fluid called the aqueous humor. The aqueous humor is secreted into the chambers by the ciliary body. It drains into the encircling canal of Schlemm, located in the angle between the cornea and the iris. This angle is called the iridiocornealis angle.

(7) Vitreous body. Behind the lens is a jellylike material called the vitreous body.

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