Figure 10-8 The retina of the left eye.
thickness and is thinnest in the region of the macula. Histologically, the retina is made up of
10 distinct layers. Basically, the retina senses light through the rods and cones in its outer layer (closest to the RPE), performs initial signal processing in its middle layer, and encodes and transmits the data in its inner layer, the nerve fiber layer. The nerve fiber layer is directly under the inner limiting membrane of the retina, the layer closest to the vitreous. These nerve fibers course along the inner portion of the retina and aggregate to form the optic nerve. On leaving the eye, the nerve fibers become myelinated.
Within the retina are several important structures: the optic disc, the retinal vessels, and the macula. Figure 10-8 illustrates the retina of the left eye.
The optic disc is located at the nasal aspect of the posterior pole of the retina. This is the head of the optic nerve, from where the nerve fibers of the retina exit the eye. The optic disc is 1.5 mm in diameter and is ovoid. It is lighter than the surrounding retina and appears yellowish-pink. The disc margins are sharp with some normal blurring of the nasal portion. African-American patients may have pigmentation at the margins. The physiologic cup is the center of the disc, where the retinal vessels penetrate. This small depression normally occupies about 30% of the disc diameter.
The retinal vessels emerge from the disc and arborize on the retinal surface. The arteries are brighter red and thinner than the veins. An artery-to-vein ratio of 2:3 is normal.
The macula is a small, round area, approximately the size of the disc, located 3.5 mm temporal to and 0.5 mm inferior to the disc. The macula is easily seen because it is devoid of retinal vessels. In the center of the macula is the fovea, a depressed area composed only of cones. Cones provide detailed vision and color perception.
The remaining areas of the retina contain mostly rods, which compose the other neurosen-sory element of the retina. The rods are responsible for motion detection and night vision. It should be remembered that the image on the retina is upside down and reversed left to right: The right world is projected on the left half of the retina, and the left world is projected on the right half of the retina. An image in the superior world strikes the inferior part of the retina, and an inferiorly positioned image strikes the superior part. This concept is illustrated in Figure 10-9.
At birth, there is little pigment in the iris, which is why all infants are born with blue eyes. By 6 months of age, the pigmentation is completed. The lens is more spherical at birth than in later life. Most infants are born hyperopic (farsighted). By 3 months after birth, the medullation process of the optic nerve is completed. As the child grows, hyperopia increases until the age of 8 years and then gradually decreases. After age 8, myopia (nearsightedness) appears to increase.
With advancing age, there is the gradual loss of elasticity of the skin around the eyes. The cornea may show an infiltration of degenerative material around the limbus, which is known as an arcus senilis. The lens consistency changes from plastic to rigid, making it progressively more difficult to change its shape to focus on near objects. This condition is presbyopia. The lens may undergo changes resulting from metabolic disorders that cause its opacification;this condition is called a cataract. The vitreous humor may develop condensations, called floaters. The retinal arteries may develop atherosclerosis, with resultant retinal ischemia or infarction.
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