Only the foveae have a sufficient cone density to provide detailed color vision, so anything we wish to view critically is focused on the foveae. Because each fovea is only about the size of the head of a pin, only a thousandth of the entire visual field is in hard focus (foveal focus) at a given moment. Consequently, for us to visually comprehend a scene that is rapidly changing (as when we drive in traffic), our eyes must flick rapidly back and forth to provide the foveae with images of different parts of the visual field.
The neural layer of the retina receives its blood supply from two sources. Vessels in the choroid supply the outer third (containing photoreceptors). The inner two-thirds is served by the central artery and central vein of the retina, which enter and leave the eye through the center of the optic nerve (see Figure 15.4a). Radiating outward from the optic disc, these vessels give rise to a rich vascular network. This is the only place where small blood vessels are visible in a living person (Figure 15.7). Physicians may observe these tiny vessels with an ophthalmoscope for signs of hypertension, diabetes, and other vascular diseases.
Figure 15.7 Part of the posterior wall (fundus) of the right eye as seen with an ophthalmoscope. Clinical: Homeostatic Imbalance 15.5 The pattern of vascularization of the retina makes it susceptible to retinal detachment. This condition, in which the pigmented and neural layers separate (detach) and allow the jellylike vitreous humor to seep between them, can cause permanent blindness because it deprives the photoreceptors of nutrients.
Retinal detachment usually happens when the retina is torn during a traumatic blow to the head or when the head stops moving suddenly and then jerks in the opposite direction (as in bungee jumping). The symptom that victims most often describe is “a curtain being drawn across the eye,” but some people see sootlike spots or light flashes. If diagnosed early, it is often possible to reattach the retina with a laser before photoreceptors are permanently damaged. Internal Chambers and Fluids As we noted earlier, the lens and its halolike ciliary zonule divide the eye into two segments, the anterior segment in front of the lens and the larger posterior segment behind it (Figure 15.8 and Figure 15.4a). The posterior segment is filled with a clear gel called vitreous humor (vitre = glassy) that binds tremendous amounts of water. Vitreous humor:
Figure 15.8 Circulation of aqueous humor. The arrows indicate the circulation pathway.
Transmits light Supports the posterior surface of the lens and holds the neural layer of the retina firmly against the pigmented layer Contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles Vitreous humor forms in the embryo and lasts for a lifetime.
The iris divides the anterior segment into the anterior chamber (between the cornea and the iris) and the posterior chamber (between the iris and the lens) (Figure 15.8). The entire anterior segment is filled with aqueous humor, a clear fluid similar in composition to blood plasma. Unlike the vitreous humor, aqueous humor forms and drains continually. In Figure 15.8, you can follow its movement from the posterior chamber to the scleral venous sinus, an unusual venous channel that encircles the eye in the angle at the corneoscleral junction.
Normally, aqueous humor forms and drains at the same rate, maintaining a constant intraocular pressure of about 16 mm Hg, which helps to support the eyeball internally. Aqueous humor supplies nutrients and oxygen to the lens and cornea and to some cells of the retina, and it carries away metabolic wastes.
Clinical: Homeostatic Imbalance 15.6 If the drainage of aqueous humor is blocked, fluid backs up as in a clogged sink. Pressure within the eye may increase to dangerous levels and compress the retina and optic nerve—a condition called glaucoma (glaw-ko′mah). The eventual result is blindness (glaucoma = vision growing gray) unless the condition is detected and treated early. Unfortunately, many forms of glaucoma steal sight so slowly and painlessly that people do not realize they have a problem until the damage is done. Late signs include seeing halos around lights and blurred vision.
The glaucoma examination is simple. The intraocular pressure is determined by directing a puff of air at the cornea and measuring the amount of corneal deformation it causes. This exam should be done yearly after age 40. The most common treatment is eye drops that increase the rate of aqueous humor drainage or decrease its production. Laser therapy or surgery can also help. Lens The lens is a biconvex, transparent, flexible structure that can change shape to precisely focus light on the retina. It is enclosed in a thin, elastic capsule and held in place just posterior to the iris by the ciliary zonule (Figure 15.8). Like the cornea, the lens is avascular; blood vessels interfere with transparency.
The lens has two regions: the lens epithelium and the lens fibers. The lens epithelium, confined to the anterior lens surface, consists of cuboidal cells that eventually differentiate into the lens fibers that form the bulk of the lens. The lens fibers, which are packed tightly together like the layers in an onion, contain no nuclei and few organelles. They do, however, contain transparent, precisely folded proteins called crystallins that form the body of the lens. Since new lens fibers are continually added, the lens enlarges throughout life, becoming denser, more convex, and less elastic, all of which gradually impair its ability to focus light properly.
Clinical: Homeostatic Imbalance 15.7 A cataract (“waterfall”) is a clouding of the lens that causes the world to appear distorted, as if seen through frosted glass (Figure 15.9). Some cataracts are congenital, but most result from age-related hardening and thickening of the lens or are a secondary consequence of diabetes mellitus. Heavy smoking and frequent exposure to intense sunlight increase the risk for cataracts.
Figure 15.9 Photograph of a cataract. The lens, not the cornea, is milky and opaque.
Oxidative stress and metabolic changes in the deeper lens fibers promote clumping of the crystallin proteins. Unexpectedly, supplementation with the antioxidant vitamin C may actually increase cataract formation. Fortunately, the offending lens can be surgically removed and an artificial lens implanted to save the patient’s sight. Check Your Understanding What are tears and what structure secretes them?
What is the blind spot and why is it blind?
Sam’s optometrist tells him that his intraocular pressure is high. What is this condition called and which fluid does it involve?