The eye, an elongated sphere about 1 inch in diameter, has three layers, or coats: the outer sclera, the middle choroid, and the inner retina. The outer sclera is a white, fibrous layer except for the transparent cornea, the window of the eye. The middle, thin, dark brown layer, the choroid, contains many blood vessels and absorbs stray light rays. Toward the front, the choroid thickens and forms the ring-shaped ciliary body containing the ciliary muscle, which controls the shape of the lens for near and far vision. Finally, the choroid becomes a thin, circular, muscular, and pigmented diaphragm, the iris, which regulates the size of the pupil, the hole through which light enters the eyeball. The lens, attached to the ciliary body by ligaments, divides the cavity of the eye into two smaller cavities. The posterior cavity behind the lens is filled with vitreous humor (vit're-us hu'mor), a viscous, gelatinous material. The anterior cavity between the cornea and the lens is filled with aqueous humor (a'kwe-us hu'mor), a watery solution secreted by the ciliary body. (The anterior cavity contains two chambers: an anterior chamber between the cornea and iris, and the posterior chamber between the iris and the lens.) A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior cavity by way of tiny ducts located where the iris meets the cornea.
When a person has glaucoma/ these drainage ducts are blocked/ and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where the sight receptors are located. The nerve fibers begin to die due to lack of nutrients/ and the person becomes partially blind. Over time/ total blindness can result.
Retina
The inner layer of the eye, the retina, has three layers of cells. The layer closest to the choroid contains the sense receptors for sight, the rods and cones; the middle layer contains bipolar cells; and the innermost layer contains ganglionic cells whose fibers become the optic nerve. Only the rods and cones contain light-sensitive pigments, and therefore, light must penetrate to the back of the retina before nerve impulses are generated.
Nerve impulses initiated by the rods and cones are sent to the bipolar cells, which, in turn, send them to the ganglionic cells. The axons of the ganglionic cells pass in front of the retina, forming the optic nerve, which turns to pierce the layers of the eye. Rods and cones greatly outnumber ganglionic cells. In fact, the retina has as many as 150 million rods but only 1 million ganglionic cells and optic nerve fibers. This means that there is considerable mixing of messages and a certain amount of integration before nerve impulses are sent to the thalamus and then on to the occipital lobe of the cerebrum. There are no rods or cones where the optic nerve passes through the retina; therefore, the optic disk is known as a blind spot, where vision is impossible.
The retina contains a very special region called the macula lutea, an ovaL yellowish area with a depression called the fovea centralis. In this region, vision is most acute because there is a great concentration of cone cells.
The retina contains the rods and cones, vvhich are the sensory receptors for sight. When either is stimulated, nerve impulses begin and are transmitted via the optic nerve to the brain.
Rods
In dim light, the iris causes the pupil to enlarge so that more light rays can enter the eye. As the faint light rays enter, they strike the rods and cones, but only the 150 million rods located in the periphery of the eyes are sensitive to faint light. The rods do not detect fine detail or color, so at night, for example, all objects appear to be blurred and have a shade of gray. However, because of their abundance and position in the eyes, rods do detect even the slightest motion.
The rods contain rhodopsin, a molecule that contains the protein opsin and the pigment retinal. When light strikes rhodopsin, rhodopsin breaks down to its components, and this generates nerve impulses. The more rhodopsin present in the rods, the more sensitive the eyes are to dim light. Therefore, during the time required for adjustment to dim light, when it is difficult to see, rhodopsin is being formed in the rods. Retinal is a derivative of vitamin A, which is abundant in carrots, so the suggestion that eating carrots helps vision is not without foundation.
The rods are responsible for vision in dim light. They do not see fine detail or color. but they do detect motion.
Cones
The cones, located primarily in the fovea centralis, function in bright light to detect fine detail and color. To perceive depth, as well as to see color, we turn our eyes so that reflected light from the object strikes the fovea centralis. Color vision depends on three kinds of cones, one kind for each of three colors: blue, green, and red. The colors we see depend on which of these cones are activated.
Complete color blindness is extremely rare. In most instances, a particular type of cone is lacking or deficient in number. The lack of either red or green cones is the most common type of color blindness, affecting about 5% of the American population. If the eye lacks red cones, the green colors become accentuated, and vice versa.
The cones are responsible for vision in bright light. They detect fine detail and color.
Stereoscopic Vision
Each eye sends its own information to the brain about the placement of an object because each forms an image from a slightly different angle. These data are pooled to produce stereoscopic vision by a two-step process. First, because the optic nerves cross at the optic chiasma, onehalf of the brain receives information from both eyes about the same part of an object. Later, the two halves of the brain communicate to arrive at a complete, three-dimensional interpretation of the whole object.
Lens
When we look at an object, light rays are bent (refracted) and focused on the retina. The cornea, vitreous humor, and lens all help in this process. Although the lens remains flat when we view distant objects, it rounds up when we view close objects, a process called accommodation. Lens shape is controlled by the ciliary muscle within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut; therefore, the lens remains relatively flat. When we view a near object, the ciliary muscle contracts, releasing the tension on the ligaments; the lens then rounds up due to its natural elasticity. Close work, which requires contraction of the ciliary muscle, often causes eyestrain.
When the eyeball is too long or too short, accommodation by the lens may not be sufficient to bring an object into focus. Also, after age 40, the lens loses some of its elasticity and is unable to accommodate as well. Then a person needs glasses .
With age, the lens also is subject to cataracts. A catara occurs when the lens becomes opaque and incapable of transmitting light rays. Recent research suggests that cataracts develop when crystalline proteins within the lens oxidize. Knowing this may help researchers find a way to treat cataracts medically. At present, surgery is the only viable cataract treatment. First, a surgeon opens the eye near the rim of the cornea. Then the enzyme zonulysin may be used to digest the ligaments holding the lens in place. A plastic lens is then implanted in the eye, and the patient does not need to wear thick glasses or contact lenses.
The lens focuses light rays on the retina. The lens is flat for distant vision and rounds up for near vision.
