ENSCONCED as it is within its rugged bony fortress, the brain must have a most efficient intelligence system. Of the five freeways that give access to this capital of our bodies, and through which it receives information about its environment, vision provides the greatest quantity of data and is the most indispensable. The mechanisms of the eye and the problems that arise when it is diseased are a most intriguing study.
Eye function falls naturally into three phases. First, the eyes must be pointed at the object the brain needs visual in formation about. Second, the lens system must focus light reflected from the object to form an image on the sensitive part of the eye (retina). Third, the retina must convert this image into electrical signals that nerves can transmit to the brain. These are fascinating functions, and discussing them prepares us to understand eye disease.
Control over what the eyes look at is remarkably sophisticated in order to meet the surprisingly complex demands of everyday life. Since vision is precise only directly in front of the eyes they must be pointed accurately at whatever target is under study. This directing of the eyes must take place rapidly. More over, throughout their entire range of motion, both eyes must remain parallel so that the images formed, when combined in the brain, are similar.
Certain eye motions are automatic, thereby making vision much more useful. For instance, when the head is turned, built-in reflexes turn the eyes the opposite direction so that they re main fixed on the same spot. When the head is tilted within a certain range the eyes rotate in order to maintain an image that is upright.
All of this eye motion is accomplished by six muscles attached to each eye. The four rectus muscles come from behind each eye and insert at the top, bottom, and sides of the globe. There is, then, an individual muscle to raise the eye, to turn it to the right, to turn it to the left, or to lower it. Under the control of complex brain centers responsible for coordinating the action of the two eyes, these muscles can act very rapidly and precisely.
Occasionally one muscle may become weakened, thereby failing to move the eye adequately in its direction. Crossed eyes may result. Besides this being a relatively unsightly condition, the visual function is greatly impaired, be cause each eye is encompassing a different scene, and it is impossible for the brain to integrate this information. The resulting visual confusion is settled by the brain in a way that can be destructive to vision. One of the eyes is selected as dominant, and the image from that eye is utilized the brain considers the image from the other eye as spurious and suppresses it. If this suppression occurs in childhood, before the age of visual maturity, it goes so far as to actually destroy vision in that eye. At times a blind eye results from the brain's at tempt to eliminate the confusion of double vision. If such problems are dis covered early enough the weak muscle can be surgically shortened, turning the eye around parallel and resulting in a single brain image. Thus the blinding effect of the brain's suppression can be prevented. This surgery is quite safe and is very useful, even in cases that have only cosmetic benefits.
Focusing the Image
The eye gathers some of the light from our environment and focuses it on the retina to form an image of our surroundings. This light enters through a clear window, the cornea, and then passes through a lens that bends the rays and focuses them onto the retina. The space between the cornea and the retina must remain clear to avoid impeding the light rays. The eye is actually a sphere with a circular clear opening in the front, a lens located approximately a quarter of the way to ward the back, and the retina, the sensitive part, is the actual lining of the back of the sphere. In front of the lens, between it and the cornea, is a watery substance called aqueous humor. Be hind the lens is a clear gel called the vitreous humor.
Transparency of the cornea is most important in order that light may enter. There are a number of disease conditions that result in obscuring or foggin the cornea. Injury can cause a scar that clouds the window. Infection with virus or bacteria may have the same result. A common offender is herpes simplex virus, the same one that produces cold sores.
An opaque cornea can be replaced by a corneal transplant operation. In this procedure a disc is cut out of the patient's cornea, which is less than a sixteenth of an inch thick. An identical disc is cut out of a donor eye and then sewed into place in the cornea of the recipient. This provides a new clear window for the patient to see through. There is a shortage of these donor eyes, and grieving relatives may gain some consolation by approving the use of their departed loved one's eyes in the hope that another person may be able to see.
Glaucoma
The waterlike fluid that occupies the space between the cornea and the lens is in constant circulation. It is produced by the tissue surrounding the lens circulating forward through the pupil out into the periphery just behind the cornea. If its movement is blocked by a plug in the outflow tracts, the pressure inside the eyeball rises, producing glaucoma. If the pressure within the eye is excessive, blood cannot circulate to the tissues of the eye, and tissue death occurs. The nerve that carries the electrical signals to the brain, the optic nerve, begins to die first, and this results in varying degrees of blindness.
Particularly in older people, this eye pressure should be checked regularly, because glaucoma can come on very slowly and insidiously without pain or any other warning sign that blindness is occurring. Patients that have this disease are often satisfactorily treated by placing drops in their eyes two to four times daily. This constricts the pupil and facilitates the drainage of aqueous humor out of the eye, thus lowering the pressure within the eyeball.
The pupil of the eye is the hole that appears black in the center of the iris. Irises are composed of muscular tissue that makes blue eyes blue and brown eyes brown. The iris is capable of be coming large or small depending on the amount of light, and automatically regulates the brightness of the image in the back of the eye.
The lens changes its shape to keep objects in focus, whether they are close or far away, analogous to focusing a pair of binoculars. As we get older the lens becomes less and less pliable and consequently less and less capable of focusing for near objects.
Another consequence of aging is that the lens occasionally becomes cloudy. This condition is known as cataract. There is no known treatment, except to remove it. But after the lens is removed, the light rays will no longer be focused on the retina, so people who have cataracts removed must either wear thick glasses or contact lenses, thus placing a lens on the outside of the eye.
It is the responsibility of the retina to convert the image focused on it into electrical signals for transmission to the brain. This sensitive membrane contains approximately 100 million receptor cells, rod cells and cone cells,, which absorb the light rays into chemical pigments. The pigments then change form, thus setting off an electrical re action that travels through connector cells and into specialized nerve cells of the retina. These have long processes that extend across the retina to the exit point of the eye, through the optic nerve, and into the brain.
Cone cells transmit color vision but require quite a bit of light. Rod cells are capable only of black and white vision and thus are well adapted to function under dim illumination. The center of the retina, which is devoted to the task of seeing fine detail, contains a great concentration of cone cells for detailed color vision. This area is called the macula. Since the edges of the retina contain more rod than cone cells it is often easier to see an object at night by looking slightly to one side of it.
The blood vessels that supply most of the retina with blood traverse over its surface in such a way that the light must pass through them to reach the rods and cones. This interference is very slight. But to obtain the maximum accuracy of vision, there is a different blood supply in the central area of best defined vision. It filters through from the back. This makes blood circulation to the central area quite critical and sensitive to inadequacies.
Visual Problems
One of the biggest visual problems yet to be solved is that of degeneration of the blood circulation in the area of the macula. This condition, which most often affects older people, results in an inability to see in the very center of one's field of view—which is the area he depends on for precise vision. Reading is nearly impossible and it is difficult to recognize faces. The process does not cause blindness because the edges of the retina are still usable, but it causes marked impairment of performance.
Another problem the retina is subject to is separation from the wall of the eye by accumulation of fluid behind it. The detached part of the retina is dam aged and will eventually lose function. This condition must be treated surgically by draining the fluid that is separating the retina from the wall of the eye and by causing the retina to be held against the wall long enough for it to stick back in place.
The eye is a remarkable instrument. We can be very grateful that most of us have eyes in which all of these complicated components work together correctly to produce the great blessing of good vision. How essential that we realize our responsibility to guard this major freeway to the brain, permitting access only to images and information that will not weaken but lend strength.
