Many will agree that the ability to see is the most critical of the five senses. Few of us, however, may be able to distinguish between sight and vision.

Sight is what takes place in your eyes when you see light; vision is what occurs when the messages triggered by that light race through the optic nerve into the visual cortex in the depths of the brain. Eyes are even known to widen in fear, boosting sensitivity and expanding our field of vision to locate surrounding danger.

Construction of the human eye and the supporting structures is most remarkable, bewilderingly complicated, elaborately constructed and composed of multiple subsystems.

The entire eyeball is protected by being almost buried in a boney socket. Even the small, exposed front section of the eye is guarded by the eyebrows, eyelashes and the eyelids, and the tear apparatus (located under the upper eyelid). The eyebrows help to protect the eyes from dust and foreign bodies. The eyelid keeps out foreign material and then in blinking and closing, spreads the tears across the eye while simultaneously wiping the surface of the eye clean, like the sweep of a windshield wiper. During sleep, the closed eyelid prevents evaporation of moisture in the eye. Eyes cannot survive unless their exposed surfaces are kept continuously moist.

The eye has been defined as an optical device that receives and recognizes light and has the ability to define spatial detail. It is made up of a conjunctiva, the lens, the iris and its center aperture the pupil, the cornea, ciliary muscles, sclera, extraocular muscles and the optic nerve.

The collective function of the nonretinal parts of the eye is to keep a focused, clear image of the outside world anchored on the retina. The cornea and lens focus light rays onto the back of the eye while the lens regulates the focusing for near and far objects by becoming more or less globular. This change of shape occurs in all mammals, reptiles and birds, in a process of deformation.

Each part of the eye plays a role in how we see. Light bouncing off an object will first contact the cornea. The cornea refracts the rays of light which then passes through the pupil and enters the lens. The lens further bends the light to an image focused on the retina. Light acts like an electric shock through the layers of cells and stimulates them to send their message to the brain. Most of the light is focused on an area of the retina called macula, where vision is the sharpest and clearest.

The normal, healthy human eye has as many as 150 million photoreceptor cells, with three visual pigments in the cones for day color vision and one pigment in the rods for night time. The primate retina is complex with many interconnecting neurons, but not as complex as the retina of many birds and even some turtles. While we do not have the best optical device in the animal world, we do have a human brain that helps us meet any deficiencies in retinal construction.  

A number of afflictions affect the eye, but cataracts are the most common; they affect almost 22 million people above the age of 40. In its early stages, however, a cataract is not a disease, but a normal part of aging.

A cataract is a loss of transparency of the normally clear lens of the eye. Aging causes chemical changes to occur that render it less transparent. The lens opacity causes light to be scattered. As a result, light from an object that should pass directly through the lens produces a degraded image or no image at all.

Unlike that of other animals, the human lens is yellow, and grows for a remarkably long period of time, increasing from about 90 milligrams at birth to 240 milligrams at age 80. Oxidation damage to the lens is one contributor to cataract formation. The oxidation process releases chemicals called free radicals. In the eye the sources of oxidation include ultraviolet radiation, X-ray radiation and possibly cosmic radiation. Because the front of the eye is so transparent, the lens is constantly bathed in light and over many years the delicate protein arrangement in the lens becomes damaged. Other causes include diabetes, where poor control of blood sugar as well as a long duration of the illness play a major role in cataract development.

Cataracts are also fairly common in individuals who must take corticosteroids for prolonged periods and in cigarette smokers.

Glaucoma is another common eye disease that causes damage to the optic nerve. You should visit your eye care professional, particularly if you are over the 40 and have a family history of the disease.

The eye’s evolution from a primitive photosensitive pigment to today’s marvelous instrument is indeed a miracle. How fortunate we are to have inherited such a wondrous gift.

Max Sherman is a medical writer and pharmacist retired from the medical device industry. He has taught college courses on regulatory and compliance issues at Ivy Tech, Grace College and Butler University. Sherman has an unquenchable thirst for knowledge on all levels.  Eclectic Science, the title of his column,  will touch on famed doctors and scientists, human senses, aging,  various diseases, and little-known facts about many species, including their contributions to scientific research. He can be reached by email at  [email protected].