The eyes of both mollusks and vertebrates operate on the same basic principle as
a camera. A single lens focuses light from all parts of the visual field onto a
sheet of light-sensitive cells.
Despite the close similarity in structure and function between the eyes of mollusks and vertebrates, all the evidence indicates that they arose and evolved quite separately in the two groups.
The human eye is roughly spherical in shape. It is bounded by three distinct layers of tissue. The outer layer, the sclerotic coat, is extremely tough. It is white in color (the „white" of the eye) except in the front. Here it forms the transparent cornea, which admits light into the interior of the eye and bends the light rays so that they can be brought to a focus. The surface of the cornea is kept moist and dust-free by the secretion from the tear glands.
Like tissues of the central nervous system, the major metabolic fuel for the tissues of the eye is glucose. The cornea, which is not a homogenous tissue, obtains a relatively large percentage of its ATP from aerobic metabolism.
The middle coat of the eye, the choroid coat, is deeply pigmented with
melanin and well supplied with blood vessels. It serves the very useful function
of stopping the reflection of stray light rays within the eye. This is the same
function that is accomplished by the dull black paint within a camera.
In the front of the eye, the choroid coat forms the iris. This may be pigmented and is responsible for the „color" of the eye. An opening, the pupil, is present in the center of the iris. The size of this opening is variable and under automatic control. In dim light (or times of danger) the pupil enlarges, letting more light into the eye. In bright light, the pupil closes down. This not only protects the interior of the eye from excessive illumination, but improves its image-forming ability and depth of field. Photographic enthusiasts, too, make a practice of "stopping down" the iris diaphragm of their cameras to the minimum permitted by the amount of light available in order to get the sharpest possible pictures.
The inner coat of the eye is the retina. It contains the visual sensing apparatus (the actual light receptors, the rods and cones, and thus functions in the same way as the film of a camera). The exterior of the cornea is bathed by tears, while the interior is bathed by the aqueous humor. It is an isoosmotic fluid containing salts, albumin, globulin, glucose, and other constituents. The aqueous humor brings nutrients to the cornea and to the lens and removes end products of metabolism from these tissues. The vitreous humor is a collagenous or gelatinouslike mass that helps maintain the shape of the eye, but also allows it to retain some pliability.
There are no mitochondria in the outer segments of the rods and cones, however, where the visual pigments are located.
The lens of the eye is located just behind the iris. It is held in position by ligaments. Ordinarily, these are kept under tension and the lens is correspondingly flattened. Hovewer, contraction of muscles attached to these ligaments relaxes them and permits the lens to take on a more nearly spherical shape. These changes in lens shape enable the eye to shift its focus (accomodate) from far objects to near objects and vice versa.
The lens of the eye is bathed on one side by the aqueous humor and supported on the other side by the vitreous humor. The lens has no blood supply but it is an active metabolizing tissue. The lens is mostly water and protein. The proteins are synthesized within the lens, occurring mostly in an epithelial layer around the edge of the lens. The center area of the lens, the core, consists of the lens cells that were present at birth. The lens grows from the periphery. The human lens increases in weight and thickness with age and becomes less elastic. On average the lens may increase threefold in size and appriximately 1,5-fold in thickness from birth to about age 80.
The proteins of the lens must be maintained in a native unaggregated state. These proteins are sensitive to various insults such as changes in the oxidation-reduction state of the cells, the osmolarity of the cells, excessively increased concentrations of metabolites, and various physical insults such as UV irradiation.
The method of changing focus by changing the shape of the lens has no parallel in photography. Focus is changed in cameras by moving the position of the entire lens with respect to teh film. This method is also used in the eyes of some fishes, amphibians, snakes, and some mollusks.
The iris and the lens divide the interior of the eyeball into two main chambers. The anterior one is filled with a watery fluid, the aqueous humor. The porterior chamber is filled with a jellylike material of marvelous clarity, the vitreous humor.
Eyes are in continuous movement durring watching. Even, when they are observing a resting object they are doing small, involuntary movements. A view on retina is still changing, removing from the center of the yellow spot in flank and comming back to it. In the meantime the eye is trembling with large frequency. If a view on the retina were immobilize, it would turn pale and disappear, and later it would appear partial or whole.
Movement of the eyeball is accomplished by three pairs of muscles, the members of each
pair working antagonistically. The coordinated action of these muscles enables the
eyeball to be rotated in any direction. Thus we are able to train both eyes in a single
direction. This produces two slighty differing views of the same scene which our brain
is able to fuse into a single, three-dimensional image.