The eyes

 The eye is an extremely complex organ, and we need not enter into all the technical details of its structure and function. Seeing however, depends on the relationship of the eye with that even more complicated structure, the brain. Mechanically speaking, the eye functions as a camera whose images are relayed to the visual centers of the brain

Protected by eyelids and eyelashes, the eye itself, set in the sockets of the skull, is a globe or sphere filled with fluid. Three coats or membranes enclose the fluid. The fluid in the rear of the eyeball is called the vitreous fluid; it has a jellylike consistency. That in the front bulge of the eye is known as the aque­ous fluid.

The outer membrane of the eyeball is known as the sclera a tough, fibrous mem­brane, it covers the entire eyeball and ap­pears as "the white" of the eye. However, at the front of the eye, this coat is crystal clear and is called the cornea.

The middle layer of membrane, called the choroid, also encloses the whole eyeball ex­cept altogether at the front of the eye, where the pupillary opening is found. The middle layer is pigmented and makes up the iris or colored part of the eye. The iris has tiny radiating and circular muscle fibers which enable it to expand when light is dim and to contract when light is bright so that light can enter its central opening, the pupil, in the right amount.

The inner layer of the eyeball is the retina; it lines the entire inner (posterior) chamber of the eye except in the region of the iris. This layer is composed of nerve tissue and millions of light sensitive receptors, known as rods and cones. These are connected with the optic nerve, which attaches to the back of the eye­ball. Where the optic nerve connects with the retina, there is a small "blind spot." The optic nerve conveys images to the brain, where they are "interpreted."

One more crucial part of the human camera must be mentioned, namely the crystalline lens of the eye. It is situated immediately be­hind the iris, which lies between the two chambers of the eye. It is held in place by suspensory ligaments and can be flattened or thickened by the operation of ciliary muscles. The lens is transparent and refracts light. The changes in its shape (degrees of convex­ity) serve to focus light rays on the retina.