325.  The Eye.  The eye, the outer instrument of vision, is a most beautiful and ingenious machine.  All its parts are arranged with such a delicate adjustment to one another, and such an exquisite adaptation of every part to the great object of the whole, that the eye is properly regarded as one of the wonders of nature.

The eyeball is nearly spherical in shape, but is slightly elongated from before backwards.  The front part is clear and transparent, and bulges somewhat prominently to allow the entrance of the rays of light.  The eye rests in a bowl-shaped socket, called the orbit, formed by parts of various bones of the head and face.  The margins of this cavity are formed of strong bone which can withstand heavy blows.  The socket is padded with loose, fatty tissue, and certain membranes, which serve as a soft and yielding bed in which the eyeball can rest and move without injury.  In a severe sickness this fatty tissue is absorbed, and this fact explains the sunken appearance of the eyes.

The orbit is pierced through its posterior surface by an opening through which the nerve of sight, the optic, passes to the eyeball.  We may think of the optic nerve holding the eyeball much as the stem holds the apple.  It is the function of this most important nerve to transmit retinal impressions to the seat of consciousness in the brain, where they are interpreted.

The eye is bathed with a watery fluid, and protected by the eyelids and the eyebrows; it is moved in various directions, by muscles, all of which will soon be described.

[Illustration:  Fig. 127.—­Section of the Human Eye.]

326.  The Coats of the Eyeball.  The eyeball proper is elastic but firm, and is composed of three coats, or layers, each of which performs important functions.  These coats are the sclerotic, the choroid, and the retina.

The sclerotic coat is the outside layer and enclosing membrane of the eyeball.  It is a tough, fibrous coat for the protection and maintenance of the shape of the eye.  It is white and glistening in appearance, and is in part visible, to which the phrase, “the white of the eye,” is applied.  To this coat, which serves as a kind of framework for the eye, are attached the muscles which move the eyeball.  In front of the globe, the sclerotic passes into a transparent circular portion forming a window through which one can see into the interior.  This is the cornea.

The cornea, a clear, transparent, circular disk, fits into the sclerotic, somewhat as the crystal fits into the metallic case of a watch, forming a covering for its dial.  It projects from the general contour of the eyeball, not unlike a rounded bay-window, and is often spoken of as the “window of the eye.”