as to
respond to the vibrations of this medium for the transmission of light.

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