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."

Lining the inner surface of the sclerotic is the second coat, the
choroid. It is dark in color and fragile in structure, and is made up
almost entirely of blood-vessels and nerves. As the choroid approaches the
front part of the eyeball, its parts become folded upon themselves into a
series of ridges, called ciliary processes. These folds gradually
become larger, and at last merge into the ciliary or accommodation
muscle of the eye. The circular space thus left in front by the
termination of the choroid is occupied by the iris, a thin, circular
curtain, suspended in the aqueous humor behind the cornea and in front of
the crystalline lens. In its center is a round opening for the admission
of light.

This is the pupil, which appears as if it were a black spot. The back
of the iris is lined with dark pigment, and as the coloring matter is more
or less abundant, we may have a variety of colors. This pigment layer and
that of the choroid and retina absorb the light entering the eye, so that
little is reflected.

The pupil appears black, just as the open doorway to a dark closet seems
black. The margin of the iris is firmly connected with the eyeball all
round, at the junction of the sclerotic and the cornea.

327. The Retina. The third and innermost coat of the eyeball is the
retina. This is the perceptive coat, without which it would be impossible
to see, and upon which the images of external objects are received. It
lines nearly the whole of the inner surface of the posterior chamber,
resting on the inner surface of the choroid. It is with the retina,
therefore, that the vitreous humor is in contact.

The retina is a very thin, delicate membrane. Although very thin, it
is made up of ten distinct layers, and is so complicated in structure that
not even a general description will be attempted in this book. It does not
extend quite to the front limits of the posterior chamber, but stops short
in a scalloped border, a little behind the ciliary processes. This is the
nerve coat of the eye, and forms the terminal organ of vision. It is
really an expansion of the ultimate fibers of the optic nerve, by means of