form of the
eyeball and protects the portions within. It is pierced at the back by a
small opening which admits the optic nerve, and in front it becomes
changed into the peculiar tissue that makes up the cornea.

The _cornea_ forms the transparent covering over the lesser spherical
segment of the eyeball, shading into the sclerotic coat at its edges. It
has a complex structure, consisting in the main of a transparent form of
connective tissue. It serves the purpose of admitting light into the
eyeball.

*The Middle Coat* consists of three connected portions--the _choroid coat_,
the _ciliary processes_, and the _iris_. These surround the larger
spherical segment. All three parts are rich in blood vessels, containing
the blood supply to the greater portion of the eyeball.

The _choroid coat_ lies immediately beneath the sclerotic coat at all
places except a small margin toward the front of the eyeball. It is
composed chiefly of blood vessels and a delicate form of connective tissue
that holds them in place. It contains numerous pigment cells which give it
a dark appearance and serve to absorb surplus light. Near where the
sclerotic coat joins the cornea, the choroid coat separates from the outer
wall and, by folding, forms many slight projections into the interior
space. These are known as the _ciliary processes_. The effect of these
folds is to collect a large number of capillaries into a small space and
to give this part of the eyeball an extra supply of blood. Between the
ciliary processes and the sclerotic coat is a small muscle, containing
both circular and longitudinal fibers, called the _ciliary muscle_.

The _iris_ is a continuation of the choroid coat across the front of the
eyeball. It forms a dividing curtain between the two spherical segments
and gives the color to the eye. At its center is a circular opening,
called the _pupil_, which admits light to the back of the eyeball. By
varying the size of the pupil, the iris is able to regulate the amount of
light which passes through and it employs for this purpose two sets of
muscular fibers. One set of fibers forms a thin band which encircles the
pupil and serves as a sphincter to diminish the opening. Opposing this are
radiating fibers which are attached between the inner and outer margins of
the iris. By their contraction the size of the opening is increased. Both
sets of fibers act reflexively and are stimulated by variations in the
light falling upon the re