specially modified to
permit the passage of light into the interior of the eye. Its outline is
elliptical, nearly circular, and its greatest diameter is transverse. At
its periphery it joins the sclerotic by continuity of tissue, and as the
edge of the cornea is slightly beveled and has the fibrous sclerotic
carried for a little distance forward on its outward surface, the cornea is
generally said to be fitted into the sclerotic like a watch glass into its
rim. The venous canal of Schlemm runs circularly around the eyeball at the
line of junction of the sclerotic and cornea. The anterior surface of the
cornea is exquisitely smooth, and is kept moist by the lacrimal secretions.
Its posterior surface forms the anterior boundary of the chamber in which
the aqueous humor is contained. The cornea is of uniform thickness and is
of a dense, almost horny, consistence. Save a few capillary loops of blood
vessels at its margin the cornea is without vessels. Its structure is
comprised of five distinct layers.

The aqueous humor occupies a chamber which is bounded in front by the
posterior surface of the cornea and behind by the capsule and suspensory
ligament of the lens and by the ends of the ciliary processes. It is across
this chamber that the iris extends. The aqueous humor is composed of water,
with a small proportion of common salt in solution.

The iris is a muscular, pigmented curtain extending across the interior of
the eye and having about its center an aperture termed the pupil. By
variations in the size of this aperture the amount of light transmitted to
the retina is regulated. It varies somewhat in color, but is most
frequently of a yellowish-brown tint. Its anterior face is bathed by the
aqueous humor. The greater part of the posterior surface is in contact with
the capsule of the lens and glides on it during the movements of the
curtain. The circumferential border is attached within the junction of the
sclerotic cornea. The inner border circumscribes the pupil, which varies in
outline according to its size. When much contracted the pupil is a very
elongated ellipse, the long axis of which is in the line joining the nasal
and temporal angles of the eyelids. It contains muscular tissue, which, by
contracting or relaxing, lessens or dilates the pupillary opening.

The choroid coat is a bell-shaped, dark membrane which lines the sclerotic.
Its outer surface has a shaggy appearance, caused by the tunica fusca,
which u