nites the two coats. Between the two the ciliary vessels and nerves
pass forward. Behind it is pierced by the optic nerve; in front it is
continued as the ciliary processes, which form, as it were, the rim of the
bell. The ciliary processes form a fringe around the slightly inverted rim
of the choroid.

The retina is the most delicate of the coats of the eyeball. It is formed
by the expansion of the optic nerve on the inner surface of the choroid,
and, like that coat, it is bell-shaped. Its inner surface is molded on the
vitreous humor. The nervous structures of the retina terminate at a wavy
line, the ora serrata, behind the ciliary processes. Ten distinct layers
are described as composing the thickness of the retina.

The lens is situated behind the pupil and is contained within a capsule of
its own.

The capsule is a close-fitting, firm, transparent membrane. The anterior
surface forms the posterior boundary of the cavity containing the aqueous
humor, and the iris in its movement glides on it. The posterior surface is
in contact with the vitreous humor.

The vitreous humor occupies four-fifths of the interior of the eyeball. It
is globular in form, with a depression in front for the lodgment of the
lens. It is colorless, transparent, and of a consistency like thin jelly.
It is enveloped by a delicate capsule--the hyaloid membrane--which is
connected in front with the suspensory ligament of the lens, and ends by
joining the capsule behind the lens.

The orbital cavity, at the side of the head, is circumscribed by a bony
margin; posteriorly, however, there are no bony walls, and the cavity is
often confounded with the depression above and behind the orbit--the
temporal fossa. A fibrous membrane completes this cavity and keeps it
distinct from the temporal fossa. This membrane--the ocular sheath or
periorbita--is attached posteriorly around the opening in the back part of
the orbital cavity (the orbital hiatus) and anteriorly to its inner face;
then it becomes prolonged beyond the margin to form the fibrous membrane of
the eyelids. When complete the orbital cavity has the form of a regular
hollow cone, open at its base and closed at the apex. The opening of this
cone is directed forward, downward, and outward. Independently of the globe
of the eye, this cavity lodges the muscles that move it, the membrana
nictitans, and the lacrimal gland.

The muscles of the eye are seven in number--one retractor, four straight,