shell which encloses fluid or semisolid parts. The shell is anteriorly made
up of a transparent convex membrane, the cornea, while the remainder of its
wall is formed by three opaque layers or tunics.

The external tunic is the _sclerotic_. It is a white, solid membrane,
forming about four-fifths of the external shell. Its external face is
related to the muscles and fatty cushion. It receives posteriorly, a little
lower than its middle portion, the insertion of the _optic nerve_, which
passes through the shell and spreads out to form a very thin membrane, the
retina or internal coat.

_The retina_ lines about two-thirds of the posterior portion of the shell
of the eye. It is made up of seven layers. The essential layer is named
from its appearance, rods and cones.

The middle coat is the _choroid_. This is a dark, pigmented, vascular and
muscular membrane. The posterior portion is in contact with the retina.
Anteriorly it forms the ciliary processes and the iris.

_The media_ of the eye are the crystalline lens, vitreous and aqueous
humors. The _crystalline lens_ is a transparent, biconvex body sustained by
the ciliary processes. The _vitreous humor_ is a transparent jelly-like
substance that fills all the cavity of the eye posterior to the lens. The
_aqueous humor_ is a liquid, contained in the anterior and posterior
chambers of the eye in front of the lens. This fluid separates the iris
from the front of the lens.

EXAMINATION OF THE EYE.--In examining and treating the eye we should avoid
rough and hasty manipulation. The animal should be approached slowly. It is
best for the attendant who is familiar with the animal to hold it for the