Use any specimen on which one side of the head has been left intact.
Remove the head from the body by cutting through the neck a little
craniad of the first rib.
Study the eye externally. Observe the eyelids, the conjunctiva,
nictitating membrane with its cartilage, and the Harderian gland (Fig.
165); the two openings of the lachrymal duct, the Meibomian glands if
possible.
Remove the zygomatic arch and expose the orbit and the structures which
it contains. Study the periorbita (p. 409), and find the lachrymal gland
(p. 410).
Study the muscles of the eyeball (p. 411, and Fig. 166). The lateral
rectus on the lateral surface will perhaps be first found, and its
tendon traced beneath the inferior oblique. Use great care not to injure
the levator palpebræ superioris.
After studying the muscles, find the optic nerve. Cut it and the
muscles, and remove the eyeball for farther study.
_The Eyeball_ (p. 412, and Fig. 167).--For an examination of the eye it
is well to have a fresh specimen and one hardened in formalin or
alcohol. The hardened specimen is more essential, however. An eye from
one of the specimens used in dissection is usually satisfactory. All
accessory portions should be trimmed from the eyeball, leaving only the
spherical ball with a short stalk formed by the optic nerve.
Observe such features of the eye as can be seen externally: the optic
nerves, sclerotic, cornea, iris, and pupil. This should be done on a
fresh specimen, if one is at hand. The changes in size and form of the
pupil can be observed in the living cat by changing it from a light to a
dark place and _vice versa_.
_Dissection._--Examine the internal structures on a preparation. Then
with fine scissors and forceps remove from the eye about one-fourth of
the wall, in the form of a quadrant having one point at the optic nerve,
the other at the centre of the cornea. The coats of the eye can then be
studied on the piece removed, while the other structures will be visible
within the eyeball. No special directions are necessary for observation
of the structures described, unless it be the capsule of the lens and
the zonula ciliaris. The capsule of the lens may be demonstrated by
tearing a bit of it off with fine forceps. The zonula ciliaris is easily
seen by divaricating the edges of the cut made in removing the quadrant,
so as to stretch the fine fibres of which the zonula is formed.