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.
II. The Ear ([p. 415]).
(The muscles of the external ear are of little practical importance, and will doubtless usually be omitted.)
(1) The External Ear. Read the description of the external ear ([p. 415]), verifying it by examination and comparison of an ear still covered with integument, in the natural condition, and of a preparation of the isolated cartilages of the ear ([Fig. 168]). The latter may be obtained by dissecting the skin and muscles from a fresh ear.