its own axis, the upper one turning its outer
surface upward and inward, and the lower one turning it downward and
inward.

THE HAW (THE WINKING CARTILAGE, OR CARTILAGO NICTITANS).

This is a structure which, like the retractor muscle, is not found in
the eye of man, but it serves in the lower animals to assist in removing
foreign bodies from the front of the eyeball. It consists, in the horse,
of a cartilage of irregular form, thickened inferiorly and posteriorly
where it is intimately connected with the muscles of the eyeball and the
fatty material around them, and expanded and flattened anteriorly where
its upper surface is concave, and, as it were, molded on the lower and
inner surface of the eyeball. Externally it is covered by the mucous
membrane which lines the eyelids and extends over the front of the eye.
In the ordinary restful state of the eye the edge of this cartilage
should just appear as a thin fold of membrane at the inner angle of the
eye, but when the eyeball is drawn deeply into the orbit the cartilage
is pushed forward, outward, and upward over it until the entire globe
may be hidden from sight. This protrusion of the cartilage so as to
cover the eye may be induced in the healthy eye by pressing the finger
and thumb on the upper and lower lids, so as to cause retraction of the
eyeball into the socket. When foreign bodies, such as sand, dust, and
chaff, or other irritants, have fallen on the eyeball or eyelids it is
similarly projected to push them off, their expulsion being further
favored by a profuse flow of tears.

[Illustration: PLATE XXII.

DIAGRAMMATIC VERTICAL SECTION THROUGH HORSE'S EYE.]

This is seen, to a lesser extent, in all painful inflammations of the
eye, and to a very marked degree in lockjaw, when the spasm of the
muscles of the eyeball draws the latter deeply into the orbit and
projects forward the masses of fat and the cartilage. The brutal
practice of cutting off this apparatus whenever it is projected
necessitates this explanation, which it is hoped may save to many a
faithful servant a most valuable appendage. That the cartilage and
membrane may become the seat of disease is undeniable, but so long as
its edge is thin and even and its surface smooth and regular the mere
fact of its projection over a portion or the whole of the eyeball is no
evidence of disease in its substance, nor any warrant for its removal.
It is usually but the evidence of the presence of some pain