276 DISEASES OF THE HORSE. 
(capsule) and is maintained in position by a membrane (suspensory 
ligament) which extends from the margin of the lens outward to the 
sclerotic at the point of junction of the choroid and iris. This liga- 
ment is, in its turn, furnished with radiating, muscular fibers, which 
change the form or position of the lens so as to adapt it to see with 
equal clearness objects at a distance or close by. 
Another point which strikes the observer of the horse’s eye is that 
in the darkness a bright, bluish tinge is reflected from the widely 
dilated pupil. This is owing to a comparative absence of pigment 
in the choroid coat inside the upper part of the eyeball, and enables 
the animal to see and advance with security in darkness where the 
human eye would be of little use. The lower part of the cavity 
of the horse’s eye, into which the dazzling rays fall from the sky, is 
furnished with an intensely black lining, by which the rays penetrat- 
ing the inner nervous layer are instantly absorbed. 
MUSCLES OF THE EYE. 
These consist of four straight muscles, two oblique, and one re- 
tractor. The straight muscles pass from the depth of the orbit 
forward on the inner, outer, upper, and lower sides of the eyeball, 
and are fixed to the anterior portion of the fibrous (sclerotic) coat, 
so that in contracting singly they respectively turn the eye inward, 
outward, upward, and downward. When all act together they draw 
the eyeball deeply into its socket. The retractor muscle also consists 
of four muscular slips, repeating the straight muscles on a smaller 
scale, but as they are only attached on the back part of the eyeball 
they are less adapted to roll the eye than to draw it down into its 
‘socket. The two oblique muscles rotate the eye on 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 remov- 
ing foreign bodies from the front of the eyeball. It consists, in the 
horse, of a cartilage of irregular form, thickened inferiorly and pos- 
teriorly where it is intimately connected with the muscles of the eye- 
ball 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 cov- 
ered 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 
