DISEASES OF THE EYE. 253 



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 

 penetrating 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 crbit 

 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 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 



