DISEASES OF THE HORSE. 253 



iris. This lig-ament 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 obserA'cr 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 cavit}^ of the horse's 

 eye., into which the dazzling- raj^s fail 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 retractor. 

 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 contract- 

 ing singly the}" respectiveh" turn the eye inward, outward, upward, 

 and downward. When all act together the}' 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 ite 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, OK 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 Avith 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 line's 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 



