257 



(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 ligament 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 ijigment 

 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 fur- 

 nished 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 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 aiM inward, and the lower one turning it downward 

 and inward. 



THE HAW — THE WINKING CARTILAGE — CARTILAGO NICTATANS. 



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, moulded on the lower and inner surface of the eyeball. Exter- 

 nally, it is covered by the mucous membrane which lines the ej'elids 

 and extends over the front of the eye. In the ordinarj^ 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. 

 5961 — HOR 9 



