249 



Anotlier point wbicli strikes the observer of tlie horse's eye is that in 

 the darkuess a bright bluish tinge is reflected from the widely-dilated 

 pupil. This is owing to a comparative absence of pigment in the cho- 

 roid coat inside the upper part of the eyeball, and enables the animal 

 to see and advance with security in darkuess 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 in- 

 tensely 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 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— CARTILAGO NICTATANS. 



This is a structure, wliich, 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 j)osteriorly 

 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. 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 carti- 

 lage, 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 its 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. 



