DISEASES OF HORSES 347 



upward, and downward. When all act together they draw the eye- 

 ball 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 re- 

 moving 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 flat- 

 tened anteriorly where its upper surface is concave, and, as it were, 

 molded on the lower and inner surface of the eyeball. Externally it 

 is covered by the mucous membrane which lines the eyelids and ex- 

 tends 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 mem- 

 brane 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 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. 



This is seen, to a lesser extent, in all painful inflammations of 

 the eye, and to a very marked degree in lockjaw, when the spasm of 

 the muscles of the eyeball draws the latter deeply into the orbit and 

 projects forward the masses of fat and the cartilage. The brutal prac- 

 tice of cutting off this apparatus, whenever it is projected, necessi- 

 tates this explanation which, it is hoped, may save to many a faithful 

 servant a most valuable appendage. That the cartilage and mem- 

 brane may become the seat of disease is undeniable, but so long as its 

 edge is thin and even, and its surface smooth and regular, the mere 

 fact of its projection over a portion or the whole of the eyeball is no 

 evidence of disease in its substance, nor any warrant for its removal. 

 It is usually but the evidence of the presence of some pain in another 

 part of the eye, which the suffering animal endeavors to assuage by 

 the use of this beneficent provision. 



LACHRYMAL APPARATUS. 



This consists, first, of a gland for the secretion of the tears, and, 

 second, of a series of canals for the conveyance of the superfluous 

 tears into the cavity of the nose. The gland is situated above the 

 outer part of the eyeball, and the tears which have flowed over the 

 eye and reached the inner angle are there directed by a small conical 



