250 DISEASES OF THE HOUSE. 



the retina. It is fixed in a socket of the head, moved by 

 various muscles, and bedded in fat, in order to aiford facility 

 to its motions. It is nearly globular in shape, or rather, it 

 resembles parts of two globes, a segment of a smaller one being 

 appai-ently joined to the frustrum of a larger one. It consists 

 of several humours, and of various coats or membranes, some 

 being transparent and others opaque. In describing the ana- 

 tomy and physiology of the eye, the most convenient method is 

 to divide the subject into two parts, that is, the eye itself, and 

 its appendages. Under the latter head is comprehended the 

 eyelids, the muscles which move them, the eyelashes, the la- 

 chrymal gland, the puncta lachrymalia, and lachrymal duct, the 

 caruncle of the eye, the haw, and the membrane named con- 

 junctiva, and the muscles by which the eye is moved. 



The palpebrjB, or eyelids, may be considered as the shutters 

 of the eye, their office being to exclude light when so required. 

 They consist of cartilaginous bodies covered with soft and elastic 

 skin, and lined internally by the conjunctive membrane. They 

 are moved by two kinds or orders of muscles. Tlie one, com- 

 posed of circular fibres, and named orbicularis, is situated im- 

 mediately under the skin, and, by contracting, closes the eye- 

 lids ; tlie other, named levator, is composed of straight fibres, is 

 situated under the upper eyelid, and, by contracting, opens the 

 eyelids. The margin of the eyelids is smooth and thickened, 

 and secretes a mucous fluid, as well as the hairs, named cilia, or 

 eyelashes. In the horse there is only one eyelash, that is, the 

 superior, which serves to keep out dust from the eye, and mode- 

 rate the light which comes directly from the sun, without offering 

 any impediment to the reflected light which comes from the 

 surface of the earth. Immediately under the superior part of 

 the orbit, or socket of the eye, and rather towards the inner 

 corner, there is a gland of considerable size, which forms the 

 fluid named tears. This fluid is conveyed to the under surface 

 of the upper eyelid, and is then diffused all over the eye, by the 

 motion of the lids, as well as by the motion of the eye itself. 

 In this action the upper eyelid is principally employed. As 

 the tears accumulate, they are directed, by the closing of the 

 eyelids, towards the inner corner, where they are received by 

 the open extremities of two ducts, which, soon uniting, form the 

 lachrymal duct, a long membranous tube which terminates in 

 the lower part of the nostril, where it may be distinctly seen, 

 with the fluid named tears flowing from it. The inner surface 

 of the eyelids is formed by the membrane named conjunctiva, 

 which, passing inward, embraces the globe of the eye, and serves 

 to keep it in its situation ; at the same time it is sufficiently loose 

 to admit readily of all its various motions. This membrane, in 

 its healthy state, is nearly white ; but in all cases of internal 



