252 DISEASES OF THE HOESE. 



one on the under, and one on each side. Then- use is apparent : 

 acting simultaneously they pull the eye within the orbit ; and 

 acting separately they draw its transparent surface to one side 

 or the other, upward or downward, according to the situation of 

 the acting muscle. Besides these, there are two oblique muscles, 

 the superior and the inferior ; the former rises from the back of 

 the orbit, goes to the inner side, where it passes through a pulley ; 

 its tendon then traverses in an opposite direction, and is in- 

 serted into the upper and outer part of the eye. The inferior 

 is a shorter and stronger muscle, but, like the former, to which 

 it is an antagonist, takes an oblique course. These muscles 

 furnish, in some measure, a rotatory motion to the eye, and thus 

 fiving expression to these organs, are exercised particularly in 

 vicious horses. The seventh muscle, which is peculiar to brutes, 

 is called the retractor, and is situated within the straight 

 muscles, and immediately surrounds the optic nerve. Its use is 

 to draw the eye farther into the orbit, and by pressing on the 

 fat, which forms a soft cushion for this organ, forces the haw to 

 protrude itself more readily over it. This muscle possesses con- 

 siderable power. 



Having described the appendages, as they are termed, I shall 

 proceed to a description of the eye itself, the structure and 

 economy of which is most curious and interesting. It is said to 

 be composed of coats and humours, and this, perhaps, is the best 

 manner of considering it. The first coat that appears is the 

 cornea, or glass of the eye, which forms the anterior part, and 

 is beautifully transparent. It may be considered as the window 

 of the eye, being that part of the case formed to admit light. It 

 is not of a circular form, as in man, but of an irregular oval or 

 rather oblong form when examined out of the socket ; but in its 

 natural situation in the living horse, that part which projects 

 beyond the eyehds is a regular oval, or rather of an oblong 

 form, and corresponding, in some degree, with the form of the 

 pupil. It forms a larger portion of the globe of the eye than in 

 man, and by its convexity, causes the rays of light which pass 

 through it to converge towards the pupil. This convexity may 

 be too great or too little, and, in either case, renders vision 

 somewhat imperfect, and causes starting. The convexity of 

 the cornea is preserved by the fluid which it incloses, named 

 aqueous humour. On puncturing the cornea this fluid escapes, 

 and then the cornea becomes flat and wrinkled. 



The second humour of the eye is situated immediately behind 

 the pupil, and is named the crystalline humour, or crystalline 

 lens. On taking it out, it appears to be a solid and beautifully 

 transparent double convex lens, the posterior surface of which is 

 more convex than the anterior one. It is found to become 

 denser and denser from the circumference to the centre ; and 



