THE EYE. 67 



01 enable the animal to look downwards. A third, e, is inserted 

 at the outer comer, and by means of it the eye is turned outward, 

 and, from the situation of the eye of the horse, considerably back- 

 ward ; and the fourth is inserted at the inner corner, turning the 

 eye inward. They can thus rotate or turn the eye in any direc- 

 tion the animal wishes, and by the action of one, or the combined 

 power of any two of them, the eye can be immediately and ac- 

 surately directed to every point. 



These muscles also assist to support the eye in its place. They 

 are aided in this, especially when the head is depressed, by the 

 retractor (drawer back) muscle, g, which has already been al- 

 luded to. The power of this muscle is very great, and it renders 

 some operations on the eye almost impossible. It is an admirable 

 substitute for hands to defend the eye from many things that 

 woidd injure it. Being partially separated into four divisions, it 

 assists the straight muscles in tumiiig the eye. 



The muscles we have described, perform another important 

 office. By drawing back the eye, and slightly flattening it, they 

 brmg the lens nearer the retina, and adapt the eye to the obser 

 vation of more distant objects. There are two other muscles, 

 used solely in turning the eye, called oblique muscles, because 

 their course is obliquely across the eye. The upper one, a, h, is 

 most curiously constructed. It comes from the back part of the 

 orbit, and takes a direction upwards and towards the inner side, 

 and there, just under the ridge of the orbit, it passes through a 

 perfect mechanical puUey, and, turning round, proceeds across the 

 eye, and is inserted rather beyond the middle of the eye, towards 

 the outer side. Thus the globe of the eye is evidently directed 

 ir.ward and upward. Something more", however, is accomplished 

 by this singular mechanism. When it is necessary to bring the 

 eye forward in its socket,- to enlarge the field of vision, the object 

 is readily effected by this singular pulley, h, c. By the poWer of 

 this muscle — the trochlearis, or pulley-muscle — and the straight 

 muscles at the same time not opposing it, or only regulating the 

 direction of the eye, it is reaUy brought somewhat forward. The 

 lower oblique muscle rises just within the lachrymal bone (i. Fig. 

 2), and, proceeding across the eye, is fixed mto the part of the 

 sclerotica opposite to the other obhque muscle, and it turns the 

 eye in a contrary direction, assisting, however, the upper oblique 

 ip bringing the eye forward from its socket. 



