THE MUSCLES OF THE EYE. 99 



tracts, the eye must be drawn upward. Another, f^ is inserted exactly 

 opposite, at the bottom of the eye ; and its ofhce is as clearly to depress the 

 eye, or enable the animal to look downwards. A third, e, is inserted at the 

 outer corner, and by means of it the eye is turned outward, and, from the 

 situation of the eye of the horse, considerably backward ; and the fourth is 

 inserted at the inner corner, turning- the eye inward. They can thus rotate 

 or turn the eye in any direction the animal wishes. If the upper and outer 

 muscles are called into action, the horse looks upward and outward, and 

 more upward than outward in proportion as the upper muscle acts, at the 

 will of the animal, more powerfully than the outer ; and thus, by the action 

 of one of them, or the combined action of any two of them, the eye may 

 be immediately and accurately directed to every point. 



These muscles, however, have another duty to discharge. They support 

 the eye in its place. In the usual position of the head of the horse, they must 

 be to a certain degree employed for this purpose ; but when he is grazing- or 

 feeding, the principal weight of the eye rests upon them ; and then, lest 

 from this slanting and downward position of the head, when the horse is 

 so often and so long employed in feeding, they should be fatigued, another 

 muscle is added, peculiar to quadrupeds, called the retractor {drawer- 

 back), or the s2ispe7isoriiis (sjispensory) muscle, g. It arises from the edge 

 of the foramen or hole through which the optic nerve enters the orbit — sur- 

 rounds the nerve as it proceeds forward, and then, partially dividing into 

 four parts, is attached to the back part of the eye. Its office is evidently 

 to support the eye generally, or, when it is suddenly called into powerful 

 action, and assisted by the straight muscles, it draws the eye back out of 

 the reach of threatening danger, and in the act of drawing it back causes 

 the haw to protrude in the manner which we have already described, as an 

 additional defence. 



The power of this muscle is very great. It has been proved, in at- 

 tempted operations for cataract, to exert a force equal to more than twenty 

 pounds ; and it renders an operation on the eye almost impossible. It 

 is an admirable substitute for the want of hands, to defend the eye from 

 many things that would injure it; and, being partially separated hito four 

 divisions, it is also enabled to assist the straight muscles in turning the eye. 



These muscles discharge another and a most important office. If we 

 examine near and distant objects through a telescope, we must alter the 

 focus, i. e. we must increase or diminish the length of the tube. We must 

 shorten it a little when we examine distant objects, because the rays coming 

 to us from them, in a less divergent direction, are sooner brought to a 

 point by the power of the lens ; so the straight and retractor muscles 

 drawing back the eye, and forcing it upon the substance behind, and thus 

 in a slight degree flattening it, bring the lens nearer to the retina, and adapt 

 the eye to the observation of distant objects. 



Still, however, being employed in supporting the weight of the eye, these 

 muscles might not be able to turn it so rapidly and so extensively as 

 the wishes or wants of the animal might require ; therefore, two other 

 muscles are given, which are used solely in turning the eye. They are 

 called oblique muscles, because their course is obliquely across the eye. 

 The upper one is most curiously constructed, a, h. 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 pulley, 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 inward and 



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