MUSCLES OF THE EYE. 99 



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

 opposite at the bottom of the eye ; and its office is as clearly to depress tlie 

 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 suspensorius (suspensory) muscle, g. It arises from 

 the edge of the foramen or hole through which the optic nerve enters the 

 orbit — surrounds 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 attempted 

 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 admira- 

 ble substitute for the want of hands, to defend the eye from many things 

 that would injure it; and, being partially separated into 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 vhe 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 upward and toward the inner side, and 

 there, just under the ridge of the orbit, it passes through a perfect mechan- 

 ical pulley, and, turning round, proceeds across the eye, and is inserted 

 rather beyond the middle of tha eye, towards the outer side. Thus the 

 globe of the eye is evidently directed inward and upward. Something 



