92 



THE SENSORIAL FUNCTION. 



turn this little hut important orffan ; and, that they may act with sufficient power and 

 quickness, no fewer than six nerves are directed to the muscles of the eye generally, 

 or to particular ones — while the eye rests on a mass of fat, that it may be turned with 

 little exertion of power, and without friction. 



5IUSCLES OF THE EVE. 



There are four straight muscles, 

 three of which. (/, c. and/, are repre- 

 sented in our cut, rising from the 

 back of the orbit, and inserted into 

 the ball of the eye, ( pposite to, and 

 at equal distances I'roiii each other. 

 • One, f/, runs to the u[)}ier part of the 

 eye, just behind the transparent and 

 visible portion of il, and its office is 

 clearly to raise the eye. When it 

 contracts, the eye must be drawn 

 upward. Another./, is inserted ex- 

 actly opposite, at the bottom of the eye ; and its office 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, and by the action of one, or the combined power of any two of 

 them, the eye can be immediately and accurately directed to every point. 



These muscles, however, have another dutj' 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. Another muscle is therefore added, peculiar to 

 quadrupeds, called the relracinr [droiccr-bac/f), or the suspcjisorius {suspensory) muscle, 

 i^. It arises from the edge of the foramen through which the optic nerve enters the 

 orbit — surrounds the nerve as it proceeds forward, and then, partially dividing into 

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

 the eye generally, or, when suddenlj' 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, as an additional defence. 



The power of this muscle is very great. It renders some operations 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 into four 

 divisions, it assists 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 a/kr t/ie focus ; i. e., we must increase 

 or diminish the length of the tube. We must shorten it a little when we examine dis- 

 tant objects, because the ra3's, coming to \is from them in a less divergent direction, 

 are sooner brought to a point by the power of the lens. Thus the straight and retractor 

 muscles drawing back the eye, and forcing it u])on the substance behind, and 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 constantly employed in supporting the weight of the eye, these 

 muscles may not be able to turn it so rapidly and so extensively as the wishes or 

 wants of the animal require; therefore two others are superadded which aroused 

 solely in turning the eye. Thejr are called oblique muscles, because their course is 

 obliquely across the eye. The upper one is most cviriously constructed, a, h. It 

 comes from the back part of the orbit, and takes a direction upwurds and towards the 

 inner side, and there, just under the ridge of the orbit, it passes l!iro>iiih a perfect me- 

 chanical pulley, and. turning round, proceeds across the eve, and is inserted rather 

 beyond the middle of the eye, towards the outer side, 'i'hus the glolie of the eye is 

 evidently directed inward and upward. Something more, however, is accom])lished 

 l)y this singular mechanism. The eye is naturally deep in the orbit, that it may he 



