134 THE SENSORIAL FUNCTION. 



the rays, and throwing the perfect image beyond, and not on, the retina. Ther ® 

 is a striking difference in the convexity of the cornea in the colt and the old 

 horse; and both of them, probably, may shy from opposite causes— the one 

 from a cornea too prominent, and the other from one too flat. In the usual 

 examination of the horse previously to purchase, sufficient attention is not always 

 paid to the convexity of the cornea. 



The remedy for shying will be considered when we speak ot the vices ot 



horses. 



There is a provision yet wanting. The horse has a very extended field of view, 

 but many persons are not perhaps aware how little of it he can command at a 

 time. There is not one of our readers who can make out a single line of our 

 treatise without changing the direction of the eye. It is curious to follow the 

 motion of the eyes of a rapid reader. Nature has given no less than seven 

 muscles to the horse, in order to turn this little but important organ ; 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 — wlsile the 

 eye rests on a mass of fat, that it may be turned with little exertion of power, 

 and without friction. 



MUSCLES OF THE EYE. 



There are four straight muscles, three of which, d, e, and /, are represented 

 in our cut, rising from the back of the orbit, and inserted into the ball of the 



eye, opposite to, and at equal dis- 

 tances from each other. One, d, 

 runs to the upper part of the eye, 

 just behind the transparent and vi- 

 sible portion of it, and its office is 

 clearly to raise the eye. When it 

 contracts, the eye must be drawn 

 upward. Another, f, is inserted 

 exactly 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 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. Another muscle is therefore 

 added, peculiar to quadrupeds, called the retractor (drawer-back), or the sus- 

 pensorius (suspensory') muscle, g. It arises from the edge of the foramen through 

 which the optic nerve enters the orbit — surrounds the nerve as it proceeds for- 

 ward, 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 sud- 

 denly 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. 



