166 



THE SEXSOIUAL FU.NX'TIOX. 



— a loss of convexity in the eye, lessening the convergency of the rays, 

 and throwing the perfect image beyond, and not on the retina. There 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 vriW be considered when we speak of the vices of 

 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 hne 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 — while the eye rests on a mass of fot, 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 repre- 

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



ball of the eye, opposite to and 

 at equal distances from each 

 other. One, d, runs to the upper 

 part of the eye, just behind the 

 transparent and visible portion 

 of it, and its office is clearly to 

 raise the eye. When it contracts, 

 the eye must be dra^vn upward. 

 Another, /, 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 outwai-d, and fi-om 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 tui'n 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 siip- 

 port 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 pui^pose ; but when he 

 is grazing or feeding, the principal weight of the eye rests upon them. 

 Another muscle is therefore added, pecuhar to quadrupeds, called the 

 retractor {drawer-back^, or the suspensorkis (s7ispensorij) muscle, g. 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 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, as an additional defence. 



