66 



THE EYE. 



a jelly, and of beautiful transparency ; but if it is punctured a 

 fluid escapes from it as limpid and as thin as water, and when 

 this has been suffered completely to ooze out, a mass of membra- 

 neous bags or cells remains. 



Last of all, between the vitreous humor and the choroid coat, 

 is the retina, o, or net-like membrane. It is an expansion of the 

 substance, g, of the optic nerve. 



On this expansion of nervous pulp, the rays of light from sur- 

 rounding objects, condensed by the lens and the humors, fall, and, 

 producing a certain image corresponding with these objects, the 

 animal is conscious of their existence and presence. 



It may, however, so happen that from the too great or too little 

 convexity of the eye or a portion of it, the place of most distinct 

 rision may not be immediately on the retina, but a little before 

 or behind it. In proportion as this is the case, the sight will be 

 indistinct and imperiect ; nor shall we be able to offer any remedy 

 for this defect of sight. It is this chat occasions shying in the horse, 

 and as he grows older and the eye becomes less convex, the habit of 

 shying will increase. 



Nature has given seven muscles to the horse to enable him to 

 turn his eye, so that he can command the whole of that extended 

 field of view which the position of the organ enables it to take 

 in. And that they may act with sufficient power and quickness, 

 no fewer than ^ix nerves are directed to the muscles of the eye 

 generally, or t'> particular ones — while 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 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 

 drawn upwards. Another, /, is inserted exactly opposite, at the 

 bottom of the eye ; and its office is as clearly to depress the eye. 



