VOL. IJCXXVI.] THILOSOPHICAL TRANSACTIONS. 665 



that substance is possessed of the same refractive power with the surrounding 

 fluid. 



To avoid confusion in so extensive a field of inquiry, I shall separately consi- 

 der the peculiarities in the eyes of these different classes of animals, so far as 

 they appear to be concerned in producing the adjustment to different distances. 

 Quadrupeds have 3 modes of procuring their food ; one by their fore-paws only, 

 which they use like hands, as all the monkey tribe ; the 2d, by their fore-paws 

 and mouths, as the lion, and cat tribe ; the 3d, by the mouth only, as all rumi- 

 nating animals. These 3 different modes require the food being brought to dif- 

 ferent distances from the eye ; and it is curious, that the muscles of the eye are 

 different in all the 3 tribes. In the monkey tribe, the muscles of the eye are 

 exactly the same as in the human. In the lion tribe, they are double in num- 

 ber, and the 4 intermediate muscles are lost in the sclerotic coat, at a greater 

 distance from the cornea than the others. In the ruminating tribe, there are 4 

 muscles, as in the human eye; but there is also a muscle surrounding the eye- 

 ball, attached to the bottom of the orbit, round the hole through which the 

 optic nerve passes, and lost on the sclerotic coat immediately before the broadest 

 diameter of the globe of the eye ; the upper portion of this muscle is rather the 

 longest, its insertion being nearly in a circular line at right angles to the axis of 

 vision, but not to the axis of the eye from the entrance of the optic nerve. 



In quadrupeds in general, the ball of the eye is broader in proportion to its 

 depth, than in the human subject ; in the bull the proportion is If inch to I4-. 

 The cornea is larger and more prominent ; its real thickness is hardly to be de- 

 termined, since, as well as that of the human eye, it readily imbibes moisture 

 immediately after death. When dried, it is thinner than the sclerotic coat in 

 the same state. In ruminating animals, it appears externally of an oval form ; 

 it is not however really so, the cornea itself being circular, as in other animals; 

 but a portion of it is rendered opaque, by a membrane which covers its external 

 surface, and produces an oval appearance. This circular form of cornea is ne- 

 cessary, that when it is stretched it may form a regular curve. The ciliary 

 processes, as in the human eye, are connected with the choroide coat ; but they 

 are larger, and are united at their origin with the iris. This structure of the 

 eye in quadrupeds, so far as it differs from that of the human eye, appears cal- 

 culated to increase the power of adjusting it to see near objects, and from the 

 mode of life which these animals pursue, such additional powers appear necessary 

 to enable them with ease to procure their food. 



Birds in general procure their food by means of their beak ; and the distance 

 between the eye and the point of the beak is so small, that they must have a 

 power of seeing very near objects. From living in air, and moving through it 

 with great velocity, they require for their own defence, as well as to assist them 



VOL, XVII. 4 Q 



