204 ANATOMY AND PHYSIOLOGY. 



ing into tne nostril of the horse. The two edges of the eyelids, 

 when closed, form a channel, along which the tears flow. Birds 

 have a third eyelid, at the inner angle of the two others, which they 

 may often be seen moving. Fishes have neither eyelids nor 

 lachrymal apparatus. 



Others of the appendages are the muscles that move the eye, six 

 in number. There are, besides these, two that move the eyelids. 

 A broad circular one, which closes the eyelids, lies immediately 

 under the skin. The other, which raises the upper eyelid, is a long 

 muscle, and is attached to the bone deep behind the eyeball. 



We now come to consider the globe of the eye, the parts com- 

 posing which are seen in the following figure, representing a hori- 

 zontal section of it. C, the cornea, is the transparent part of the 

 eye in front, which, it will be seen, forms part of a lesser circle, and 

 therefore projects more than the rest of the globe. It is set into 

 the white part of the ball of the eye, and after steeping, can be 

 taken out of it like a watch-glass. S, the sclerotic or hard coat, is 

 the outermost one, or the white part of the eye seen in front. It 

 extends over the whole ball posteriorly, and, from its toughness, 

 forms its principal support. In the tortoise and in birds, this part 

 anteriorly has bony matter in its composition ; and in the immense 

 eye of the extinct reptile called the ichthyosaurus, it appears to 

 have been composed of bony plates. The coat (X), which lies 

 internal to the sclerotic, is called the choroid coat. It is lined on 

 its inner surface, in the human eye, by a brownish-black paint (con- 

 tained in hexagonal cells), which we see when we look deep into 

 the eye. Its use seems to be to absorb the rays of light not re- 

 quired in vision. The color of this paint is, as every one has seen, 

 yellowish-green in the eye of the cat. It is chocolate-brown in the 

 hare and rabbit, silvery-blue in the horse, and pale golden yellow 

 in the lion and bear. In general it is of a light shade in such ani- 

 mals as prowl by night. This paint is wanting altogether in albino 

 animals, such as white rabbits or ferrets, and the red blood-vessels 

 can then be seen in the eye. This coat seems to be continuous 

 with a number of foldings called ciliary processes (K). The inner- 

 most of the coats of the eye (R) is called the retina, from its netted 

 appearance. It consists of a very fine membrane, with the pulpy, 

 half transparent substance, which is continuous with the optic 

 nerve (0), expanded upon it. This is the seat of vision. All 

 visual impressions must, in the first place, be made upon this ex- 

 pansion, and are then conveyed by the optic nerve to the mind. 



