THE NERVOUS SYSTEM. 223 



ternally by dark pigment cells, except in the caudal area 

 around the optic nerve, which has a metallic luster. This 

 portion is called the tapetum. It causes the shining ap- 

 pearance of the cat's eyes in the dark. The choroid is a 

 vascular membrane, being supplied by the ophthalmic artery, 

 a branch of the internal carotid. 



The iris is attached by its peripheral margin to the 

 sclerotic and choroid coats, and hangs free in the aqueous 

 humor. It gives color to the eye. In the cat it is yellowish, 

 while in man it is frequently blue or black. The iris is 

 merely a curtain to regulate the amount of light admitted 

 to the retina. There is a sphincter muscle lying in it, which 

 by contraction renders the pupil very small. There is prob- 

 ably no dilating muscle of the iris present in the cat. The 

 short, thickened, radial projecting folds of the choroid are 

 the ciliary processes, which contain numerous blood-vessels, 

 and in some mammals a gland. The ciliary muscle arises 

 from the sclerotic coat near its junction with the cornea, 

 and is inserted into the cranial part of the choroid coat. 



The inner membrane of the eye is the retina, which is of 

 a light gray color in a fresh specimen and seems quite free 

 from the choroid. It is thickest in the caudal two-thirds 

 of the cavity of the eyeball (Fig. no). At the base of the 

 ciliary bodies it seems to end with a free margin, called the 

 ora serrata. In reality it becomes very thin here and is 

 prolonged over the ciliary bodies and covers the caudal 

 aspect of the iris. The blind spot is the point of entrance 

 of the optic nerve, laterad of which is the yellow spot, or 

 macula lutea, containing the fovea centralis, or acute point 

 of vision. This is the point on which the rays of light are 

 focused when the cat sees distinctly. 



The three humors of the eye are the aqueous ', the crystal- 

 line lens, and the vitreous humor. The aqueous humor is a 

 watery fluid occupying the cavity between the cornea and 



