236 ELEMENTS OF MAMMALIAN ANATOMY. 



This portion is called the tapetum. It causes the shining 

 appearance 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 probably 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 cephalic 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. 106). 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 

 crystalline lens, and the vitreous humor. The aqueous 

 humor is a watery fluid occupying the anterior chamber 



