248 ELEMENTS OF MAMMALIAN ANATOMY 



by cutting away the lateral and dorsal walls of the orbital 

 cavity. The retractor oculi originates on the boundary of the 

 optic foramen and divides into four heads which are inserted 

 into the sclerotic around the entrance of the optic nerve. 

 This muscle is partly hidden by the recti muscles. 



The eyeball is composed of three membranes and three 

 humors. The outer coat, the sclerotic, consists of the opaque 

 portion forming the caudal two-thirds, and the transparent 

 portion, or cornea, forming the remainder (Fig. 119). The 

 optic nerve pierces the sclerotic a little mediad of the longi- 

 tudinal axis of the eye. The surface of the cornea is more 

 strongly curved than that of the opaque portion of the sclerotic, 

 and contains no blood-vessels. The second or middle coat of 

 the eye is formed by the choroid membrane and its extension, 

 the iris. This coat is incomplete, as there is an aperture, the 

 pupil, through the iris for the admission of light. The choroid 

 appears as a jet-black membrane, less than half as thick as 

 the sclerotic, lying closely appressed to the latter. The choroid 

 is lined internally 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 appear- 

 ance 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 junc- 



