THE NERVOUS SYSTEM 249 



tion with the cornea, and is inserted into the cranial part of 

 the choroid coat. 



The inner membrane of the eye is the rdma, 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. 119). 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 refracting media of the eye are the aqueous humor, the 

 crystalline lens, and the vitreous body. The aqueous humor is 

 a watery fluid occupying the cavity between the cornea and 

 crystalline lens. It always escapes as soon as the cornea is 

 punctured. The aqueous humor appears to be supplied by 

 the ciliary processes. The vitreous body is of a jelly-like con- 

 sistency, filling the part of the eyeball caudad of the lens. It 

 is perfectly transparent and is surrounded by a delicate cap- 

 sule, the hyaloid membrane. 



The crystalline lens is a transparent biconvex tissue having 

 a vertical diameter of about one centimeter and a shorter 

 diameter through its optical axis. It is enclosed in a trans- 

 parent elastic capsule, some of whose fibers are continued 

 peripherad as the suspensory ligament which is inserted in the 

 choroid coat (Fig. 119). 



The Auditory Organ. — The organ of hearing is composed of. 

 three parts — the external ear, middle ear, and internal ear. 

 The first consists of the pinna and the auditorius meatus exter- 

 nus. The auricle is the projecting portion of the ear cap- 

 able of being moved by muscles, and is composed of integument 

 strengthened by fibrocartilage. The auditorius meatus externus 

 extends from the base of the auricle to the tympanic membrane 



