THE EAR. 415 



than the part in front of the lens. It is lined chiefly by the 

 retina, its front boundary being the lens. It contains a jelly- 

 like substance, the vitreous humor (or vitreous body, corpus 

 vitreum). 



II. THE EAR. ORGANON AUDITUS. 



The ear of the cat, like that of man, is composed of three 

 parts, the external, middle, and internal ear. 



The external ear is limited internally by the tympanic 

 membrane. 



The middle ear extends from the tympanic membrane to 

 the fenestra cochleae and fenestra vestibuli. It contains the 

 bones of the ear, and is connected with the pharynx by the 

 Eustachian tube. 



The internal ear is wholly contained within the petrous 

 bone, and on the surface of the membrane lining its cavities 

 are distributed the branches of the auditory (eighth) nerve. 



The External Ear. 



The external ear consists of an expanded portion, the 

 auricle (auricula) or pinna, and of a canal leading from the 

 auricle to the tympanic membrane, known as the external 

 auditory meatus. The concave surface of the auricle is 

 directed craniolaterad, and its edges form the dorsal and lateral 

 boundaries of the auditory opening. From the bottom of the 

 cavity which it partly encloses, the external auditory meatus 

 passes medioventrad. 



The auricle (Fig. 168) is composed of a thin curved plate 

 of cartilage (cartilage auriculae), to which numerous muscles 

 are attached and which is covered on both sides by integument 

 intimately attached to the cartilage. Distad the edges of the 

 cartilage form a free border (^); proximad the plate is rolled 

 together in such a way that its edges (g and It) overlap on the 

 craniomedial side, thus enclosing a tube or funnel (e) (concha) 

 which forms the enlarged outer portion of the auditory passage. 



Within the cavity of the auricle, on its medial surface, 

 somewhat below the level of the lowest portion of the margin 

 of the auditory opening is seen a very prominent pedun- 



