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 cochlee 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 (cartilago auricule), 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 (4); proximad the plate is rolled 
together in such a way that its edges (g and 4) overlap on the 
craniomedial side, thus enclosing a tube or funnel (¢) (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- 
