THE EAR. 



389 



V. THE EAR. 



(Re-written by the translator from Das Gehiirorgan der Wirleltbiere, 

 by G. Retzius, 1881.) 



The organ of hearing- is divisible into two parts^ the tympanum 

 or middle ear^ and the labyrinth or internal ear ; an external ear is 

 absent^ unless a very slight depression of the tympanic membrane 

 be regarded as such. 



A. The tympanum [Cavum timpani) is a cavity, bounded ex- 

 ternally by the tymj^anic membrane and internally by the capsule 

 of the internal ear ; it communicates by means of the Eustachian 

 tube (tuba Eustachii) with the pharyngo-oral cavity (Fig, 178). 



a. The tympanic membrane lies immediately underneath the 

 skin, but can be separated from that structure; externally it 



Fig. 242. 



possesses a very slight depression, the only 

 trace of an external ear fomid in the frog. 



The tympanic membrane is of a rounded 

 oval form, being* a little wider in the trans- 

 verse than in the longitudinal diameter; the 

 membrane is directed outwards. 



After detaching the skin (Fig. 243 I) the 

 membrane is seen to be attached by its circtmi- 

 ference to a ring of cartilage [Â?inuli!S mem- 

 hranae tympani ; see also p. 26) {at) ; the ring 

 is attached anteriorly and above to the squa- 

 mosal bone {s<i), and in the rest of its circum- 

 ference to the surrounding soft parts, /. e. the 

 M. dep-essor maxillae {(hu), the M. temporalis 

 (/), and their fasciae. In the middle of the 

 membrane is a small, rounded, white part {col) 

 which can be traced backwards and upwards 

 towards the circumference. 



The membrane consists of fibres of con- 

 nective-tissue which radiate peripherally from 

 the central point of attachment of the colu- 

 mella; in the peripheral portion unstriated muscular fibres are 

 also found (Ley dig). 



Internally the tympanic membrane is covered by columnar epi- 



I. The tympanic membrane 



of Rana esciilenia ; 



natural size. After 



Retzius. 

 II. The tymixmum as seen 



after removing tlie 



tympanic membrane ; 



natural size. After 



Retzius. 

 Annuliis membranae 

 tympani. 

 cdl Columella. 

 ct Fissure-like cavity. 

 rim M. depressor maxillae. 

 mt Tympanic membi-ane. 

 .«/ Squamosal. 

 t M. temporalis. 



at 



