THE EAR. 



V. THE EAR. 



(Re-written by the translator from Das Gehororgan der Wirbelthiere, 

 byG. 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 verv slight depression of the tympanic membrane 

 be regarded as such. 



A. The tympanum (Cavum tyrnpani} is a cavity, bounded ex- 

 ternally by the tympanic 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 

 possesses a very slight depression, the only Fig. 24.'. 



trace of an external ear found 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. 242 I) the 

 membrane is seen to be attached by its circum- 

 ference to a ring of cartilage (Annvlus mem- 

 branae tytiipani; see also p. 26) (at); the ring 

 is attached anteriorly and above to the squa- 

 mosal bone (#^), and in the rest of its circum- 

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

 31. depressor maxillae (dm), the 31. temporal^ 

 (f), 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 Kana etculenta ; 



natural size. After 



Retzius. 

 II. The tympanum as seen 



after removing the 



tympanic membrane ; 



natural size. After 



Retzius. 

 at Annultis membranae 



tympani. 

 <).' Colnmella. 

 c/ Fissure-like cavity. 

 dm M. depressor maxillae. 

 ml Tympanic membrane. 

 tq Squamoeal. 

 t M. temporally. 



