AUDITORY ORGAN 



301 



Especially in Crocodiles and Birds, in which the two Eustachian 

 canals open by a single median aperture into the pharynx, 1 the 

 tympanic cavity is complicated, and is continued into pneumatic 

 cavities in the neighbouring bones. The osseo-cartilaginous 

 columella is well developed in most of the Sauropsida, and is not 

 distinct from the stapedial plate ; in Hatteria it is continuous 

 distally with the hyoid (p. 115). 



In most Lizards the .tympanic membrane is on a level with the 

 skin, but in certain forms (e.g. Ascalabota, Lacerta, Monitor) an 

 indication of the develop- 

 ment of an external audi- 

 tory passage is seen, the 

 tympanic membrane being 

 partially covered posteri- 

 orly by a small fold of 



skin, usually enclosing the 



-s^i 



ma 



mu 



rec 



7RS 



OF Turdus musieus, from the inner side. 

 (After G. Retzius.) Letters as in previous 

 Figures. 



anterior border of the di- 

 gastric muscle : in Croco- 

 diles there is a definite in- 

 tegumentary valve moved 

 by muscles (abductor of 

 the mandible, supplied by 

 the facial nerve) and en- 

 closing a dermal bone, and Fia 2 _20. RIGHT MEMBRANOUS LABYRINTH 

 in certain Birds also (e.g. 

 Owls) there is a movable 

 valve. The tympanic 

 membrane in Birds is situated some distance from the surface in 

 an external auditory passage, and is stretched on a ring formed 

 by several bones of the skull. 



Mammals. The auditory organ of Mammals reaches a much 

 higher stage of development, but in Monotremes it shows many 

 points of resemblance to that of Reptiles and Birds. 



The cochlea now reaches its highest development, and forms 

 a long tube which becomes spirally coiled on itself (Figs. 221 

 and 222).' 2 In this, as well as in the more highly-specialised 

 histological structure of the cochlea, lies the characteristic peculi- 

 arity of the auditory organ of Mammals. The auditory nerve 

 forms the axis of the spiral. 



In consequence of this development of the cochlea, the papilla 

 basilans acustica, or, as it is called in Mammals, the organ of Corti, 



i In Crocodiles this aperture leads into three canals, a median and a paired, 

 all of which communicate with the tympanic cavity in a complicated manner. 



- In Man it forms nearly three coils, and in other Mammals from one and a 

 halt (Cetacea) up to as many as four or more. Thus in the Rabbit there are two 

 and a half, in the Ox three and a half, in the Pig almost four, arid in the Cat 

 three coils. Ihe cochlea, as well as the sacculus and all parts of the pars superior 

 of the membranous labyrinth, vary considerably both in form and arrangement ill 

 the various types. 



