AUDITORY ORGANS. 



I8 7 



and may have to do with the recognition of sound waves of different 

 rapidity. It has recently been shown that the membrana tectoria is 

 connected with -the hairs of the hair cells. The fact that in birds, where 

 pitch is certainly recognized, there is no organ of Corti, renders 

 all speculation doubtful. 



he 



FIG. 187. Organ of Corti of guinea pig, after Schneider, d, Deiter's cells; he, Henson's 

 cells; ih, inner hair cells; ip, inner pillar cells; Is, limbus spiralis; mt, membrana tectoria; n, 

 nerve fibres; oh, outer hair cells; op, outer pillar cells; si, inner sulcus; st, scala tympani; t, 

 tunnel; In, tunnel nerve. 



The Middle Ear or tympanum first appears in the anura. It con- 

 sists of a cavity (cavum tympani) in front of and below the otic 

 capsule, connected by a slender duct, the Eustachian tube, with the 

 pharynx. Externally it is separated from the outer world by a thin 

 partition, the tympanic membrane, from which a chain of bones, the 

 ossicula auditus (p. 73), extends across the cavity to the fenestra ovale, 

 and serves to transmit the sound waves to the inner ear. The tympanic 

 cavity is the homologue of the spiracular cleft of the elasmobranchs 

 (see respiration), which never breaks through. The tympanic mem- 

 brane, covered externally with ectoderm, on the inner surface with 

 entoderm, represents the imperforate wall of the cleft, while the Eusta- 

 chian tube is the narrowed internal end of the spiracle. The chain of 

 ear bones has already been described. It is to be noted that the 

 chain consists of columella and stapes in anura and sauropsida, 

 while in the mammals columella is replaced by incus and malleus. 

 In the urodeles and gymnophiones, where no tympanic cavity is devel- 

 oped, the quadrate articulates with the stapes. 



The External Ear. In the anura and in many reptiles the tym- 

 panic membrane is flush with the surface of the head, but in other rep- 

 tiles and in birds it is at the bottom of a canal, the external auditory 

 meatus, the simplest expression of an external ear. In the mammals 



