468 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Vertebrates, now useless visceral clefts. While on its 

 outer side this cleft gives rise to the tympanic cavity 

 (Fig. 197 ; c) and part of the external meatus, on its 

 inner side it continues to communicate with the 

 pharynx, and so forms the Eustachian tube (e,u), 

 by means of which air can enter into the otherwise 

 closed ear chamber. The vibrations of the tympanic 

 membrane have to be conveyed to the inner ear, and 

 this is effected by one or more bones, the outer end of 

 which is fixed to the tympanic membrane, while the 

 other impinges on the membrane which closes the en- 

 trance to the internal ear (fenestra, ovalis, Fig. 197). 

 In the Amphibia and Sauropsida there is but a single 

 bone in the middle ear ; the origin of this columella 

 is not satisfactorily established, but it is, probably, the 

 upper end of the hyoid arch. (See page 327.) In the 

 Mammalia this single bone is replaced by three, 

 which are known respectively as the stapes, incus, 

 and malleus ; the discussion of the homologies of 

 these bones has been deep and protracted, but a con- 

 sideration of it would be beyond our scope here. We 

 must be content to say that, according to the latest 

 views of Parker, the stapes is the uppermost element 

 of the hyoid, and that the incus and malleus belong to 

 the mandibular arch. According to these views it is 

 the incus, and not, as is ordinarily taught, the malleus, 

 that is the homologue of the quadrate bone of the 

 Sauropsida. In the Prototheria the stapes is not 

 hollowed out at one end so as to have the form of a 

 stirrup, nor is the incus of the " normal mammalian 

 form " ; Parker further finds that the malleus still 

 forms the hinder part of the mandible. 



We have in this history of the auditory ossicles 

 (which must not by any means be confounded with 

 otoliths) one of the most remarkable examples of the 

 way in which parts useless for one are gradually ap- 

 propriated to another function. The cleft between 



