1910.] Cartilages of the External Ear; the Tymqmnic Cavity. 125 



III. Evidence from Comparative Anatomy and Ontogeny on the 

 ORIGIN of the Middle and Outer Ear. 



The Cartilages of tJte External Ear. 



Numerous attempts have been made, so Kingsley informs us (1900, 

 ]). 253), to derive the cartilages of the external ear of mammals from "opercu- 

 lar structures like those of the teleostomes," and more especially from branchi- 

 ostegal rays. But the gap between mammals and fishes is so extremely wide 

 that it is no wonder these attempted homologies have never proved convinc- 

 ing. Huge however (1S9S) showed that in the Monotremes the cartilaginous 

 passage of the external ear is continuous with the dorsal portion of the hyoid 

 arch, from which, he inferred, the ear-cartilages must have been derived. 

 The figures given by Versluys (1899) show that in many lizards the proximal 

 portion of the hyoid arch is attached to the skull and to the extracolumella, 

 or tympanic portion of the columella auris, just above the external auditory 

 passage, where it would be in a position to support an ear conch if such were 

 developed. In Sphenodon (Howes and Swinnerton, 1901, pi. IV, figs. 4-9) 

 the hyoid is attached to the extra-columella on the outer border of the skull. 

 It may be remarked however that Ruge did not demonstrate that the cartilage 

 of the ear conch in Monotremes was a derivative of the hyoid cornu, since 

 he dealt only with structures already formed. Parker (1886, p. 270) found 

 that in developing Marsupials "a folded cartilage protects the Eustachian 

 tube [ef. Weber, 1904, p. 145], and outside the former the meatus externus 

 is protected by a more or less segmented tube of cartilage, which ends outside 

 in the continuous concha auris." The possibility is thus suggested that the 

 ear conch is serially homologous with these rings, which in turn appear to be 

 neomorphs, perhaps analogous to the rings of the trachea. The develop- 

 ment of a cartilaginous ear conch, through the hypertrophy either of the 

 proximal end of the hyoid arch or of the rings of the meatus, was at any rate 

 only one incident in a remarkable series of improvements and changes of 

 function in the accessory auditory apparatus, whereby the relatively simple 

 mechanism of reptiles gave rise to the intricate and delicately adjusted 

 mechanism of mammals. 



The tympanic cavity. 



The external auditory meatus and the Eustachian tube, which together 

 form the tubo-tympanal canal, are generally considered to be the homologue 

 of the first gill cleft. When the tubo-tympanal canal first appears in the 



