224 COMPARATIVE ANATOMY 



obliterated. The several portions of the sensory plate or macula 

 acustica, which are originally continuous, become later disconnected 

 from one another, and except in Cyclostomes are seen as separate 

 maculae acusticse. 



The higher we pass in the Vertebrate series, the greater share 

 Joes the mesoblast take in the formation of the auditory organ. 

 At first that is, in Fishes the membranous labyrinth or internal 

 ear lies close under the roof of the skull, and is thus easily 

 accessible to the waves of sound, which are conducted partly 

 through the operculum (when present), and partly through the 

 gill-slits or spiracle. As we pass to the higher animals, however, 

 the auditory organ gradually sinks further and further inwards from 

 the surface, so that a new method for conducting the sound-waves 

 becomes necessary, and certain accessory structures are developed 

 (Fig. 179). A canal, the external auditory passage or meatus, passes 

 .inwards from the surface ; this opens into a spacious chamber, the 

 tympanic cavity, in which are situated the auditory ossicles, and 

 which is connected by the Eustacliian tube with the pharynx. The 

 whole of this canal, which is divided into outer and inner portions 

 (external and middle ear) at the junction of the external auditory 

 passage and tympanic cavity by a vibratory membrane, the tympanic 

 membrane, lies in the position of the first embryonic visceral 

 (hyoid or spiracular) cleft. From Reptiles and Birds onwards the 

 first indications of a pinna (that is, the part of the external ear 

 which projects from the head) are seen, but this only reaches a 

 full development in Mammals. 



Cyclostomes. In Petromyzon there are only two (the vertical) 

 semicircular canals, and in Myxine only one canal is present, which, 

 as it possesses two ampul Ia3, probably represents the two fused 

 together (Fig. 180A). 



Fishes and Dipnoans, The auditory organ of all the true 

 Fishes (Fig. ISOA^c) follows the general plan given above, and the 

 same may be said for all higher Vertebrates. Almost without 

 exception we meet with a division into a pars superior represented 

 by the utriculus and semicircular canals, which remains essentially 

 much in the condition already described, and a pars inferior 

 constituted by the sacculus and lagena, which gradually becomes 

 more differentiated, and attains to a higher and higher degree of 

 development and functional perfection. In Fishes the lagena 

 consists simply of a small knob-like appendage of the sacculus, 

 which opens freely into the main cavity of the latter by means 

 of the sacculo-cochlear canal : it is absent in Chimsera. The 

 utriculus and sacculus also communicate with one another by 

 the sacculo-utricular canal. In Elasmobranchs the ductus endo- 

 lymphaticus opens dorrally on the posterior part of the head, and is 

 thus in free communication with the sea- water. 



