808 



THE SENSORIAL FUNCTIONS. 



§ 3. Comjjarative Physiology of Heariiig. 



The structure of the organs of hearing in the lower ani- 

 mals presents a regular gradation from the simple vestibule, 

 with its membranous sac, supplied with nervous filaments, 

 which may be regarded as the only essential part of this or- 

 gan, through the successive additions of semicircular canals, 

 fenestra ovalis, tympanic cavity, ossicula, ear-drum, meatus 

 auditorius, cochlea, and concha, till we arrive at the combi- 

 nation of all these parts in the higher orders of the Mam- 

 malia. The simpler forms are generally met with in aqua- 

 tic animals, probably because the sonorous undulations of 

 water are communicated more readily, and with greater 

 force, than tiiose of air, and require no accessory apparatus 

 for their concentration. The loftster, for instance, has a ves- 

 tibular cavity (seen at v, in Fig. 399,) containing a membra- 

 nous sac, with a striated groove (g,)* and receiving the fila- 

 ments of the auditory nerve. This vestibule is protected 

 by tho shell on all sides, except at one part, where it is 

 closed only by a membrane (e,) which may therefore be 

 considered as corresponding to the fenestra ovalis. The 

 outer side of this membrane in the Jlstaciis Jluviatilis, or 

 cray-fish, is seen at f in Fig. 401; while Fig. 402, shows an 



ear-drum be deslroyed, and the ossicula lost, an almost total deafness g-ene- 

 rally ensues. After a time, however, the hearing- may be in a great measure 

 recovered, with an undiminished power of distinguishing musical tones. See 

 two papers by Sir Astlcy Cooper, in the Phil. Trans, for 1800, p. 151; and 

 for 1801, p. 437. 



♦ This g-roove is represented magnified in Fig. 400. 



