434 THE SENSORIAL FUNCTIONS. 



mentary impression ; the prolongation of the 

 undulations of the fluid of the labyrinth being 

 prevented by their mutual collision and neutral- 

 ization.* 



§ 3. Comparative Physiology of Hearing. 



The structure of the organs of hearing in the 

 lower animals 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 organ, 

 through the successive additions of semicircular 

 canals, fenestra oval is, tympanic cavity, ossicula, 

 ear-drum, meatus auditorius, cochlea, and con- 

 cha, till we arrive at the combination of all 

 these parts in the higher orders of the Mam- 

 malia. The simpler forms are generally met 



* The preliminary steps in the process above described are not 

 absolutely essential to hearing, for many instances have occurred 

 in which the power of hearing has been perfectly retained after 

 the membrane of the ear-drum, and also the ossicula had been 

 destroyed by disease. A small aperture in the membrane does 

 not interfere with its power of vibration ; but if the whole ear- 

 drum be destroyed, and the ossicula lost, an almost total deafness 

 generally ensues. After a time, however, the hearing may be in 

 a great measure recovered, with an undiminished power of dis- 

 tinguishing musical tones. See two papers by Sir Astley Cooper, 

 in the Phil. Trans, for 1800, p. 151 ; and for 1801, p. 437. 



I 



