388 



THE SENSORIAL FUNCTIONS. 



improbable supposition that the return into the ves- 

 tibule, of undulations which have passed through 

 these canals, has the effect of putting a stop to all 

 further motion of the fluid ; thus preventing all 

 interferences among successive undulations, and 

 preserving the distinctness of each impression made 

 on the nerves. The same use may be assigned to 

 the double spiral convolutions of the tubes of the 

 cochlea : for the undulations of the fluid in the 

 tympanic tube, received from the membrane of the 

 fenestra rotunda, will meet those proceeding along 

 the vestibular tube, derived from the membrane of 

 the fenestra ovalis, and like two opposing waves, will 

 tend to destroy one another. Thus each external 

 sound will produce but a single momentary impres- 

 sion ; the continuance of the undulations of the 

 fluid of the labyrinth being prevented by their 

 mutual collision and consequent destruction.* 



Sounds may produce impressions on the auditory 

 nerves by vibrations communicated directly to them 

 through the solid bones of the skull ; and indepen- 

 dently of the tympanic apparatus. This is shown 



animals in whom these canals are highly developed possess the 

 greatest power of" distinguishing the direction of sounds. The same 

 view of the subject is taken by Professor Wheatstone. Todd, 

 Cyclopsedia of Anat. and Physiology, ii. 570. 



* 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 re- 

 covered, with an undiminished power of distinguishing musical tones. 

 See two papers by Sir Astley Cooper, in the Phil. Trans, for 1800, 

 p. 151 ; and for 1801, p. 437. 



