420 THE SENSORIAL FUNCTIONS. 



pending on the force of the vibrations ; the 

 latter, on their frequency. These acoustic prin- 

 ciples are to be borne in mind in studying the 

 comparative physiology of hearing ; and since 

 the functions of the different parts of the organ 

 of this sense are, as yet, but imperfectly under- 

 stood, I shall, in treating of this subject, deviate 

 from the plan I have hitherto followed, and pre- 

 mise an account of the structure of the ear in its 

 most perfectly developed state, as it appears to 

 be in Man. 



§ 2. Physiology of Hearing in Man. 



That part of the organ of hearing, which, above 

 all others, is essential to the performance of this 

 function, is the acoustic nerve, of which the 

 fibres are expanded, and spread over the surface 

 of a fine membrane, placed in a situation 

 adapted to receive the full impression of the 

 sonorous undulations which are conveyed to 

 them. This membrane, then, witli its nervous 

 filaments, may be regarded as the immediate 

 organ of the sense ; all the other parts consti- 

 tuting merely an accessory apparatus, designed 

 to collect and to condense the vibrations of the 

 surrounding medium, and to direct their concen- 

 trated action on the auditory membrane. 



