THE SENSE OF HEARING. 649 



vestigation shows that there are somewhat similar structures in 

 the ear. The free ends of all the nerve-fibers are connected with 

 small elastic particles which we must assume are set into sympathetic 

 vibration by sound-waves." (Stirling.) 



The mechanism might be regarded, therefore, somewhat as 

 follows : The sound-waves received by the membrana tympani and 

 transmitted by the chain of bones to the fenestra ovalis produce 

 variable pressures in the fluids of the internal ear; these pressures 

 vary in intensity, in number, and in quality, and correspond with 

 the intensity, pitch, and quality of the tones. If, therefore, a com- 

 pound wave of pressure be communicated by the base of the stapes, 

 it will be resolved into its constituents by the different transverse 

 fibers of the basilar membrane, each picking out its peculiar portion of 

 the wave and thus stimulating corresponding nerve filaments. Thus 

 different nerve impulses are transmitted to the brain, where they are 

 fused in such a manner as to give rise to a sensation of a particular 

 quality, but still so imperfectly fused that each constituent, by a 

 strong effort of attention, may be still recognized. The transverse 

 fibers of the basilar membrane vary in length from 0.04125 mm. at 

 the base of the cochlea to 0.495 mm - a ^ tne apex, and, according to 

 Retzius, are about 24,000 in number. As the human ear usually 

 cannot distinguish more than 11,064 tones, it is evident that there 

 is a sufficient anatomic basis for this theory. 



The functions of the semicircular canals have already been 

 stated in connection with the chapter relating to the functions of the 

 cerebellum. 



