THE SENSE OF HEARING. 687 



their arrangement and power of sympathetic vibration, with the strings of a 

 piano. He said : " If we could so connect every string of a piano with a nerve- 

 fiber that the nerve-fiber would be excited as often as the string vibrated, 

 then, as is actually the case in the ear, every musical note which affected the 

 instrument would excite a series of sensations exactly corresponding to the 

 pendulum-like vibrations into which the original movements of the air can 

 be resolved; and thus the existence of each individual overtone would be 

 exactly perceived, as is actually the case with the ear. The perception of 

 tones of different pitch would, under these circumstances, depend upon 

 different nerve-fibers, and hence would occur quite independently of each 

 other. Microscopic investigation 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 compound 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 0.04155 mm. at the base of the 

 cochlea to 0.495 mm - at tne apex, and, according to Retzius, are about 24,000 

 in number. As the human ear usually cannot distinguish more than 1 1 ,oco 

 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. 



