738 TEXT-BOOK OF PHYSIOLOGY 



The Functions of the Internal Ear. From the anatomic arrange- 

 ment of the structures of the internal ear it is evident that if the vibrations 

 of the stapes bone are to reach the peripheral organs the hair cells of 

 both the vestibular and cochlear nerves, they must traverse successively 

 the perilymph, the membranous walls, and the endolymph. As the perilymph 

 is incompressible, the inward movement of the stapes would be prevented 

 were it not for the elastic character of the membrane closing the foramen 

 rotundum. The pressure wave occasioned by each inward movement of the 

 stapes is transmitted through the scala vestibuli, the helicotrema, and the 

 scala tympani, to this membrane, which by virtue of its elasticity is pressed 

 into the tympanic cavity. With the outward movement of the stapes, 

 equilibrium is at once restored. 



The Functions of the Cochlea. The cochlea is the portion of the in- 

 ternal ear which is concerned in the perception of tones. The arrangement 

 of the histologic elements of the organ of Corti indicates that they in some 

 way respond to the vibrations of varying frequency and form, and through 

 the development of nerve impulses, evoke the sensations of pitch and quality. 

 The manner in which this is accomplished is largely a matter of speculation. 

 While many theories have been offered in explanation of the power to distin- 

 guish the pitch and the quality or timbre of a tone, most physiologists prefer 

 that of Helmholtz, who regarded the transverse fibers of the basilar mem- 

 brane as the elements immediately concerned, and compared them, both in 

 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 



