648 TEXT-BOOK OF PHYSIOLOGY. 



1. By closing the mouth and nose and then forcing air from the 



lungs through the Eustachian tube into the tympanum, thus in- 

 creasing the pressure. 



2. By closing the mouth and nose and then making an effort of 



deglutition. As this act is attended by an opening of the phar- 

 yngeal end of the Eustachian tube, the air in the tympanum is 

 partly withdrawn and the pressure lowered. In each instance 

 hearing is impaired. After either experiment the normal con- 

 dition is restored by swallowing with the nasal passages open. 

 The Functions of the Internal Ear. From the anatomic 

 arrangement 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 move- 

 ment 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, 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 internal 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 im- 

 pulses, 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 

 distinguish the pitch and the quality or timbre of a tone, most physiol- 

 ogists prefer that of Helmholtz, who regarded the transverse fibers 

 of the basilar membrane as the elements immediately concerned, 

 and compared them, both in their arrangement and power of sympa- 

 thetic 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 in- 



