686 TEXT-BOOK OF PHYSIOLOGY. 



inward, make it a fixed point, and place the anterior part in such a position 

 that it will vibrate freely and accurately. 



The Function of the Eustachian Tube. In order that the tympanic 

 membrane may vibrate freely it is essential that the air pressure on both 

 sides shall be equal at all times. This is made possible by the Eustachian 

 tube. Were it not for this passageway, with each inward swing of the mem- 

 brane the air in the tympanic cavity would be condensed and its pressure 

 raised, in consequence of which the movement of the membrane would be 

 retarded; with each outward swing, the air would be rarefied and its pressure 

 lowered below that of the atmosphere, and in consequence the movement 

 outward would be retarded; the maximum response, therefore, of the mem- 

 brane to a given vibration could not be attained and the resulting sound 

 would be muffled and indistinct. But as with each vibration of the 

 membrane the air can pass into and out of the tympanum through this 

 partially closed tube, inequalities of pressure are prevented and a free 

 vibration permitted. 



The impairment in the acuteness of hearing which is caused by either 

 a rise or fall of pressure in the middle ear can be shown 



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



through the Eustachian tube into the tympanum, thus increasing 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 pharyngeal 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 Eustachian tube, the air in the tympanum is partly withdrawn and the 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 condition is restored by swallowing with 

 the nasal passages open. 



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 



