FUNCTIONS OF THE COCHLEA 323 



vibrate separately or in correctly associated collections (like the 

 strings of a harp), and thus reproduce communicated vibrations, 

 and so give rise to impressions which, conveyed by the auditory 

 nerve to the center, are there recognized as sounds of different 

 degrees of intensity, pitch and quality. This theory may be 

 true, but its correctness is probably beyond the range of experi- 

 mental proof. 



While the usual mode of conduction of sound waves to the 

 cochlea is through the external ear, they may reach it in other 

 ways, as through the bones of the head, or through the Eustach- 

 ian tube. Nor is the integrity of the membrana tympani actually 

 necessary to the production of sound; although practically speak- 

 ing a person in whom this organ is destroyed is deaf, he can hear 

 if the ossicles can in some way be placed in vibration by sound 

 waves, as by the intervention of an artificial membrane. In- 

 deed it has already been seen that none of the parts of the ex- 

 ternal or middle ear are actually necessary to hearing. They 

 are only accessory conveniences for the better t.ansmission of 

 impressions to the filaments of the auditory nerve. 



The (so-called) tensor and laxator tympani muscles make 

 tense or lax the membrana tympani, thus influencing the rapidity 

 and amplitude of its vibrations, and therefore the pitch and in- 

 tensity of the sound. The stapedius prevents too great move- 

 ments of the stapes. The free communication of the air in the 

 tympanum with that in the mastoid cells and pharynx insures 

 an approximately constant internal pressure upon the membrane, 

 and thus precludes accidents which would otherwise interfere 

 with its proper vibration. 



The auditory center in man is in the first and second temporal 

 convolution of the temporo-sphenoidal lobe. 



Briefly then, the physiology of hearing is as follows: Sound 

 waves collected by the pinna enter the external auditory canal 

 and impinge upon the membrana tympani. The drum is thus set 

 to vibrating and communicates its movements to the ossicles, 



