324 THE SENSES 







organ of Corti therein. That is to say, loss of the sense of 

 hearing supervenes upon destruction of this part of the in- 

 ternal ear. In physics it is known that for a sound, for ex- 

 ample of a piano string, to be heard the membrana tympani 

 must vibrate in unison with the sonorous vibrations of the 

 cord; that is, "consonating bodies" repeat sonorous vibra- 

 tions, giving them their proper pitch and quality. It has 

 been supposed that the thousands of rods of Corti, of vary- 

 ing length and size, in the cochlea are made to vibrate separ- 

 ately 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 differ- 

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

 be true, but its correctness is probably beyond the range of 

 experimental 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 

 Eustachian tube. Nor is the integrity of the membrana tym- 

 pani actually necessary to the production of sound ; although 

 practically speaking a person in whom this organ is de- 

 stroyed 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. Indeed it has already been seen 

 that none of the parts of the external or middle ear are actu- 

 ally necessary to hearing. They are only accessory conveni- 

 ences for the better transmission of impressions to the fila- 

 ments 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 intensity of the sound. The stapedius pre- 

 vents too great movements of the stapes. The free com- 

 munication of the air in the tympanum with that in the mas- 

 toid cells and pharynx insures an approximately constant 



