228 PHYSIOLOGY FOR BEGINNERS 



" quality " the same note struck on a piano and on a violin 

 seems to us in some way different. This difference is due to 

 the fact that very many musical sounds consist not of one set 

 of vibrations only, but of several, of a main set of vibrations 

 called the fundamental tone, and of others, and these vary, 

 called partial tones or overtones. 



The Transmission of Vibrations in the Ear. 

 Sound waves are collected by the external ear, pass along 

 the external auditory canal, and striking on to the tympanic 

 membrane, set this vibrating at a corresponding rate and 

 intensity. The membrane in its vibration carries with it the 

 handle of the malleus, and this leads to a similar movement 

 of the long process of the incus, and so of the stapes, which 

 this carries. The movement of the stapes sets vibrating the 

 membrane which, with the foot-plate of the stapes, closes the 

 fenestra ovalis. The to-and-fro movement of this membrane 

 sets up vibrations, which travel through the perilymph, sur- 

 rounding the vestibule, semicircular canals, and canal of the 

 cochlea, till they strike finally the membrane covering the 

 fenestra rotunda. The vibrations in the perilymph are trans- 

 mitted through the thin walls of the vestibule, semicircular 

 canals, and canal of the cochlea, and lead to vibrations of the 

 endolymph. These affect the cells of the auditory epithelium 

 in such a way as to give rise to nervous impulses in the fibres 

 of the auditory nerve, and these impulses, on reaching the 

 brain, excite in us the sensation of sound. It is undoubtedly 

 the hair cells of the organ of Corti in the cochlea which are 

 affected when we hear ; whether we also hear by means of the 

 auditory epithelium of the patches on the semicircular canals 

 and vestibule is not so clear. 



The lowest note that most persons can hear is one of thirty 

 vibrations a second. The highest note varies very much. 

 Some persons can hear a note of 30,000 vibrations a second, 

 while others cannot hear the squeak of a bat or mouse or the 

 chirp of a sparrow, which are much lower notes. Very likely 

 many insects make sounds which are not heard by us. 



