766 PHYSIOLOGY. 



ments is really mechanical in nature, due to the wavelike motion of 

 the serous fluid of the membranous labyrinth. 



It is common to divide auditory stimuli into those which are 

 caused by noises and those caused by musical sounds. It is a feature 

 peculiar to musical sounds that the vibrations which form them are 

 periodical and that they recur at regular intervals. When neither 

 of these two conditions is present, there results a noise. From the 

 sensory impulses to which the several vibrations give rise are gen- 

 erated our sensations of noise or of sound. 



To produce a sensation certain conditions in the excitation of 

 the auditory nerve are necessary. 



The sound-wave must exist for a certain length of time ; it must 

 not be greater than Vso nor l ess than 1 Aoooo second. In the piano 

 the lowest base (C, 33 vibrations) and the highest treble (C, 4224 

 vibrations) exist. A certain number of impulses must be made 

 within a given interval of time to excite a sensation of tone. The 

 lower limit is about 30 vibrations, the upper limit about 40,000, per 

 second. Visual sensations separated by less than a tenth of a second 

 are fused, but auditory sensations separated by 1 / 133 second remain 

 distinct. 



Theory of Hearing. If you sing a note into a piano, the cords 

 of the piano tuned for this note only respond. Now the basilar 

 membrane is supposed, like a harp, to represent a series of cords 

 which, like the piano-strings, respond to the sounds striking them. 

 This membrana basilaris is striated in a radiating direction, and 

 these striations increase as it ascends toward the helicotrema. Un- 

 like the harp, the cords are joined together by their edges; but, as 

 they are stretched only in a radiating direction, they can vibrate as 

 though they were separate cords. Now, the cords are very short, 

 being at most not over 1 / 12 inch in length; so that they would be 

 expected only to vibrate for high sounds; but it must be remem- 

 bered that these cords are weighted with the arches and cells of 

 Corti, which lower their sound. Hence we have a series of cords in 

 the basilar membrane vibrating separately to musical sounds. We 

 know that there are in man about 3000 arches of Corti, and as at least 

 two of the cords correspond to an arch of Corti, we have 6000 cords. 

 Now, the scale of musical sounds extends to seven octaves, and we 

 have 400 arches of Corti to 1 octave. In 1 octave there are 12 semi- 

 tones, and we have 66 cords corresponding to a semitone; so that 

 we have sufficient cords to vibrate in unison with all possible musical 

 sounds. 



