202 SPECIAL SENSES. 



by influence, when it is brought in accord with a given 

 tone. In other words, this membrane obeys the laws of 

 consonance, and vibrates strongly by the influence of sounds 

 in unison or in harmony with its fundamental tone. The 

 laws of vibrations by influence have been fully discussed in 

 the preceding chapter ; 1 and it remains for us now to deter- 

 mine how far these laws are applicable to the physiology of 

 the vibrations of the membrana tympani, and the action of 

 these vibrations in the accurate perception of musical sounds. 



There are certain phenomena of vibration of the mem- 

 brana tympani that must occur, as a physical necessity, under 

 favorable conditions, which it is important to note in this 

 connection ; and these have hardly attracted sufficient atten- 

 tion at the hands of physiological writers. In the first place, 

 this membrane must obey the laws of vibration by influence. 

 It is undoubtedly thrown into vibration by irregular waves 

 of noise, as contradistinguished from musical tones; but 

 when a tone is sounded in unison with its fundamental 

 tone, or when the tone sounded is one of the octaves of its 

 fundamental, it must undergo a vibration by influence, like 

 an artificial membrane. If we suppose the membrane to be 

 tuned in unison with a certain tone, it will not only return 

 this tone by influence, but will repeat its quality. Not only 

 this, when a combination of harmonious tones is soimded, 

 the combined sound will be returned, with all the shades in 

 quality which the combined tones produce. On account of 

 its small size, the sound produced by the exposed membrane 

 itself cannot be heard ; but that the membrane does vibrate 

 by influence, has been proven by experiments with small par- 

 ticles of sand on its surface. 



We are certainly justified in supposing that vibrations of 

 the membrana tympani, too delicate to be revealed to the 

 eye or the ear in objective experiments, may be appreciated 

 by the auditory nerves as a subjective phenomenon. In other 

 words, we can probably appreciate vibrations in our own tym- 



1 See page 192. 



