550 THE HUMAN BODY. 



strings set in movement by it be examined, they will be 

 found to be exactly those which answer to these pendular 

 vibrations and to no others. We thus get experimental 

 grounds for believing that compound tones are really made 

 up of a number of simple vibrations, and get an additional 

 justification for the supposition that in the ear each note is 

 analyzed into its pendular components; and that the differ- 

 ence of sensation which we call timbre is due to the effect of 

 the secondary partial tones thus perceived. If so, the 

 ear must have in it an apparatus adapted for sympathetic 

 resonance. 



It may be asked why, if the ear analyzes vibrations in 

 this way, do we not commonly perceive it? How is it that 

 what we ordinarily hear is the timbre of a given tone and not 

 the separate upper partials which give it this character ? The 

 explanation is more psychological than physiological, and 

 belongs to the same category as the reason why we do not 

 ordinarily notice the blind spot in the eye, or the double- 

 ness of objects out of the horopter, or the duplicity of 

 stereoscopic images. We only use our senses in daily life 

 when they can tell us something that may be useful to us, 

 and we neglect so habitually all sensations which would be 

 useless or confusing, that at last it needs special attention 

 to observe them at all. The way in which tones are com- 

 bined to give timbre to a note is a matter of no importance 

 in the daily use of them, and so we fail entirely to observe 

 the components and note only the resultant, until we make 

 a careful and scientific examination of our sensations. 



The Functions of the Tympanic Membrane. If a 

 stretched membrane, such as a drum-head, be struck, it will 

 be thrown into periodic vibration and emit for a time a note 

 of a determined pitch. The smaller the membrane and the 

 tighter it is stretched the higher the pitch of its note; every 

 stretched membrane thus has a rate of its own at which it 

 tends to vibrate, just as a piano or violin string has. When 

 a note is sounded in the air near such a membrane, the 

 alternating waves of aerial condensation and rarefaction will 

 move it; and if the waves succeed at the vibrational rate of 

 the membrane the latter will be set in powerful sympathetic 



