196 THE HUMAN BODY 



mathematically analyzed into simple vibrations, and if the piano 

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

 be exactly those which answer to these vibrations and to no 

 others. We thus get experimental grounds for believing that com- 

 pound 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 components; and that the 

 difference 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. 



The Functions of the Tympanic Membrane. If a stretched 

 membrane, such as a drumhead, 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 vibration; if they do 

 not push the membrane at the proper times, their effects will 

 neutralize one another: hence such membranes respond well to 

 only one note. The tympanic membrane, however, responds 

 equally well to a large number of notes ; at the least for those due 

 to aerial vibrations of rates from 60 to 4,000 per second, running 

 over eight octaves and constituting those commonly used in 

 music. This faculty depends on two things: (1) the membrane is 

 comparatively loosely and not uniformly stretched ; (2) it is loaded 

 by the tympanic bones. 



The drum-membrane is a shallow funnel with its sides convex 

 towards the external auditory meatus; something like an umbrella 

 turned inside out ; in such a membrane the tension is not uniform 

 but increases towards the center, and it has accordingly no proper 

 note of its own. Further, whatever tendency such a membrane 

 may have to vibrate rather at one rate than another, is almost com- 

 pletely removed by "damping" it, i.e. placing in contact with it 

 something comparatively heavy and which has to be moved when 

 the membrane vibrates. This is effected by the tympanic bones, 

 fixed to the drum-membrane by the handle of the malleus. An- 



