THE EAR AND HEARING. 571 



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 doubleness 

 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 combined 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 alter- 

 nating 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 4000 per second, running over eight octaves and 

 constituting those commonly used in music. This faculty 

 depends on two things: (I) 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 con- 

 vex 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 centre, and it has accord- 

 ingly no proper note of its own. Further, whatever tendency 

 such a membrane may have to vibrate rather at one rate than 



