254 LIFE : OUTLINES OF GENERAL BIOLOGY 



is, it sets up a series of vibrations in the air. These vibrations have 

 a certain violence or ampHtude, determining the loudness of the 

 sound. Further, they have a definite tone or pitcli, the wave-length 

 of the vibrations, which is determined bj' the quahty, thickness, 

 and length of the string. A similar string of half the length will 

 emit vibrations twice as rapid, that is of half the wave-length, and 

 we say that the note produced is an octave higher than the first 

 one. But a plucked string vibrates not only as a whole, producing 

 the characteristic or fundamental tone, but vibrates also and 

 simultaneously as two halves, and three thirds, and so forth, pro- 

 ducing simultaneously the "overtones", an octave, an octave and a 

 fifth, two octaves, and so on, above the fundamental note. Different 

 means of producing sound differ in the strength of the different 

 overtones produced; and it is these differences, in the distribution 

 of emphasis among the overtones, that give to each musical instru- 

 ment, or to any heard note, its characteristic timbre or quahty. The 

 differences between the various vowels of speech are primarily 

 differences of timbre, that is, differences in the number and strength 

 of the accompanying overtones. A noise, as distinct from a note, is 

 a jumble of vibrations of different pitch and timbre. 



The outer ear in mammals consists of the externally visible flap 

 or pinna, which is probably almost useless in man. but in many other 

 mammals serves as a funnel to collect sound and, being movable, 

 to locate sounds. From the pinna a passage leads through the wall 

 of the skull, and is closed at the inner end by the delicate "drum" 

 of the ear, which is thrown into vibrations by sound-waves. 



The middle ear acts as an amplifier, to magnify the vibrations 

 received by the drum. The vibrations are carried from the drum 

 by a chain of delicately poised little bones, called the hammer, the 

 anvil, and the stirrup, to a similar delicate membrane which closes 

 off the inner ear. As this inner membrane, the "oval window", is 

 much smaller than the drum of the outer ear, the vibrations are 

 concentrated or strengthened in passing from the one to the other, 

 in spite of a certain loss of energy in friction. The middle ear itself 

 is merely a cavity, bridged by this chain of little bony levers; the 

 cavity communicates with the phar\'nx by the Eustachian tube. 



Of the inner ear, the only part which has to do with the sense of 

 hearing is the cochlea, which is a tube coiled in a spiral. Within this 

 tube, which is full of liquid, are transverse membranes and a com- 

 plex arrangement of sensitive and supporting cells — too complex 

 to be easily described, especially since the meaning of the com- 

 plexity is unknowTi. The generally accepted theory of the nature of 

 the process of hearing is that it depends on the resonance of the fibres 

 of one of the membranes. This, the basilar membrane, consists of a 

 very large number of fibres, some twenty thousand or more. It is 

 supposed that these fibres are so graduated in size and shape that 



