HEAKING. 801 



into corresponding vibrations the so-called auditory hairs. In the cochlea 

 the vibrations of the perilymph are supposed to throw into vibrations the 

 basilar membrane with the superimposed organ of Corti, consisting of the 

 rods of Corti with the inner and outer hair-cells. (Fig. 197.) The vibra- 

 tions thus transmitted to these structures give rise to nervous impulses in 

 the terminations of the auditory nerves, and these impulses reaching certain 

 parts of the brain produce what we call auditory sensations. We are accus- 

 tomed to divide our auditory sensations into those caused by noises and those 

 caused by musical sounds. It is the characteristic of the latter that the 

 vibrations which constitute them are periodical ; they occur and recur at 

 regular intervals. When no marked periodicity is present in the vibrations, 

 when the repetition of the several vibrations is irregular, or the period so 

 complex as not to be readily appreciated, the sensation produced is that of 

 a noise. There is, however, no abrupt line between the two. Between a 

 pure and simple musical sound produced by a series of vibrations, each of 

 which has exactly the same wave-length, and a harsh noise in which no 

 consecutive vibrations may be alike, there are numerous intermediate stages. 



713. In both noises and musical sounds we recognize a character 

 which we call loudness. This is determined by the amplitude of the vi- 

 brations ; the greater the disturbance of the air (or other medium) the 

 louder the sound. In a musical sound we recognize also a character 

 which we call pitch. This is determined by the wave-length of the vi- 

 brations ; the shorter the wave-length, the larger the number of consec- 

 utive vibrations which fall upon the ear in a second, the higher the pitch. 

 We are able to speak of a whole series of tones or musical sounds of differ- 

 ent pitch, from the lowest to the highest audible tone. And even in many 

 noises we can, to a certain extent, recognize a pitch, indicating that among 

 the multifarious vibrations there is a periodicity of certain groups of vibra- 

 tions. 



714. Lastly, we distinguish musical sounds by their quality ; the same 

 note sounded on a piano and on a violin produce very different sensations, 

 even when a series of vibrations having in each case the same period of 

 repetition is set going. This arises from the fact that the musical sounds 

 generated by most musical instruments are not sftaple but compound vibra- 

 tions. When the note C in the treble, for instance, is struck on the piano, 

 and we analyze the total sound, we find that it can be resolved partly into 

 a series of vibrations with a period characteristic of the pure tone of the 

 treble C, and partly into other series of vibrations with periods character- 

 istic of the C in the octave above, of the G above that, of the C in the next 

 octave, and of the E above that. And the sensation which we associate with 

 the sound of the treble C on the piano is determined by the characters of 

 the complex vibration arising out of these several constituent simple vibra- 

 tions. Almost all musical sounds are thus composed of what is called a 

 " fundamental tone " accompanied by a number of " overtones." And the 

 overtones varying in number and relative prominence in different instruments, 

 give rise to a difference in the sensation caused by the whole tone. So that 

 while the fundamental tone determines the pitch of the sound, the quality 

 of the sound is determined by the number and relative prominence of the 

 overtones. In a somewhat similar way we distinguish the quality of 

 noises, such as a banging, crackling, or rustling noise, by an appreciation 

 of sudden or irregular changes in the amplitude and period of the con- 

 stituent vibrations. 



715. Since we have a very considerable appreciation, capable by 

 exercise of astonishing enlargement, of the loudness, pitch, and quality 

 of a wide range of noises and musical sounds, it is clear that, within the 



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