146 ESSENTIALS OF PHYSIOLOGY. 



note, and the more rapid the vibrations, the higher the pitch. The highest 

 note which .can be appreciated by the human auditory apparatus has a 

 frequency of about 40,000 vibrations per second, but some animals can 

 detect sounds of higher pitch than this. The lowest note used in 

 music, that of the sixty-four foot organ pipe, has a frequency of sixteen 

 vibrations per second, and gives an impression rather of vibration than 

 of sound. 



(2) Intensity or loudness depends on the amplitude of the vibrations 

 giving rise to the note. This can be shown by recording the vibrations 

 of a tuning fork on a moving drum, when it is seen that the more 

 extensive the movement of the fork, the louder is the note produced 

 by it. 



(3) Quality or timbre is due to the form of the wave. In the case 

 of the tuning fork the wave is simple, and the note is a pure one 

 uncombined with any secondary vibrations. The notes produced by 

 musical instruments, on the other hand, owe their distinctive quality to 

 the production of overtones, which combine with the fundamental note 

 and produce a compound wave. A violin string, for example, not only 

 vibrates as a whole, giving the fundamental note, but also vibrates in 

 segments, producing the overtones which are due to the vibration of 

 halves, thirds, fourths, and still smaller segments of the whole string. 

 The particular quality of the tones produced by any instrument depends 

 upon the number and degree of prominence of the particular overtones. 



When two notes are sounded together, the result may be, on the one 

 hand, consonance or harmony, or, on the other hand, dissonance or dis- 

 cord. Discord is due to the fact that the two notes have nearly the 

 same vibratory period, with the result that at certain intervals the 

 summit of one wave occurs at the same instant as the trough of the 

 other, so that the two neutralise each other, causing a momentary 

 silence. Later, the summits of the two waves will correspond, and the 

 degree of sound will be momentarily increased. The rapidly alternating 

 increase and diminution in volume of the sound wave constitutes what 

 is called a beat, and gives rise to the jarring sensation known as discord. 

 When two notes sounded together give a sensation of harmony, there 

 are no such beats, the two waves being combined to form a compound 

 wave of regular rhythm. 



The auditory mechanism is not only capable of appreciating sounds, 

 but also of distinguishing differences of pitch within limits, and even, 

 in the case of trained musicians, of analysing a combination of notes, 

 sounded together, into its constituents. Various theories have been held 

 as to the part played by the different structures in the cochlea and by 

 the cerebral cortex itself in the discrimination of pitch, but it will only 



