620 THE NERVOUS SYSTEM. 



In referring to this subject, Waller compares the basilar mem- 

 brane to the membrana tympani in the following language : " It 

 is the internal drum-head, repeating the complex vibrations of the 

 membrana tympani, and vibrating in its entire area to all sounds 

 although more in some parts than in others giving what we may 

 designate as acoustic pressure patterns between the membrana 

 tectoria and the subjacent field of hair-cells. In place of an 

 analysis by sympathetic vibration of particular radial fibers, it 

 may be imagined that varying combinations of sound give vary- 

 ing pressure patterns, comparable to the varying retinal images of 

 external objects." 



There are several terms used in the discussion of the subject 

 of sound which it is important to understand ; especially is this 

 true for the medical student, for he will 

 constantly meet them in his study of 

 physical diagnosis. 



Period; Amplitude; Frequency. If a 

 weight attached to a rubber cord (Fig. 

 398) is pulled down and then released, 

 the weight and the particles composing 

 the cord will vibrate, and any particle, 

 as a, will oscillate between two extreme 

 points, as b and c, which are equidistant 

 from a. The motion of a from c to b 

 and back again to c is one vibration or 



one complete vibration, and the time this 

 FIG. 398. Weight and cord. . * ,, . 7 ' - ,, ., ,. 



occupies is the period of the vibration. 

 The distance from a, when the particle 



is in equilibrium, to b or to c is the amplitude of the vibration, 

 and the number of complete vibrations in one second is the fre- 

 quency. 



Noises. These are sounds produced by irregular vibrations 

 i. e.j wanting in periodicity ; or by discordant or dissonant sounds 

 i. e.j sounds which differ from one another in pitch ; or they 

 may be single, sudden sounds, as the report of a cannon. Noises 

 are disagreeable sounds. 



Musical Sounds. These are sounds produced by regular vibra- 

 tions, and they produce a pleasing effect upon the ear. 



It should be said, however, that what may under some circum- 

 stances be a noise may, under others, produce the effect of a 

 musical'tone. Haughton says : " Nothing can be imagined more 

 purely a noise or less musical than the jolt of the rim of a cab 

 wheel against a projecting stone; yet if a regularly repeated 

 succession of such jolts takes place, the result is a soft, deep, 

 musical sound that will bear comparison with notes derived from 

 more sentimental sources." And Zahm says : " With a sufficient 

 number of properly tuned bottles a skilful performer could, by 



