70 INTRODUCTION TO NEUROLOGY 



energy of the stimulus as to enable it to act more efficiently upon 

 the essential end-organ. 



Sherrington states the distinctive characteristic of the sense 

 organs in this form, "The main function of the receptor is, there- 

 fore, to lower the threshold of excitability of the arc for one kind 

 of stimulus and to heighten it for all others." The selective func- 

 tion of the receptors is illustrated by a consideration of the 

 different forms of vibratory energy which pervade the environ- 

 ment in which we live. 



There are, first, rhythmically repeated mechanical impacts 

 perceived through the sense of touch. This series of tactile 

 sensations extends from a single isolated contact at one extreme 

 to rhythmically repeated contacts touching the skin as fre- 

 quently as 1552 vibrations per second. 



A second series of vibratory phenomena is presented by the 

 mechanical vibrations of the surrounding medium perceived sub- 

 jectively as sound. Out of the entire series of such vibrations of 

 all possible frequencies the human ear is sensitive to a series of 

 approximately ten octaves from about 30 (in some cases 12) to 

 about 30,000 (in some cases 50,000) vibrations per second (wave 

 lengths from 1228 cm. or 40 ft. to 1.3 cm. or .5 inch in length). 

 To all other vibrations it is insensitive. Within this range the 

 average human ear can discriminate some 11,000 different pitch 

 qualities (Titchener). 



Subjectively, the series of tone sensations is broken up into 

 a number of octaves, and it is found that a given tone of the 

 musical scale is excited by vibrations of exactly twice the fre- 

 quency which excites the corresponding tone of the next lower 

 octave. By analogy with this arrangement all series of physical 

 vibrations are sometimes spoken of as divisible into octaves, 

 the octave being defined as those vibration frequencies which 

 lie between a given rate and twice that rate or half that 

 rate. 



A third type of vibratory phenomena is presented by the much 

 more rapid series of so-called ethereal vibrations, or waves in 

 immaterial media. The lower members of this series are the 

 Hertzian electric waves; the higher members are the re-rays. 

 Between these extremes lie waves perceived as radiant heat, the 

 light waves, and the ultra-violet rays of the spectrum. This 



