136 ELECTRO-PHYSIOLOGY CHAP. 



the muscle -sound in determining the discontinuous nature of 

 voluntary muscular contraction (Martins, 31), and Helmholtz 

 subsequently investigated the phenomenon more exactly. Like 

 Ermann he started from the fact that when the masticatory 

 muscles are forcibly contracted at night, with the ears closed, " a 

 dull, humming sound is heard, the ground-tone of which is not 

 intrinsically altered by increased tension, while the humming that 

 goes with it becomes stronger and louder. Helmholtz then found 

 that on tetanising his own masseter directly, and the brachial 

 muscles of an assistant from the median nerve, by means of 

 an induction coil standing in the next room, the muscle 

 gave the tone of the interrupting spring instead of the normal 

 muscle-bruit. This is a direct proof that vibrations do occur 

 within the muscle, however constant its change of form may 

 appear to be, and that a vibration actually corresponds with 

 each single stimulus, for if the number of stimuli is altered, 

 the height of the muscle-tone alters also, since within certain 

 limits it always corresponds with the stimulation-frequency. That 

 no alteration of form is to be seen in the tetanised muscle only 

 implies that vibrations occur in the smallest particles, while the 

 external shape does not alter, much as a rod that is vibrating 

 longitudinally emits a sound, although no external change of form 

 is visible. Moreover, as pointed out by Hermann, the muscle- 

 sound could still be explained if the periodic process in tetanised 

 muscle were not merely mechanical, since the rhythmical currents 

 of action to be discussed below appear to be sufficient to account 

 for them. 



The experiments of Helmholtz indicate a high degree of 

 mobility in the least particles of striated muscle, for he even 

 detected a clear muscle-tone of corresponding pitch in electrical 

 tetanisation of 240 stimuli per sec. Bernstein (33) subse- 

 quently endeavoured to determine the range in which it was 

 possible still to detect a clear muscle-tone, i.e. to what limit the 

 muscle-elements responded to the rapidity of the stimuli acting 

 on them in electrical tetanus. By means of the acoustic current- 

 interrupter (in which a vibrating spring of different tensions opens 

 and closes the primary circuit) he stimulated the gastrocnemius 

 muscles of the rabbit, partly directly, partly from the nerve, and 

 convinced himself that the muscle-bruit can reach a very consider- 

 able height, since a tone of 748 vibrations still sounds loudly, 



