44 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



nerve into the muscle. The contraction of voluntary muscle has 

 been investigated, therefore, in various ways to ascertain whether 

 there is any objective indication of the number of separate con- 

 tractions that are fused together to make this normal tetanus. In 

 the first place, the normal movements of the muscles have been re- 

 corded graphically by levers or tambours. The records thus obtained 

 show that our usual contractions are not entirely complete tetani, 

 that is, there is an indication in some part of the curve of 

 the single contractions that are being fused. According to most 

 observers,* these records show that our normal contractions are 

 compounded of single contractions following at the rate of 10 

 per second, or, in other words, the motor neurons discharge 

 about 10 impulses per second into the muscle. The so-called 

 natural muscle-tone has been used for the same purpose. When 

 one places a stethoscope or lays his ear upon a contracting muscle a 

 low tone is heard the pitch of which corresponds with 40 vibrations 

 per second. It is assumed, however, that this note does not represent 

 the actual rate of stimulation of the muscle, since the number 

 is higher than that obtained by other methods. The ear cannot 

 perceive a musical note much lower than 40 vibrations per second, 

 and if the muscle were really vibrating 10 or 20 times per second 

 we could not perceive this fact directly by the ear. Vibrating 

 bodies, however, give out overtones of a higher pitch, and it is 

 supposed, therefore, that the normal muscle tone (40) represents 

 either the first octave of the muscle vibrations, 20 per second, or 

 the second octave, 10 per second. Helmholtz made use of a simple 

 and direct method to determine this point. He utilized the prin- 

 ciple of sympathetic vibrations, according to which a vibrating 

 body will be set into movement most easily by vibrations that 

 correspond in number to its own period. Helmholtz attached to 

 the muscle watch springs that had different periods of vibration 

 and found that when the muscle was contracted the spring that 

 vibrated 20 times per second was set into most active movement. 

 He concluded, therefore, that the muscle receives 20 stimuli per 

 second in ordinary contractions and that the tone that is heard, 

 40 vibrations per second, represents the first overtone. The agree- 

 ment among the results of those who have made graphic records 

 of voluntary contractions would lead us, however, to suppose that 

 10 stimuli per second is more probably the true rate of stimulation 

 and that the muscle-tone heard represents the overtone correspond- 

 ing to the second octave of this vibration. It is to be borne in mind, 

 however, that the motor nerve cells do not necessarily discharge 

 their impulses into the muscle at a perfectly uniform rate. The 

 rate is, in fact, liable to vary in different individuals or in the same 

 *Horsley and Schafer, "Journal of Physiology," 7, 96, 1886. 



