44 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 

 individual under different circumstances. Von Kries,* for instance, 

 states that the rate of stimulation in voluntary movements may 

 vary according to the character of the movement. In slow, sus- 

 tained movements the rate is from 8 to 12 per second, while in 

 short, sharp, rhythmical movements of the fingers the rate may be 

 as rapid as 40 per second. The fact that movements of this latter 

 character the trilling movements of the fingers of the pianist, 

 for instance may last for only y 1 ^ of a second or less, is considered 

 by some authors as a proof that they are not tetanic contractions, 

 and that therefore we can voluntarily make either long-continued 

 tetanic contractions or quick, simple contractions. Von Kries 

 has shown, however, that when these quick, rhythmical movements 

 of the fingers are recorded the curves, even of such brief contractions, 

 show that they are short-lasting tetani. It is the usual belief, 

 therefore, that all voluntary movements are tetanic in character 

 and that it is not possible for us, by a so-called act of the will, to 

 cause a simple contraction, that is, to cause the motor nerve cells 

 to discharge a single motor impulse. This general conclusion is sup- 

 * Von Kries, " Archiv fur Physiologic," suppl. volume, 1886, p. 1. 



