ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 139 



per sec. Wollaston had previously attempted to determine the 

 vibration-frequency in voluntary contraction of his brachial muscles 

 by supporting his arm on a grooved board, over which a rounded 

 piece of wood passes with such rapidity that the sound is of the 

 same pitch as the muscle-sound. He found that the frequency 

 of the latter lay between 2 and 3 vibrations. Helmholtz sub- 

 sequently found, by means of the consonating spring, that in 

 voluntary innervation there was a marked and visible consonance, 

 when the spring was registered, at 1820 vibrations per sec. 



It would appear from these experiments that the vibration- 

 frequency of the natural muscular rhythm in man is not 3040, 

 but 1820. What is heard as the muscle-tone is really only the 

 first over-tone of the true muscle- vibration, the ground-tone of 

 which is no longer within the range of audible perception ; 

 according to Helmholtz it corresponds with the C of the 16 -foot 

 open organ-pipe, and is like this a resonance-tone of the ear. 

 We cannot therefore, from the pitch of the sound that is directly 

 audible in voluntary contraction of the muscle, draw any direct 

 conclusions as to the frequency of the central impulses. But the 

 objective resonance experiments with consonating springs, as well 

 as the observation of du Bois-Eeymond, to the effect that a similar 

 bruit is heard both in voluntary innervation and in artificial 

 tetanus when the current is led into the spinal cord, and not 

 directly to nerve or muscle, do notwithstanding appear to show 

 that the natural rhythm of excitation from the central nervous 

 system lies at about 18-20 per sec. According to du Bois- 

 Eeymond we hear, under these conditions, not the tone of the 

 current oscillations, but a deeper tone, corresponding in every way 

 with the muscle-bruit. Kronecker and Stanley Hall (3) obtained 

 the same results from the objective registration of variations in 

 bulk of the exposed M. biceps feinoris of rabbit, by means of 

 Marey's air-capsules. In agreement with the results of Helmholtz 

 and du Bois-Eeymond, the curve described by the muscle only 

 showed 20 shallow undulations, when the number of stimuli 

 led into the spinal cord was about 43 per sec. This seems 

 to determine objectively that the central organ (spinal cord) not 

 merely possesses an intrinsic rhythm of innervation peculiar to it 

 under all circumstances, but that the number of efferent impulses 

 also corresponds in general with the number of vibrations in the 

 natural muscle-tone. Horsley and Schafer found on tetanising the 



