662 A MANUAL OF PHYSIOLOGY 



of the frequency of stimulation (so long as the rate of stimu- 

 lation is greater than 10 or 12 a second), it has been supposed 

 to represent the rhythm with which impulses are discharged 

 from the motor cells of the cord (Fig. 249). It is probable 

 that the cortical centres discharge at about the same rate, for 

 not only is it impossible to articulate more rapidly than eleven 

 syllables per second, but it is impossible to reproduce the act 

 of articulation in thought at a greater rate than this (Richet). 

 But while this rate of 10 or 12 a second does seem to represent a 

 fundamental rhythm of the central discharge, there are facts which 

 indicate that upon this relatively slow rhythm a quicker rhythm 

 is superposed. In other words, each of these discharges is itself 

 discontinuous, and made up of a number of separate impulses. 



1 Thus, according to Piper, the total number of simple discharges, 

 each associated with an electrical change in the muscle, is 47 to 50 a 

 second. The rhythm of strychnine tetanus in the frog is about 8 to 

 12 per second. By means of the capillary electrometer (p. 621) large 

 electrical oscillations at this rate can be demonstrated, each of 

 which represents a short tetanic spasm, as is shown by the fact that 

 a number of smaller electrical oscillations are superposed upon 

 the large ones (Sanderson). The electrical changes suggest that 

 each discharge causes a simple contraction much more prolonged 

 than the twitch of a directly stimulated muscle. This removes the 

 difficulty of understanding how such a small number as 10 contrac- 

 tions per second could be smoothly fused, and indicates that even the 

 shortest possible voluntary movement, which can be executed in 

 TT> to 2\> of a second, is not caused by a single impulse, but is a 

 tetanus. For these brief movements the frequency of oscillation, 

 as shown by the action currents, is the same as for sustained con- 

 tractions. The electrical changes in the voluntarily contracted 

 muscle seem to differ in amplitude or abruptness from those pro- 

 duced in experimental tetanus. For secondary tetanus (p. 730) is not 

 caused by muscle in voluntary contraction. But this is also the 

 case with the other prolonged contractions caused by continuous 

 artificial stimulation e.g., Ritter's tetanus (p. 636) and the 

 contraction produced by sodium chloride or ammonia. We need 

 not hesitate to conclude, then, that the voluntary contraction is 

 discontinuous, in the sense that it is not a perfectly smooth 

 and uniform tonic contraction, although we still lack a decisive 

 proof that it is maintaied by a strictly intermittent outflow of 

 nervous energy, and not by a continuous outflow causing a sus- 

 tained contraction, which, it may be, remits and is reinforced at 

 intervals. The apparent discrepancies as to the rate of discharge 

 in the results obtained by different observers and by different 

 methods, far from exciting distrust of them all, really lend support 

 to the idea of a fundamental and fairly constant rhythm in the 

 outflow as soon as it is recognised that the higher rates are approxi- 

 mately multiples of the lower. Thus, the number deduced by Helm- 

 holtz from the experiment of the springs is twice the lowest rate 

 calculated from graphic records of the contraction. The rates 

 corresponding to the muscle-sound and to the frequency of the 

 electrical oscillations are about four times this number. Now, in a 

 vibrating elastic body like a contracting muscle, a simple mathe- 



