x THE FOKE-BEAIN 563 



ing is very slight. The subsequent work of Griffiths brought out 

 the fact that when this resistance increases there is a corresponding 

 increase in the frequency of contraction, up to 15-18 per second. 

 When the resistance is protracted and fatigue supervenes the 

 frequency diminishes. As shown by Fig. 286, an outstretched arm 

 holding a weight shows the same rhythm of contraction as a single 

 muscle. 



We may thus say, with Schafer, that the average frequency of 

 the discharges which produce a voluntary contraction is from 

 10 per second, with a possible increase to 20 per second, when the 

 resistance opposed to the contraction is excessive. These facts 

 harmonise well with those given by Richet for tremor, viz. 10-11 

 contractions per second. They also agree with the fact that it is 

 impossible to speak or sing more than eleven syllables or to play 

 more than eleven musical notes per second. The cortical cells 



FIG. 286. Vibrations of outstretched arm holding a weight of about six kilos. (W. Griffiths.) 

 The spaces between the vertical lines represent intervals of one sec. 



thrown into activity during these voluntary acts cannot discharge at 

 a greater rapidity than this. The most elementary psychical acts 

 of the cortical cells have therefore a mean duration of T ^ 7 sec. ; 

 but it is probable that practice and certain favourable conditions 

 may shorten this duration. 



Let us pass on to examine the characters of the motor reactions 

 artificially obtained by electrical excitation of the excitable points 

 of the cerebral cortex, and see if they differ from those of voluntary 

 action. 



Fran^ois-Franck and Pitres (1878-79) stated that stimulation 

 of the motor cortex, like that of the motor tracts of the cerebro- 

 spinal axis, gives rise to a series of contractions the rhythm 

 of which corresponds exactly to that of the stimulus adopted, just 

 as in the stimulation of a peripheral nerve. If the cortex of the 

 excitable zone is stimulated 5, 10, 20, 40 times per second, the 

 number of contractions which make up the muscular response will 

 be 5, 10, 20, 40 per second ; above 40 per second the single contrac- 

 tions fuse to form a perfect tetanus. This last statement, which is 



