MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 735 



repeated movements ctf the fingers even 40 a second. In habitual 

 movements, such as those employed by a man in his trade, the 

 tremors are much less coarse than in unaccustomed movements. 

 For this reason the tremors of the left hand are greater than those 

 of the right in executing a movement usually made with the latter 

 (Eshner). In disease these tremors are often increased e.g., in 

 the clonic convulsions of epilepsy but the frequency is the same. 



Fig. 261. Vibrations of Contracted Arm Muscles (Griffiths). The arm was stretched 

 out, holding a weight of about 6 kilos. 



Similar vibrations, and at about the same rate, are seen in curves 

 traced by muscles excited through stimulation of the motor areas 

 of the surface of the brain. Since this rate remains the same whether 

 the motor cortex, the corona radiata, or the spinal cord is excited, 

 and, unlike the rate of response to excitation of peripheral nerves, 

 is independent of the frequency of stimulation (so long as the rate 

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

 to represent the 

 rhythm with which 

 impulses are dis- 

 charged from the 

 motor cells of the cord 

 (Fig. 262). It is prob- 

 able that the cortical 

 centres discharge at 

 about the same rate, 

 for not only is it im- 

 possible to articulate 



. ,, Fig. 262. Contractions caused by Stimulation of the 



more rapidly than Spinal Cord, 



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. 



