MUSCLE 



66 1 



deduce the rate at which the muscle-substance is vibrating, it does 

 not invalidate Helmholtz's objective observations with the oscil- 

 lating reeds. 



And several observers (Schafer, Horsley, v. Kries) have noticed 

 periodic oscillations, at the rate of 10 or 12 per second, in the 



FIG. 248. VIBRATIONS OF CONTRACTED ARM MUSCLES (GRIFFITHS). 

 The arm was stretched out, holding a weight of about 6 kilos. 



curves taken from muscles (Fig. 248), contracted voluntarily 

 against a small resistance. When the resistance is greater, the 

 rate may be as much as 18 or 20 a second, and in quick and 

 rapidly repeated movements of 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 execut- 

 ing a movement 

 usually made 

 with the latter 

 (Eshner). In 

 disease these 

 tremors are often 

 increased e.g., 

 in the clonic con- 

 vulsions of epi- 

 lepsy but the 

 frequency is the same. 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 



FIG. 249. CONTRACTIONS CAUSED BY STIMULATION OF 

 THE SPINAL CORD. 



