MUSCULAR CONTRACTION 



907 



tieth of a second is required after one twitch has been initiated before 

 the tissue has recovered sufficiently to respond to a second nerve impulse. 

 Forbes and Rappleye 22 have shown that the rhythm is not due to the 

 rate of discharge of nerve impulses from the higher centers, by demonstrat- 

 ing that the rate of the oscillations of the electromyo graph is slowed by 

 chilling the muscles of the arm in ice water. Since this procedure does 







f I f If 





Fig. 223. Electromyogram of the voluntary contraction of the flexor muscles of the forearm. 



(From Forbes and Rappleye.) 



not alter the body temperature as a whole, it cannot be supposed that the 

 temperature of the centers in the nervous system are changed, or that 

 the diminished rate of the rhythm is due to any change .in the rate of 

 discharge of nerve impulses from these centers. They give evidence 

 to show, moreover, that the rate at which nerve impulses follow one an- 



Fig. 224. The contraction of a single fiber of the sartorius muscle of the frog. The appearance 

 before stimulation is shown at the left; during stimulation at the right. Note the tense appearance 

 of the muscle fiber (X) , the slight pull on the deep blood vessel (I 7 ), and the change in the relative 

 position of the surface capillaries which are indicated by the heavy, wavy lines. (From Fyisenberger.) 



other down the motor nerves is much faster than 50 per second. Many 

 nerve impulses must reach the muscle while it is still refractory and con- 

 sequently do not take effect upon it. 



Since tetanic contractions are composed of a series of twitches it is 

 instructive to study the conditions which produce and modify this form 

 of contraction in skeletal muscle. These muscles are composed of large 

 numbers of fibers, each one of which may contract quite independently 



