CONTRACTION AND WORK OF THE MUSCLES. 45 



or fatigued, heated or chilled, executes more or less rapidly 

 the movement dictated by the nerve. 



Besides this, all the influences which cause variation in the 

 moment at which the shock of the muscle appears, cause 

 variation of speed in the propagation of the wave in its 

 interior ; which proves that the conditions which accelerate 

 or retard chemical actions, the first causes of all these phe- 

 nomena, are solely concerned. 



Of the contraction oj the muscle. Hitherto, we have applied 

 to the nerve only one single excitation, to which one single 

 motion responded, the muscular shock. Notwithstanding its 

 brevity, this shock has an appreciable duration ; in man it takes 

 8 or 10 hundredths of a second for the muscle to accomplish 

 its contraction ; then a longer time for it to resume its normal 

 length ; after which, if it receives a new order from a nerve, 

 it gives a fresh shock. But if the excitations of the nerve 

 succeed each other at such short intervals that the muscle has 

 not time to accomplish the first shock before it receives a 

 second, a special phenomenon is produced ; these movements 

 are confounded and absorbed into a state of permanent con- 

 traction, which lasts as long as the excitations go on suc- 

 ceeding each other at short intervals. 



Thus the shock is only the elementary act in the function of 

 the muscle ; it plays therein, after a fashion, the same part as 

 a sonorous vibration plays in the complex phenomenon which 

 constitutes sound. When the will ordains a muscular con- 

 traction, the nerve excites in the muscle a series of shocks 

 which follow one another so closely that the first has not time 

 to end before a second begins, so that these elementary 

 movements combine together and coalesce to produce the 

 contraction. 



Volta pointed out, in a letter to Aldini, this singular fact, 

 that a frog which receives a series of excitations, by the reite- 

 rated contacts of two heterogeneous metals applied to his 

 nerve, does not react at each of these contacts, but undergoes 

 a sort of permanent contraction. Ed. Weber shows that the 

 action of successive induced currents is of the same kind, 

 and he has given the name of tetanus to the state of the 

 muscle thus excited. Helmholtz perceived that the muscle 



