29 



a distinct class, and subservient alone to 

 what he calls the organic life. The con- 

 tractile power of muscles is also capable 

 of remaining in vehement action for a 

 great length of time, as w r e see in some 

 cases of cramps, and still more in some 

 cases of tonic tetanus, 



Yet though the irritable power is no$ 

 incapable of continued exertion, it seems 

 evidently to be in general susceptible of 

 fatigue, and inclines to be at rest. If 

 we stimulate the muscles of a limb of a 

 frog severed from the body, by voltaic 

 electricity, the muscular actions are at 

 first vivid and forcible, but they grow 

 fainter and feebler on repeated excitement. 



Yet if we wait a little till thev seem to 



*/ 



regain their power, they become vivid 

 and forcible as at first from the same de- 

 gree of excitement. Such actions may be 



