522 MUSCLE-NERVE PHYSIOLOGY 



35 C. up to 37 to 38 C., where heat rigor begins and the muscle per- 

 manently shortens. Heat rigor is usually complete at 40 to 41 C. A 

 muscle cannot recover its irritability after heat rigor has set in strongly. 



If the time of the contraction is measured at different temperatures it will 

 be found to be greatly delayed at 2 to 4 C., and very much quicker than nor- 

 mal at 33 to 35 C. As in fatigue, the effect falls chiefly on the contraction 

 and relaxation phases and only slightly on the latent period. The latent 

 period is more sharply influenced by temperature than by fatigue. 



FIG. 329. Influence of Temperature on the Duration of the Contraction of the Frog's 



Gastrocnemius. 



Influence of Blood Supply. In the normal human muscle there is a 

 delicately balanced vaso-motor mechanism by which the amount of blood 

 flowing through a muscle is immediately increased when the muscle is in con- 

 traction. This blood stream is of course carrying nutritive materials to the 

 muscle and taking away wastes. If the blood supply to a muscle is cut off, 

 then the muscle can only draw on its stored supply of potential energy, which 

 in active contraction is sooner or later exhausted. Under such conditions 

 the muscle increases in irritability for a few minutes and then gradually loses 

 both its irritability and its power to contract. Even mammalian muscles 

 have been kept alive and normal in their activity for several hours by irri- 



