THE INFLUENCE OF TEMPERATURE 457 



temperature be raised too high (40 C. fcr frog, 50 C. for mammal), the 

 muscle enters into a condition of heat rigor and its irritability is forever lost. 

 After cooling, unless the cold be too severe and prolonged, the irritability re- 

 turns as the temperature is raised. A series of vertical records of simple contrac- 

 tions beginning at room temperature and decreasing with a contraction at each 

 fall of one degree reveals the fact that the amplitude falls off slowly until a 

 temperature of 12 to 10 C. is reached, then as gradually increases until 4 to 

 2 C., after which the amplitude drops off sharply to about 1 C. However, 

 this phenomenon is partly one of irritability, since a very strong stimulus will 

 produce a vigorous contraction until the muscle begins to freeze. If at the 

 freezing temperature the muscle be slowly and carefully increased in tem- 

 perature it will recover from the effects of the cooling without apparent injury, 

 and will give a reverse series to the one obtained by decreasing the temperature. 



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



Gastrocnemius. 



As the increase of temperature is continued above room temperature the 

 amplitude of the contractions very greatly increases (also the elasticity), 

 reaching a maximum in the frog's gastrocnemius at about 35 to 36 C. The 

 amplitude sharply decreases above 35 C. up to 37 to 38 C., where heat 

 rigor begins and the muscle permanently shortens. Heat rigor is usually 



