THE INFLUENCE OF TEMPERATURE 487 



increased as the contractions are repeated. The latent period changes very 

 little. The contraction phase is considerably prolonged, but the relaxation 

 phase is very greatly increased. As fatigue progresses, the total time of the 

 simple contraction may be two or three times longer than the normal. The 

 ability of the muscle to do work falls off rapidly, of course; and the greater 

 the load during the time fatigue is coming on, the more quickly complete 

 fatigue approaches. 



The Influence of Temperature. The irritability of muscle is in- 

 creased by raising its temperature slightly above that of the animal from 

 which it has been taken, while it is decreased by cooling. If, however, the 

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

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



FIG. 328. Contractions of the Gastrocnemius Muscle to Show the Influence of 

 Temperature on the Amplitude of the Contractions. At 40 C. the muscle has begun to 

 pass into rigor mortis, yet is able to give short contractions. The steps on the curve of rigor 

 at the right occur at temperatures of 41, 42, and 43 C. 



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 con- 

 tractions beginning at room temperature and decreasing w r ith 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 temperature it will recover from the effects of the cooling with- 

 out apparent injury, and will give a reverse series to the one obtained by 

 decreasing the temperature. 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 gastroc- 

 nemius at about 35 to 36 C. The amplitude sharply decreases above 



