456 



MUSCLE-NERVE PHYSIOLOGY 



of sustained contractions, and this finally by a diminishing series of amplitudes 

 until the muscle fails to respond. After a few minutes' rest a muscle will 

 again give strong contractions, but only for a brief series. 



If the time of the simple contractions is measured, it will be found, figure 

 327, that not only is the amplitude decreased but the duration is greatly 

 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 



FIG. 327. Contractions of the Gastrocne"mius Muscle to Show Fatigue. The numbers printed 

 on the figure indicate the contractions in the series which is recorded. (Lee.) 



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 



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 tem- 

 peratures of 41, 42, and 43 C. 



