HUMAN PHYSIOLOGY 



mena may be induced by the application of dilute acids and 

 certain other drugs, and may be removed for a time_by.jyash- 



out the muscle with salt 



FIG. 23. Influence of continued 

 Exercise on Skeletal Muscle (1) 

 the first trace ; (2) a trace after 

 moderate exercise ; (3) a trace 

 when fatigue has been induced. 



solutions^ (Fig. 23). (Practical 

 Physiology, Chap. III.) 



(2) Temperature. If a muscle 

 be warmed above the normal 

 temperature of the animal from 

 which it is taken, all the phases 

 ofcontraction become more rapid, 

 and the~ contraction is jit first 

 increased in extent, but subse- 

 quently decreased in force. If, 



on the other hand, a jmu_sdfi_hfi_ cooleil, the various periods 

 are prolonged. At first the contraction becomes greater 

 and more powerful, but as the cooling process goes on 

 it becomes less and less, until finally the most powerful 

 stimuli produce no effect. Cooling has thus practically the 

 same effect as fatigue (Fig. 23). (Practical Physiology, Chap. 

 III.) 



4. Drugs. Many drugs modify muscular contraction, e.g. 

 veratrin enormously prolongs the relaxation period. (Prac- 

 tical Physiology, Chap. IV.) 



5. Strength of Stimulus. A stimulus must have a certain 

 intensity to cause a 



contraction. The pre- 

 cise strength of this 

 minimum stimulus 

 depends upon the con- 

 dition of the muscle. 

 The application of 

 stronger and stronger 

 stimuli causes the 

 muscular contraction 

 to become more and 

 more rapid, more and 

 more complete, and 



Con. 



Flo. 24. Influence of increasing the Strength of 

 the Stimulus upon the contraction of Skeletal 

 Muscle. St., the stimulus; Con., the result- 

 ing contraction. A , a subminimal stimulus ; 

 B, the minimum adequate stimulus ; C, tbe 

 optimum stimulus. 



more and more powerful. But increase in 



the contraction 

 is notjproportionate to the increase in the stimulus. If the 

 stimulus is steadily increased, the increase in contraction 



