8o 



TEXT-BOOK OF PHYSIOLOGY. 



The Load. The extent to which a muscle is loaded or weighted 

 will not only determine the height of the contraction, but also the 

 time relations of all its phases. This is apparent from an ex- 

 amination of Fig. 28, in which it is shown that with an increase 

 in load there is a decrease in the height of the contraction, an 

 increase in the latent period, and a general decrease in the dura- 

 tion of both the periods of rising and falling energy. 



FIG. 27. SINGLE CONTRACTIONS OF THE GASTROCNEMIUS AT DIFFERENT TEMPERA- 

 TURES. TIME TRACING, 200 PER SECOND. (Brodie.} 



Muscle Fatigue. Prolonged or excessive activity of our own 

 muscles is accompanied by a feeling of stiffness or soreness and 

 lassitude. There is at the same time a diminution in the rate and 

 vigor of the contractions and the power of doing work. To this 

 condition the term fatigue has been given. The c^auseuiljhe 

 fatigue is attributed to a diminution_in the amount_ofjthe energy- 

 holoihg compounds as well as to the production and accumulation 



FIG. 28. CONTRACTIONS OF A GASTROCNEMIUS WITH DIFFERENT LOADS. (Brodie.) 



ff of waste products resulting from activity. Among the waste 



pfWlUClS phosphoric acid, p6t^ltrm~p!Tosphate, lactic acid, and 

 carbon dioxid are the most important. \\The more rapidly they 

 are removed, the sooner is a fatigued muscle restored to its normal 

 condition. It is highly probable that the nerve-centers are more 

 easily fatigued than the muscles. The condition of fatigue with 



