GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 65 



the muscle suddenly contracts and passes into the condition of heat 

 rigor or rigor caloris. The protein constituents of the muscle are 

 coagulated and the irritability destroyed. (Fig. 26.) 



Variations in 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 exami- 

 nation of Fig. 27, 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 increase in the duration of both the periods 

 of rising and falling energy. 



Rapidly-repeated Stimulation. Prolonged or excessive activity of our own 

 muscles is accompanied by a feeling of stiffness or soreness and lassi- 

 tude. There is at the same time a diminution in the speed and vigor 

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

 the term fatigue has been given. The cause of the fatigue is attributed 

 to a diminution in the amount of the energy-yielding compounds as 

 well as to the production and accumulation of waste products resulting 

 from katabolic activity. Among the waste products, mono-potassium 

 phosphate, paralactic acid, and carbon dioxid are the most important. 

 These substances, when present in small amounts or in larger amounts 

 for a short time, increase the irritability of the muscle, but when they 

 accumulate more rapidly than they are removed, as is the case during 

 excessive activity, they exert a depressive influence on the irritability of 

 the muscle and thus diminish its contractile power and its capacity 

 for doing work. The more rapidly they are removed, the sooner is 

 a fatigued muscle restored to its normal condition. The condition 

 of fatigue with its attendant phenomena is shown by stimulating 

 through its nerve an excised frog muscle with induced electric currents 



FIG. 28. FATIGUE CURVES. EVERY TWENTIETH CONTRACTION RECORDED. 



at intervals of one second. In a variable period of time the muscle 

 shows an increase in the duration of the latent period, a diminution 

 of the height of the contraction, in the power of doing work, and an 

 increase in the time required for relaxation. (Fig. 28.) If the stimu- 

 lation is continued the contractions gradually decline as the muscle 

 becomes exhausted. When a muscle will no longer respond to stimu- 

 lation through its related nerve, it can be made to respond to direct 

 stimulation with the electric current. This taken in connection with 

 the fact that stimulation of a nerve-trunk even for several hours does 

 not fatigue it, leads to the inference that the cause of the cessation of 

 contraction does not lie wholly in the muscle but partly in the nerve 

 endings in the muscle. These structures it is believed fatigue more 

 readily than the muscle structures, and hence fail to conduct the nerve 

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