THE INFLUENCE OF REPEATED ACTIVITY 455 



multiplied many fold. The range between the strengths of the minimal and 

 maximal stimuli is very restricted indeed. The absolute strength of a mini- 

 mal stimulus varies exceedingly for a given muscle, depending on its degree 

 of irritability. This narrow range between minimal and maximal stimuli 

 serves as a convenient means for detecting the variations in irritability. One 

 should count on a continued decrease in irritability in isolated muscles, hence 



FIG. 326. Contraction of the Gastrocnemius Under the Influence of Variation of Strength of 

 Stimulus. The muscle was stimulated by Petr.old's inductorium, graduated to show units of 

 current. The figures 6, 7, 8, 9, 10, etc., indicate relative strength of stimulus. 



should choose a supramaximal stimulus for all such preparations when other 

 conditions surrounding the muscle are under investigation. 



The Influence of Repeated Activity. The irritability of muscle is 

 decreased by undue functional activity. The cause of the diminished ir- 

 ritability is twofold: when a muscle contracts, part of its substance is ex- 

 pended, part of its store of nutriment is exhausted, and it cannot contract so 

 vigorously again until the loss is made up. To this extent fatigue has much 

 the same effect as cutting off or diminishing the blood supply. The other 

 cause for the diminution of irritability is the accumulation of poisonous prod- 

 ucts in the muscle, substances generated during contraction, probably sar- 

 colactic acid chiefly. In a living animal these poisonous products exert their 

 influence not only upon the muscle or muscles immediately concerned in 

 contraction, but upon the musculature of the body generally, and the effect 

 remains until they are eliminated from the body. Massage of the muscles 

 increases the passage of waste products into the general blood stream and the 

 rapidity of their elimination. 



In the first few simple contractions, repeated in series, there is an increase 

 in the amplitude of the contractions resulting in the phenomenon known 

 as staircase contractions or "Treppe." This stage is followed by a period 



