520 MUSCLE-NERVE PHYSIOLOGY 



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 

 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 



FIG. 327. Contractions of the Gastrocnemius Muscle to Show Fatigue. The numbers 

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



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 

 of sustained contractions, and this finally by a diminishing series of ampli- 

 tudes 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 



