THE NATURE OF FATIGUE 183 



examine it first in the tissue of voluntary muscle, which affords certain 

 advantages for study over other tissues, because of the ease of employ- 

 ing the graphic method and other physical, as well as chemical, methods. 

 If, shortly after the death of an animal, a single muscle, such as a 

 muscle of a leg, be removed from the body, be attached to the usual 

 muscle lever of the physiological laboratory, and be stimulated, in the 

 usual manner at regular intervals, beginning when the muscle is fresh 

 and continuing until it is well fatigued, the graphic record of the 



Fig. 1. Series of contractions of a frog's sartorius muscle, excised and stimu- 

 lated at intervals of two and one half seconds. Each successive vertical line is the 

 record of a single contraction. The contractions at first increase in extent, this stage 

 constituting the treppe, and later decrease, this stage constituting fatigue. 



series of resulting contractions presents a striking picture. Both the 

 extent and the duration of the contractions may be affected. There ap- 

 pears early an increase in the extent of the contractions, which proceeds 

 gradually to a maximum. This is shown in the graphic record as an in- 

 crease in the height of the successive muscle curves (Fig. 1) and has been 

 called, not inappropriately, the staircase, or treppe. The treppe signi- 

 fies that in the early stages of muscular activity the working power of 

 the muscle is progressively augmented — there is a temporary improve- 

 ment in the power to work. This in turn means that what physiologists 

 call the irritability of the muscle, or, in other words, its power of re- 

 sponding to a stimulus, has become greater; hence the same stimulus 

 is followed by a greater contraction. A progressive improvement in the 

 power to work in the early portion of a task, I may say, is not pecu- 

 liar to muscle. We all must have noticed it in our own experience, 

 witli both physical and mental labor. It has also been demonstrated by 

 laboratory methods in nerves, the central nervous system, and other 

 animal and plant tissues; and it is probably a characteristic of all 

 living substance. An analogous phenomenon is observed when living 

 substance is \mt under the influence of certain drugs — a small quan- 

 tity of alcohol, for example, often effects a temporary improvement in 

 the individual's power of performing work. 



Following the treppe, the muscle may perform maximal contrac- 

 tions for a considerable time; it is in its best working condition; its 

 irritability is such that a given stimulus calls forth the greatest contrac- 

 tion of which it is capable. But sooner or later the contractions begin 

 to diminish in extent ; they sink to the level of the original amount and 

 below it; the muscle becomes gradually weaker and weaker, until, with 

 long-continued effort, it may finally cease altogether to lift the weight. 

 This decrease in working power from the maximum characterizes the 



