74 HUMAN PHYSIOLOGY 



indicates that the capacity to respond to stimuli is not 

 at its best when the preparation is absolutely fresh but 

 rather after it has done some work. We have here an 

 instance of " warming up" although it may not be 

 closely comparable with the experience for which that 

 termjs used. The word staircase, or its German equiva- 

 lent Treppe, is applied to the early contractions which 

 show this gain in height. The Treppe is usually limited 

 to a small number of twitches; a maximum is then 

 reached and for a time the contractions are nearly of 

 uniform extent. 



Presently, however, the height of the tracings begins 

 to denote a slow decline. We say that the muscle has 

 begun to exhibit fatigue. If we continue stimulating 

 at regular intervals a time will come when no move- 

 ment can be evoked, but a vigorous muscle will make more 

 than 1000 simple contractions before this limit is reached. 

 If one of these later and reduced contractions is traced 

 over a tuning-fork record, it is found to be prolonged as 

 well as low. Closer observation shows that the short- 

 ening has taken place almost as rapidly as in an un- 

 fa tigued muscle, but that the period of the relaxation has 

 been much lengthened. The frog's muscle when fatigued 

 is accordingly said to show "contracture," that is, de- 

 layed relaxation. 



What has happened in a muscle which has been forced 

 to work until quite fatigued? To answer this question 

 we must consider the source of the energy which has been 

 set free. The general truth to be borne in mind is that 

 all such energy must be referred to chemical decom- 

 position. A muscle has often been compared with a 

 steam engine in which fuel is burned, heat produced, and 

 a share of the heat transformed to do mechanical work. 

 Recent studies have served to show that the parallel is 

 not so close as had been assumed. Still it has its value. 



A muscle removed from the body can receive nothing 

 of importance from its environment except oxygen. 

 Its fuel supply is strictly limited. As it works this 



