66 EXPERIMENTAL PHYSIOLOGY 



CHAPTEE V 



FATIGUE OF MUSCLE 



A perfectly fresh rnuscle possesses in itself a certain store of 

 potential energy in the form of chemical substances by whose decom- 

 position the muscle is able to perform a definite amount of work, or 

 exert a definite force. If the muscle be caused to contract and produce 

 either a series of twitches or a prolonged tetanic contraction, this 

 chemical energy becomes more and more used up, and the muscle 

 passes into a state in which it becomes more and more difficult for it 

 to respond to the stimuli sent to it. This difficulty is due primarily 

 to the using up of the initial store of chemical energy, and secondarily 

 to the accumulation of chemical bodies of the nature of waste 

 products which obstruct the action of the physico-chemical processes 

 which are the basis of a muscle's activity. In this state a muscle 

 is said to be fatigued. We can study this condition of fatigue, from 

 a chemical standpoint, and again we may study it by contrasting its 

 performances while in this state with its bebaviour under similar 

 conditions when in a normal state. 



To show how a single twitch becomes modified as the muscle 

 passes into fatigue proceed in the following way. 



Experiment 1. — Fit up the apparatus in a similar manner to that 

 employed when studying the effect of two successive stimuli upon muscle 

 (fig. 58), but remove the second contact M 1 , so that only a single stimulus is 

 sent into the muscle with each revolution of the drum. The rate of rotation 

 of the drum should be arranged so that a complete revolution occupies about 

 two-thirds of a second. In this way the contact m- by touching x closes the 

 primary circuit three times in two seconds, and the induced shocks of the 

 secondary circuit are at this rate. A muscle preparation is now got ready 

 and fitted into a myograph, and the electrodes fixed in position. The 

 secondary coil is so placed that make shocks are ineffective. The point of 

 stimulation and an abscissa line are now drawn. It is of great importance 

 that the writing point should in this experiment only draw a very fine line. 

 The drum can now be set in rotation. With each revolution a twitch is 

 caused which is recorded. After about ten to fifteen minutes the drmn may 

 be stopped, and the record examined. During this time from 1,000 to 1,500 



