38 PEACTICAL PHYSIOLOGY 



a machine for doing work is found to have its output of energy 

 regulated, not merely by the strength of the stimulus reaching it, 

 but also to a large extent by the amount of work it is called upon to 

 do (see p. 60). 



Effect of Fatigue. When discussing the fatigue of muscle it is 

 necessary to draw a distinction between the fatigue of a movement 

 produced by the voluntary contractions of the muscle concerned in it 

 (see p. 70), and the fatigue of a muscle caused by the artificial stimula- 

 tion of the muscle itself or of the nerve supplying it (see p. 72). 

 Further, in the second case there is a marked difference in the effect of 

 continued stimulations on a muscle whose circulation is still intact (see 

 p. 74), and on one which has been excised from the body. Here we 

 shall deal only with the simplest case of a muscle excised from the 

 body and stimulated directly and not through its nerve, in order to 

 exclude any possibility of fatigue of nerve or of nerve endings. 



Prepare either a hyoglossus preparation to be stimulated by two 

 needle electrodes, or a gastrocnemius-sciatic preparation to be stimulated 

 by one needle-electrode and by fine capillary copper wire threaded 

 through the tendo-Achillis, as the other electrode. The drum is placed in 

 the primary circuit, so that each time it revolves the muscle receives a 

 maximal make induction-shock ; it should revolve at such a speed that 

 the muscle will be stimulated once or twice a second. Weight the muscle 

 near the axis of the lever, using 20 grms. for a hyoglossus and 50 

 grms. for a gastrocnemius preparation. With the Du Bois key closed, 

 describe a base line and mark on it the point at which the stimulus will 

 enter the muscle. Now open the Du Bois key, allow the drum to 

 revolve, and record the first contraction and every tenth or twentieth 

 subsequent contraction. For this purpose, directly the first contraction 

 is over, the writing point is swung away from the drum, which goes 

 on revolving and causing the muscle to contract. The base of the 

 stand carrying the myograph must not be moved so that for each con- 

 traction the point of entrance of the stimulus will be the same. The 

 writing point should be a fine one, otherwise the number of super- 

 imposed curves will to some extent obliterate each other. 



When a series of curves taken in this way is examined (Fig. 43) it 

 is seen that they show the following changes as fatigue progresses, 

 the latent period becomes slightly longer, the shortening of the muscle 

 takes place more slowly and reaches its maximum more gradually, but 

 the actual height of the curves does not begin to decrease much until 

 the other features of fatigue are well marked ; the lengthening out of 

 the period of relaxation is the most marked feature, it is evident from 

 the first, and, as it progresses, a ' contraction remainder ' also appears. 



