FATIGUE OF MUSCLE AND NERVE 43 



metronome), to close and open a gap between two mercury cups 

 in the circuit. Use maximal stimuli. Keep the point of the lever 

 which must be free-weighted against the smoked paper, and 

 record every contraction. In this way a continuous fatigue curve 

 is obtained, exhibiting the effect of fatigue, not only in the extent of 

 contraction but also on the extensibility of the muscle both in rest 

 and in contraction. Notice the " staircase " (gradual rise of the 

 ordinates) at the beginning of the curve and the " contracture " 

 (permanent contraction remainder) near its termination. Carry the 

 experiment to complete exhaustion i.e., until the stimuli produce no 

 further perceptible effect. Then allow the preparation to rest, 

 keeping it moist with Ringer's solution. After 15 or 20 minutes 

 again test the effect of a stimulus. Notice that there is a certain 

 amount of recovery from the fatigue, even in a preparation such as 

 this in which no blood is circulating. In muscle in which the circula- 

 tion is maintained fatigue comes on more slowly and is more rapidly 

 recovered from, since the circulating blood removes the fatigue 

 products. 



Reaction of the fatigued muscle. Cut across a muscle which is completely 

 fatigued, and apply blue litmus paper to it. Notice that the paper is reddened 

 (production of acid fatigue products). Compare the reaction with that of a 

 piece of fresh, unfatigued muscle. Muscle which has died and passed into 

 rigor whether natural or the effect of heat (see p. 41) is also acid. 



Onset of the fatigue in a muscle-nerve preparation : Bernstein's 

 experiment. Arrange an experiment in the manner shown in the 

 diagram (fig. 40). Dissect out both nerve-muscle preparations of a 

 frog and place both nerves near the spinal cord on a single pair of elec- 

 trodes connected through a short-circuit key with the secondary coil. 

 Another pair of electrodes which should be non-polarisable (p. 7) 

 is connected with one of the nerves in such a manner that the current 

 furnished by three Daniell cells (shown below in the diagram) 

 can be led into the nerve in an ascending direction. When the 

 mercury key of this circuit is closed the constant current from the 

 Daniell cells blocks the nerve, and the nerve impulses generated by 

 the induction shocks fail to pass to the muscle. Stimulate both nerves 

 by induction shocks. The muscle the nerve of which is free to conduct 

 is speedily fatigued : the other one shows no signs of contraction ; 

 on removing the block by opening the mercury key it at once 

 responds to the stimuli, and its contractions show no evidence of 

 fatigue even after long faradisation of its nerve. 



Fatigue in voluntary contraction. This is investigated by the 



