MUSCLE 653 



except perhaps in their synapses (p. 775) in the cord, which corre- 

 spond to the endings of the peripheral fibres in the muscles. That 

 the synapses easily lose their power of conducting nerve impulses 

 under the influence of repeated excitations is indicated by the ex- 

 periments of Sherrington on fatigue of reflex mechanisms in which 

 two or more afferent paths can cause discharge along a common 

 efferent path (p. 800) . When excitation of one of the afferent paths 

 has ceased to be effective, the reflex contractions can still be 

 obtained on exciting the other. In this case the motor neuron from 

 cell-body to nerve-ending and the muscle are eliminated as the seats 

 of the fatigue block. Whether the temporary loss of conduction 

 in this case is comparable to the fatigue of muscle, or is a perfectly 

 different phenomenon (' pseudo-fatigue ' of Lee), scarcely bears 

 on our present question. For if ' pseudo -fatigue ' of afferent 

 synapses can cause a reflex to miss fire, this at least shows that the 

 conductivity of the synapse is very easily affected by repeated 

 excitation, just as it 

 is known to be very 

 easily affected by 

 anaemia. The only 

 other portions of the 

 voluntary motor 

 path besides these 



synapses that seem 



,., . r FIG. 243. VERATRIXE CURVE: FROGS 



likely to become GASTROCNEMIUS. 



easily fatigued are The curve shows a peak> the lever falling a linle 



the nerve - cells Of before the sustained contraction begins. 



the cortex and the 



cord. These central structures are usually considered the 

 weakest links in the chain, the next weakest link being the 

 motor endings in the muscles, and the strongest the nerve-fibres. 

 The motor endings do not, in general, break down in voluntary 

 contraction, because the central apparatus becomes sooner 

 fatigued. It is not inconsistent with these facts that a muscle, 

 fatigued by direct electrical stimulation, can still be voluntarily 

 contracted. For the voluntary excitation may be more effective 

 than any artificial stimulus. 



It has been shown that the injection of the blood of an animal 

 exhausted by running or other muscular effort into the circula- 

 tion of a normal animal produces in the latter all the symptoms 

 of fatigue. Here the fatigue-producing substances will have 

 the opportunity of acting on both the central and the peripheral 

 mechanisms. There are reasons for believing that the fatigue 

 process is fundamentally the same in different tissues. The 

 fatigue substances produced in muscle, and not immediately 

 eliminated or transformed during active muscular exertion, may 



