MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 727 



lated continuously with interrupted currents of equal strength, 

 while the excitation is prevented from reaching the muscles of one 

 limb till those of the other cease to contract, it will be found that 

 when the ' block ' is removed the corresponding muscles contract 

 vigorously on stimulation of their nerve. The passage of a constant 

 current through a portion of the nerve or the application of ether 

 between the point of stimulation and the muscles may be used to 

 prevent the excitation from passing down (p. 786). Or a dose of 

 curara just sufficient to paralyze the motor inner vation may be 

 given to a rabbit, and the animal kept alive by artificial respiration. 

 The sciatic is now stimulated for many hours. As soon as the 

 influence of the curara begins to wear off, the muscles of the leg 

 contract. 



The possible seats of fatigue caused by voluntary muscular con- 

 traction are (i) the muscle; (2) the nerve-endings (or the receptive 

 substances in the muscles, p. 713); (3) the nerve-trunk; and (4) the 

 path of the voluntary motor impulses in the central nervous 

 system, which includes the pyramidal cells in the motor region of 

 the cerebral cortex (p. 847), the fibres of the pyramidal tract, and 

 the motor cells in the anterior horn of the spinal cord. 



The two weak links in this chain appear to be the motor nerve- 

 endings and the muscles. The nerve- fibres, whether peripheral 

 or central, are certainly the strongest link. Ergographic experi- 

 ments have hitherto yielded results too discordant to justify any 

 very definite statement as to the point at which the chain snaps in 

 complete fatigue, if, indeed, it always necessarily breaks at the same 

 point. The muscles and motor endings appear to be always affected. 

 The position of the nerve centres, including the synapses (p. 824), 

 is in doubt. That the synapses easily lose their power of con- 

 ducting nerve impulses under the influence of repeated excitations 

 is indicated by the experiments of Sherrington on fatigue of reflex 

 mechanisms in which two or more afferent paths can cause discharge 

 along a common efferent path (p. 874). When excitation of one 

 of the afferent paths has ceased to be effective, the reflex contrac- 

 tions 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 tem- 

 porary 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 fact that a muscle, completely fatigued by direct 

 electrical stimulation, can still be voluntarily contracted, has been 

 supposed to indicate that the voluntary excitation is more effective 



