FATIGUE 339 



tion in the height of contraction, and a very considerable 

 prolongation of the period of relaxation. In fact, the 

 muscle fails to recover its original length and undergoes 

 gradual and permanent shortening. Eventually it fails 

 to respond altogether : the muscle has lost its capacity for 

 contraction. 



When a muscle is made to undergo repeated volimta/nj 

 contractions, these diminish to extinction ; but when it is in 

 this state the muscle has not lost its capacity to contract, 

 for it responds briskly to electrical stimulation appUed 

 to the muscle itself or to its nerve. Under physiological 

 conditions, then, loss of functional capacity is not to be 

 entirely or even primarily located in the muscle, nor in 

 the nerve-ending, nor, again, in the nerve-trunk. As regards 

 the last, nerve-fibres are beUeved to be almost immune to 

 fatigue. ^, 



We have seen that the reflex arc is more Uable to fatigue 

 than the nerve-fibre. This greater susceptibility of the 

 arc must be attributable to the nerve-cell, or to the synapse, 

 or to the receptor organ. 



It is not the nerve-cell, for the final common path (p. 278), 

 when stimulated to fatigue by one receptor, responds with 

 undiminished vigour to another. Nor is there any evidence 

 that the receptor is specially prone to fatigue. Fatigue 

 must therefore be located in the synapses between the 

 neurones. As to the cells of the brain, histological changes 

 have been described in them as the result of prolonged 

 activity. The diminished capacity to function seems, 

 therefore, to occur in the synapses and in the higher nerve- 

 cells. 



The Cause of Fatigue. — The signs of fatigue in the nerve- 

 muscle preparation are associated with the accumulation 

 of lactic acid, and are, indeed, in large measure due to it, 

 for if the muscle is perfused with a fluid not containing 

 any food substances or oxygen, recovery ensues. If, on 

 the other hand, fresh muscle is perfused with lactic acid, it 

 becomes more prone to fatigue. This accumulation of 



