270 THE BODY AT WORK 



nervous system. Tired muscles contract energetically when 

 the impulses which reach them are sufficiently urgent. 



Nothing so definitely removes muscle from the category of 

 machines as its liability to fatigue. To speak of a muscle as 

 tired is, of course, to transfer to an object a term which is 

 applicable only to a phenomenon of consciousness ; but it is 

 necessary, unless a cumbrous expression is to be used, to 

 designate thus the effect upon the muscle of prolonged activity. 

 The petrol may be low in the tank, but the quantity burnt in 

 the cylinder at each stroke is not reduced. If an isolated 

 muscle is repeatedly stimulated by an electric current of a 

 certain strength, the response which it makes improves for 

 the first two or three induction shocks ; then it begins to 

 weaken. At each succeeding spasm the muscle shortens a 

 trifle less than before. More remarkable than the diminution 

 in the amount of work done by a muscle which is growing 

 tired is the prolongation of the time taken both in contracting 

 and in relaxing. Further, it has been shown that the fatigue 

 which accompanies the contraction of an isolated muscle is 

 not a condition dependent upon the shrinking of the store of 

 energy which it possessed when it was first thrown into activity . 

 Muscles undisturbed as to blood-supply, and contracting under 

 the direction of the Will, also exhibit it. Speaking generally, it 

 may be said that the tiring of muscle is not so much due to the 

 exhaustion of its store of food as to accumulation of products of 

 action. Vigour is restored to a tired muscle by passing through 

 its bloodvessels a stream of salt-solution, which brings it no 

 food, but washes away some of its waste. But the problem is 

 far more complex than this. The machinery is not simply 

 clogged with the products of its own activity. If the blood 

 of a tired animal is injected into the vessels of one that is 

 rested, the muscles of the latter exhibit the phenomena of 

 fatigue. Evidently muscle is self -protective. During activity 

 it prepares a " fatigue-substance " which poisons its own 

 nerve-endings, making them worse conductors from nerve to 

 muscle of the commands which descend from the brain. Not 

 only does the fatigue-substance dull the nerve-endings in 

 the particular muscle which has contracted, but, being dis- 

 tributed by the blood to the whole body, it produces a general 

 effect. If the legs have been severely worked, they exhibit 



