46 THE BODY AT WORK 



contract until it refuses to work any longer, it again responds 

 to stimulation after a solution of salt has been passed through 

 its bloodvessels. The salt solution brings no food ; the only 

 thing it can do is to wash away waste products. But this 

 experiment upon a tired, isolated muscle does not necessarily 

 throw light upon the nature of fatigue in muscles under normal 

 conditions. The isolated muscle is using up, in contracting, 

 food which it has stored. Cut off from the circulation, it has 

 no means of getting rid of the lactic acid and other products 

 into which food is changed. They may well have accumulated 

 to a poisonous extent long before all the food has been used up. 

 Hardly more cogent is the argument based upon the benefit 

 which a tired man experiences from hot baths, massage, and 

 the like. They take away the feeling of tiredness, but it 

 does not follow that this result is due to the removal of waste 

 products. Quickening the circulation of blood brings about 

 renewal of the lymph. Renewal of lymph means fresh supplies 

 of food as well as removal of waste products. Even human 

 muscles are not perfect as machines. They will not work for 

 an unlimited spell. There comes a time when they must have 

 rest. Something goes wrong in the admirable adjustment 

 which has hitherto provided exactly the right amount of food 

 and exactly the necessary freedom from the products of action. 

 A feeling of fatigue is the signal that the apparatus is not in 

 a condition to work longer ; but whether this feeling is due 

 to a dislocation of the balance of supply and loss, or to some 

 deterioration of the apparatus which calls for rest and renova- 

 tion, it is at present impossible to say. It is not due to the 

 exhaustion of muscle food. A more powerful stimulus, the 

 urgency of fright or some other strong emotion, or an electric 

 current applied directly to the muscle or its nerve, will still 

 induce vigorous contraction. The muscles of a hare that has 

 been coursed until it can run no farther still contain glycogen, 

 muscle-food. 



Glycogen is stored in the liver. Fat, if it is assimilated in 

 excess of the needs of the body, accumulates in the connective 

 tissues. Proteins, if in excess, are either destroyed by oxida- 

 tion, or partly destroyed and partly converted into fat. In- 

 creasing the amount and richness of the food does not, if 

 nutrition is already at its best, improve the quality of the 



