268 THE BODY AT WORK 



iron and brass. These materials are subject to wear and tear ; 

 but they are not the source of its power. Its power is due 

 to the combustion of fuel. Muscle, physiologists formerly 

 said, is made of protoplasm. This wears down when it works, 

 setting free creatin and other nitrogenous debris. Its fuel is 

 glycogen. This is not the way, however, in which the matter 

 is now regarded. Protoplasm is not the machine only, but 

 also the source of power. Glycogen is not burnt in a frame- 

 work of protoplasm. When muscle contracts, protoplasm 

 casts out CO 2 and H 2 0. Glycogen is the food readiest to 

 restore to it the atoms which it has lost. 



Another consideration opposed to the hypothesis of the 

 conversion of glycogen into lactic acid is the uselessness of 

 such a transformation from a physical point of view. The 

 stability of the atoms of C 3 H 6 3 is so little greater than that of 

 the atoms of C 6 H 12 6 that practically no energy is set free when 

 the one substance changes into the other. We cannot, how- 

 ever, overlook the fact that the formation of acid may be a 

 means of profoundly altering the state of the colloid substances 

 dissolved in cell-juice. The casein of milk coagulates when milk 

 turns sour. The neutralization of a faintly alkaline solution of 

 a protein (and muscle is faintly alkaline) will throw it out of 

 solution. The appearance of lactic acid may be intimately 

 associated with movement of protoplasm, and yet the change 

 of glycogen into lactic acid not be the source of the energy 

 which muscle expends. 



Fatigue. For its continued activity muscle needs an ade- 

 quate supply of food and oxygen. If the blood which distributes 

 food is circulating properly, and the liver, the great depot of 

 food, is well stored, fresh supplies are brought to the muscles 

 as they are needed. There are muscles those of the eye and 

 of the heart, for example which never become exhausted. 

 However continuous their activity, they take food from the 

 blood as rapidly as they waste it ; a statement which, perhaps, 

 needs qualifying by the addition, " so long as the work exacted 

 of them is such as may be reasonably expected." If, in a 

 picture-gallery, one keeps the eyes elevated for an hour or 

 more a headache follows. Our eye-muscles have taken over 

 their duties on the understanding that we look down or straight 

 forwards far more often than we look up. If a long-sighted 



