48 THE HUMAN MECHANISM 



beneath obvious and important differences there are profound 

 and fundamental similarities in the processes which occur in 

 the two organs during activity. Like the gland, the muscle 

 is set to work or stimulated by a nervous impulse ; its con- 

 traction is accompanied by an increased blood supply; and, 

 most important of all, the work, or contraction, is accom- 

 panied indeed, preceded by chemical changes much more 

 profound than that of the transformation of zymogen into 

 enzyme. These chemical changes supply the power for the 

 work. 



That some chemical change has taken place when the 

 muscle contracts is proved by the fact that certain new sub- 

 stances then make then* appearance in the muscle and are 

 given off to the blood flowing through it. The most impor- 

 tant of these are carbon dioxide, the gas which is formed 

 whenever wood or coal is burned, and an acid substance 

 known as lactic acid. These substances were not present 

 in the resting muscle, or else were present in very small 

 quantities. With the act of contraction relatively large 

 quantities of them make their appearance. They are gener- 

 ally spoken of as waste products, and it is known that they 

 are the result of a chemical change in the muscle fiber, or cell, 

 precisely as the enzymes are the result of chemical changes 

 in gland cells. Just as glandular activity produces an out- 

 put called a secretion, so muscular activity produces an 

 output consisting of substances usually described as waste 

 products. 



8. The storage of fuel within the muscle fiber. The source 

 of the carbon dioxide and lactic acid produced by the active 

 muscle must in the long run be the matter taken into the 

 body in the form of food. After undergoing in the stomach 

 and intestine relatively simple changes, which do not pro- 

 foundly affect its chemical constitution, this food is absorbed 

 into the blood and through this channel delivered to the 

 cells. Thus far, however, the food material does not differ 



