366 HUMAN PHYSIOLOGY 



have seen in the frog, muscles are well supplied with blood vessels: 

 under the compound microscope, these blood vessels are observed 

 to break up into tiny capillaries so that the fibers are literally 

 bathed in blood and lymph. The protoplasm of the muscles 

 takes up oxygen from the blood; this oxygen unites with the 

 material of which the muscle is formed or with food which is brought 

 there by the blood; as a result of the oxidation, carbon dioxide 

 is given up and carried off by the lymph; nitrogenous wastes 

 (such as urea) are also removed in the lymph. 



How Bodily Heat is Produced. — When coal is burned, heat is 

 given off; this heat is a form of energy. Food and tissues, when 

 oxidized, also give off heat. When muscles are exercised, oxida- 

 tion takes place more rapidly, and more heat is thus produced. 

 From one sixth to one third of the energy produced by oxidation 

 of muscles results in work; two thirds to five sixths of it is given 

 off in the form of heat. It is evident that the high body tempera- 

 ture is thus directly due to the oxidation of foods or tissues. The 

 heat thus produced in muscles is largely dissipated to other parts 

 of the body by means of the blood. 



When an extra amount of heat is released in the body, the blood 

 is allowed to go to the skin, and heat is there set free : An increased 

 amount of blood sent to the skin favors perspiration by increasing 

 the rate of evaporation; this leads to an increased loss of heat 

 from the body. 



Necessity of Food and Fresh Air for Muscles. — From the facts 

 given above, it is evident that muscles need food and a supply of 

 oxygen in order to perform their work. Muscle, although a large 

 per cent is made up of water, contains proteid as its principal 

 solid ingredient. (This may be proved by testing a bit of frog 

 muscle with nitric acid and ammonia.) In order to build up new 

 muscle to replace the cells which are worn out or used up, proteid 

 food is a necessity. Sugars, which are energy-releasing foods, 

 can be utilized in moderate amounts. 



Fresh air is a necessity because all muscular energy is released as 

 a result of oxidation. In order to perform muscular work, fresh 

 air containing a large per cent of oxygen is necessary. It is essen- 

 tial to the health of every one, even a sedentary person, that a 



