38 NUTRITIONAL PHYSIOLOGY 



as it produces motion and does physical work at the cost of 

 fuel destroyed. The resemblance goes farther than this, 

 for both with the engine and with the muscle the ac- 

 complishment of a measured amount of work is attended 

 by seemingly wasteful evolution of heat. An engine is 

 considered economical if it turns 15 per cent, of the energy 

 resident in its fuel into horsepower. Muscles sometimes 

 do better than this, but much of the time they are even less 

 efficient. It is fair to point out that the heat set free as 

 an accompaniment of muscle contraction is often of value 

 to the animal. In our own case the temperature of the 

 body must be kept above that of its usual surroundings. 

 By far the largest part of the heat devoted to this main- 

 tenance of a relatively high body temperature is produced 

 in connection with muscular activity. Muscles are 

 thus seen to be organs of heat production as well as 

 organs to carry out movements. When the external 

 temperature is high or the degree of muscular contrac- 

 tion is greatly above the average, the heat evolved 

 does become distinctly an embarrassment to the organ- 

 ism. 



The source of the energy displayed in muscular activ- 

 ity is chiefly the disruption of carbohydrate molecules. 

 Sugar appears to be the preferred fuel of the muscular 

 machine, though other foods are known to be available also. 

 When sugar is completely oxidized the only end-products 

 are carbon dioxid and water, the same substances which 

 would be formed by the literal burning of the sugar with 

 an adequate supply of oxygen. These waste-products are 

 very readily removed from the muscle, when its situation 

 is normal, by the circulating blood. The carbon dioxid 

 will almost immediately escape from the blood when it 

 passes through the lungs. The water becomes part of 

 the large total volume which is always passing into and out 

 of the body, and may leave by all the main channels of 

 excretion the respiratory passages, the kidneys, and the 

 skin. It will be noted that the quantity of water leaving 

 the body is constantly in excess of the income. Ordinar- 



