CHAPTER VIII 

 METABOLISM 



HAVING traced the food to its _ reception in the blood, it 

 will be proper to consider the facts that are known concerning 

 its further history. The absorbed materials are rapidly carried 

 to all portions of the body, and in the capillaries are trans- 

 ferred through their walls to the lymph or tissue fluid, which, 

 in turn, brings them into intimate contact with the tissue cells. 

 Each cell extracts from the fluid which bathes it the substances 

 that it needs for its nourishment. Then, under the influence 

 of living matter or its products (enzymes), these substances 

 undergo a series of changes, anabolic and katabolic in nature, 

 which converts them finally to simple stable bodies possessing 

 little energy. 



Energy of Food. The law of the conservation of energy 

 teaches that the sum total of energy of the universe is constant, 

 and that it can be neither created nor destroyed, or, in other 

 terms, increased or diminished. This law is as rigorously 

 true for the body as for any physical system, so that the mani- 

 festations of living bodies must be the transformations of 

 energy brought to them in some form or other. Of all the 

 sources of energy to the body, the chemical energy of the food 

 is the most important. It is in a potential form, and reappears 

 in the body in kinetic form as heat, electricity, and mechanical 

 work. By far the greatest amount appears as heat. An adult 

 man in the course of twenty-four hours will liberate about 

 2,400,000 calories of heat a calorie being equal to the amount 

 of heat which is required to raise one cubic centimeter of water 

 one degree Centigrade. 



Since the energy of foodstuffs is set free by their physio- 

 logical oxidation, it is obvious from the standpoint of the 

 doctrine of the conservation of energy that it may be measured 

 by burning the foodstuffs outside of the body. This is done 



