318 HUMAN PHYSIOLOGY 



stances contain the element nitrogen. Hence such foods are 

 called nitrogenous foods. Although about four fifths of the at- 

 mosphere is composed of nitrogen, yet plants are unable to take 

 it from the air, but are forced to absorb it through the roots in the 

 form of nitrates dissolved in the water in the soil. Herbivorous 

 animals eat the plants, take into their bodies the stored nitroge- 

 nous foods, and change this food into protoplasm. Man himself 

 must form the protoplasm of his body (that is, the muscles, ten- 

 dons, nervous system, blood corpuscles, the living parts of the bone 

 and the skin, etc.) from nitrogenous food. Some of this he obtains 

 by eating the flesh of animals, and some he obtains directly from 

 plants (for example, peas and beans). Because of their chemical 

 composition, proteids are considered to be flesh-forming foods. 

 They are, however, oxidized to release energy if occasion requires it. 



Organic Fats and Oils. — Fats and oils, both animal and vegetable, 

 are the materials from which the body derives much of its energy. 

 The chemical formula of a fat shows that, compared with other 

 food substances, there is very little oxygen present ; hence the 

 greater capacity of this substance for uniting with oxygen. 

 The rapid burning of fat compared with the slower combustion 

 of a piece of meat or a piece of bread illustrates this. A pound 

 of butter releases over twice as much energy to the body as does 

 a pound of sugar or a pound of steak. Human fatty tissue is 

 formed in part from fat eaten, but carbohydrates and even pro- 

 teids may be changed and stored in the body as fat. The Arctic- 

 living tribes exist almost entirely on the blubber of the walrus or 

 the whale. The blubber not oxidized in the body is stored as fat, 

 thus forming an insulation under the skin, which aids in keeping 

 the body warm. 



Carbohydrates. — We see that the carbohydrates, like the fats, 

 contain carbon, hydrogen, and oxygen. Here, however, the 

 oxygen and hydrogen are united in the molecule in the same 

 proportion as are hydrogen and oxygen in water. Carbohydrates 

 are essentially energy -producing foods. They are, however, be- 

 lieved to be of some use in building up or repairing tissue. Some 

 experiments seem to indicate that carbohydrates may be formed 

 directly into tissue. It is certainly true that in both plants and 



