312 STUDIES IN ADVANCED PHYSIOLOGY. 



in mind that this temperature is maintained by a continued 

 radiation of heat from the body, or by a loss of heat in the 

 evaporation of the sweat of the skin. Such a continued 

 loss during twenty-four hours would not be inconsiderable. 

 Evidently, therefore, to replace this loss of energy there 

 must be introduced into the body as food, substances which 

 when they are burned will give the amount of heat required, 

 and of course in addition, the energy required for the move- 

 ment of the muscles, which, as stated, is about one-fifth 

 more in amount. 



In giving as the losses in energy muscular activity and 

 heat, no attention is paid to other possible forms of energy 

 in connection with secretion, or with the nervous system. 

 In the former there are probably no other forms of energy 

 concerned, while what the nature of the energy is in nerves 

 and psychic states we are at present perfectly unable to 

 state. That such states are accompanied by the production 

 of heat is a known fact, but that all the energy is trans- 

 formed into heat is another question. 



The exact manner, now, in which the body is able to 

 appropriate these foods and build them up into its own tis- 

 sues, or the manner in which it derives from these foods 

 the energies it expends, will be more fully discussed in the 

 chapter on nutrition. 



THE CLASSES OF FOODS. 



It would be entirely out of place here to enumerate the 

 large list of substances which figure as foods. These are 

 sufficiently familiar. A study of the varied menu of our 

 tables shows that all foods may be divided into a few 

 typical classes in which the foods of a class not only close- 

 ly resemble each other, but in which those of one class 

 are clearly distinguishable from those of any other. These 

 classes are, first, albumens, or proteids; second, albumin- 

 oids; third, carbohydrates; fourth, hydrocarbons; fifth, in- 

 organic salts; sixth, zvater. 



