300 THE ANIMAL AS A PRIME MOVER. 



as stimulants of digestive and other physiological processes, even when 

 not themselves in any degree digestible, and are, lor that reason, essen- 

 tial constituents of food. It is for this reason largely that vegetable 

 foods are indispensable to perfect action of the functions and to bodily 

 and mental health. The vegetable food also, especially the fruits and 

 grains, contain the required elements of all the compositions of the 

 animal system in best proportion and best arranged for utilization by 

 man and all other except the purely carnivorous animals. Could the 

 whole animal be used as food, including blood and nerve and bone and 

 brain, animal food would be substantially correct in composition; but 

 it would still lack the stimulating property of the other class of foods 

 which comes of the presence of the mineral and indigestible elements. 



The uses of food are mainly two: (1) To supply material for the build- 

 ing up and the repair and maintenance of the tissues of the body. (2) 

 To furnish the energies required in the operation of the animal machine 

 in their various forms and in due proportion. 



The first is the direct application of a portion of the food. The sec- 

 ond disposition of the elements of the food and their potential energy 

 may be direct, as probably in the use of the fats, the combustion of 

 which to carbon-dioxide and water results in the production of imme- 

 diately available energy; or it may be indirect, as where the carbohy- 

 drates are first digested and, later, consumed in similar manner to the 

 fats ; or, still more indirectly, as where the protein becomes first a part 

 of the flesh or of the nervous system and, later, broken down in the 

 course of the work of the body, serves as fuel or otherwise in the pro- 

 duction of heat or other energy by its oxidation. 



Protein forms tissue — muscular, nervous, and other. Fats form a part 

 of the nervous tissue, and carbohydrates are converted by the digest- 

 ive organs into fats, and then serve the same purposes. Both the latter 

 compounds serve as fuel or energy- storing reservoirs, contain a quantity 

 of potential energy, which is sooner or later drawn upon in the develop- 

 ment of the various energies utilized in the operations of the body and 

 the brain. It is usually assumed that the energy demanded is that of 

 thermal molecular motion, and that the value of the foods may be meas- 

 ured by their calorific content or potential thermal energy. Until it is 

 known just what energies are employed in the numerous and varied 

 operations of the living creature, and to what extent they are severally 

 derivable from the potential energy of the various foods and transform- 

 able from one into another, and especially from or into thermal energy, 

 no better method can be devised than that which assumes the value of 

 foods properly compounded in imitation of nature's known propor- 

 tions — as in milk for children, fruit of palatable character, and grains 

 for adults — to be proportional to their calorific measure. Pure carbon 

 or the pure carbohydrates, however, are not foods in a proper sense, as 

 they could not be converted into muscle, nerve, or bone, and only serve 

 in themselves for energy storage for use by a body composed of protein 



