136 FOODS 



General Requirements in a Food. The total food of an animal at 

 different periods of his life must meet three requirements (as, indeed, 

 Aristotle long ago pointed out), each indispensable: (1) Throughout 

 life the food must be able to supply energy, by expending which the 

 animal lives. Previous to its birth this energy, like the body- tissues, 

 was derived ready-made from the maternal organism. (2) Until 

 maturity the food must afford the materials for constant bodily growth 

 that is, the tissues must be built up faster than they are worn away by 

 use. (3) At all periods of life the food must furnish a continuation of 

 the tissue-material used up and worn out by the universal wear and 

 tear to which all material objects are subject in some degree or other. 



All food taken into the body and digested is reduced to the simplest 

 terms in which it can retain its characteristics. These food-elements are 

 then reconstructed by the eating animal into his own sort of tissues or 

 proximate principles. In other words, there is never any direct trans- 

 ference of food -materials from the flesh of one animal to that of another. 

 When a man eats pork, his tissues do not become more like those of a 

 pig; and sheep-fat is never to be found in a dog's adipose tissue after 

 being fed even wholly on mutton (except under the abnormal conditions 

 of forced feeding far beyond the limits of normal digestion). Just as in 

 a paper-mill all sorts of paper and rags are macerated together and then 

 run out as one fresh homogeneous product, so in organisms protein, fat, 

 carbohydrate, albuminoid, inorganic salts, and water are run through the 

 mechanism of the individual's digestion and absorption and are assimi- 

 lated to the special likenesses of the particular tissues of which perhaps 

 they are to form a part. Chemically as well as histologically the muscle of 

 no two sorts of animals is exactly alike, yet every one of them may serve 

 in part as the chief and adequate food of any other carnivorous animal. If 

 the flesh of seagulls tastes like that of fish it is because these birds at times 

 gorge themselves with fish far beyond even their powers of assimilation, 

 although not beyond their digestive powers. Only recently has it been 

 learned how complete is the tearing-down of food in the alimentary 

 canal. The absorbing wall of the intestines has, therefore, powers of 

 recombining these food -elements, "proximate principles," to a degree 

 much greater than was a few years ago suspected. 



By the definition in common use, any substance which produces in 

 an organism animal tissue and energy or either of these is a food. Neither 

 of these results, however, can be effected until the nutrient has been actu- 

 ally absorbed, after its digestion, at least into the circulating fluid of the 

 body, the blood, and has thus become a part of the organism. Thus, a 

 food is really a food only after it has been absorbed . Two requisites of 

 a food, then, are digestibility and absorbability. The prerequisite of 

 digestibility is obviously essential. No substance, candidate for use as 

 a food, unless directly soluble in water or the normal alkaline salines of 

 the body (saliva, the blood and lymph, etc.), can be digested except those 

 proximate principles of nutrients so often rehearsed namely, proteids, 

 fats, carbohydrates, and albuminoids. These predominant components 



