NUTRITION. 453 



condition by plants. Nevertheless the Body has certain con- 

 structive powers: it, at least, builds up protoplasm from 

 proteids and other substances received from the exterior; 

 and there is reason to believe it does a good deal more of the 

 same kind of work, though never an amount equalling its 

 chemical destructions. Given one single proteid in its food, 

 say egg albumen, the Body can do very well; making serum 

 albumen and paraglobuliri out of it for the blood, myosinogen 

 for the muscles, and so on : in such cases the original proteid 

 must have been taken more or less to pieces, remodelled, and 

 built up again by the living tissues; and it is extremely 

 doubtful if anything different occurs in other cases, when 

 the proteid eaten happens to be one found in the Body. In 

 fact, during digestion the proteids arc broken down some- 

 what and turned into peptones; in this state they are absorbed 

 and must somewhere again be built up into the proteids of 

 the tissues. 



The constructive powers of the Body used to be rather 

 too much ignored. Foods were divided into assimilable and 

 combustible, the former serving directly to renew the organs 

 or tissues as they were used up, or to supply materials for 

 growth; these were mainly proteids and fats; no special 

 chemical synthesis was thus supposed to take place, the living 

 cells being nourished by the reception from outside of mole- 

 cules similar to those they had lost. Fat-cells, it was sup- 

 posed, grew by picking up fatty molecules like their contents, 

 received from the food; and albumen-rich tissues by the re- 

 ception of ready-made proteid molecules, needing no further 

 manufacture in the cell. The combustible foods, on the other 

 hand, were the carbohydrates and some fat: the carbohy- 

 drates, according to the hypothesis, were incapable of being 

 made into parts of a living tissue, and were merely oxidized 

 in order to maintain the bodily warmth. It having been 

 proved, however, that more fat might accumulate in the body 

 of an animal than was taken in its food, this excess was ac- 

 counted for by supposing it was due to excess of com- 

 bustible foods, converted into fats and stored away as oil- 

 droplets in various cells; but not actually built up into true 

 living adipose tissue. Liebig, somewhat similarly, classed all 

 foods into plastic, concerned in making new tissue, and 

 respiratory, directly oxidized before they ever constituted 

 part of a tissue. The plastic foods were the proteids, but 



