DIGESTION 143 



purpose of setting their energy free ; others slowly disintegrated 

 as the result of tissue " wear and tear." Gradually it was 

 realized that many chemical changes occur in the body which 

 cannot be viewed as merely exhibitions of its analytical 

 capacity. The tissues were recognized as laboratories in 

 which reactions occur which consist in something more than the 

 splitting of complex into simpler molecules. The instances 

 earliest understood were connected with the history of carbo- 

 hydrates and fats. In the disease diabetes an enormous 

 quantity of sugar is excreted, amounting in extreme cases to 

 between 1 and 2 pounds per diem. When carbohydrates are 

 present in the food, the amount of sugar excreted in diabetes 

 is greater than it is when they are withheld ; on an almost 

 exclusively proteid diet the amount of sugar excreted far 

 exceeds the amount of carbohydrates in the food. Another 

 illustration of the power of making sugar possessed by the 

 animal economy is afforded by a dog fed upon lean meat, and 

 nothing else. Sugar is found in its blood, and a store of carbo- 

 hydrate (glycogen) in its liver. The formation of fat is an 

 instance of constructive metabolism. There is abundant 

 evidence that the quantity of fat produced may greatly exceed 

 the quantity contained in the food. Animals are fattened 

 for the market on a diet which contains less fat than that 

 which accumulates in their bodies. When nursing her young, 

 an animal may secrete in her milk much more fat than she 

 obtains as such in food. It was a great mistake to suppose 

 that the body is dependent upon its tradesmen for fat and 

 sugar. It can make either of these substances out of a mixed 

 diet in which it is relatively deficient. It must, however, be 

 a mixed diet. An animal cannot live exclusively on fat or 

 exclusively on carbohydrate. It is impossible, therefore, for 

 us to determine whether, if given the one alone, it can turn it 

 into the other. Chemists were very unwilling to credit the body 

 with the power of performing even the simpler of these trans- 

 formations the conversion of carbohydrate into fat. Proteins 

 are essential constituents of a fattening diet. Their immensely 

 complex molecule has always afforded a tempting field for 

 arithmetical ingenuity. It is easy to remove from it the 

 atoms needed for the composition of fat, and yet to leave such 

 groups of atoms as might reasonably be supposed to con- 



