142 THE BODY AT WORK 



The History of the Foods after Absorption. All foods, 

 with the exception of inorganic salts and salts of various 

 vegetable acids, fall into three classes : (1) Proteins sub- 

 stances of complex chemical constitution, containing nitro- 

 gen.; (2) carbohydrates so called because hydrogen and 

 oxygen, in the proportions in which they enter into the forma- 

 tion of water, are united with carbon ; (3) fats. Proteins of 

 various kinds are consumed as food. The peptones produced 

 from them by digestion also vary. Yet very little is known as 

 to the differences in physiological value which distinguish the 

 various kinds of protein when absorbed into the fluids of the 

 body (cf. p. 134). All carbohydrates after digestion and absorp- 

 tion appear as dextrose. The various fats preserve their in- 

 dividuality until they are taken up by the tissues. When fixed 

 in the tissues, they assume, except under somewhat abnormal 

 conditions, the composition characteristic of the fat of the 

 animal which has eaten them. If a dog which has been severely 

 starved is fed upon mutton-fat, it puts on in the first instance 

 fat which resembles that of a sheep rather than the normal fat 

 of a dog. As soon, however, as it is well nourished (which 

 would never occur unless some protein and carbohydrate were 

 added to the mutton-fat), its fat assumes the usual form. 



For practical purposes we are obliged to speak of the three 

 classes of food proteid, carbohydrate, and fatty as if there 

 were but one member in each class. And we have abundant 

 evidence that such a simple classification is fully justified. The 

 body has so large a power of altering chemically the nature of 

 the food which it absorbs that it makes little difference in the 

 further history of the food whether the protein supplied to it 

 be an albumin or a globulin ; the fat, stearin, palmitin, or 

 olein ; the carbohydrate, starch or sugar. 



In earlier days it was customary to regard the body as the 

 receiver of a. variety of foods which it could break down into 

 simpler substances by oxidation, but could not reconstruct. 

 Plants were regarded as the manufacturers of organic com- 

 pounds, animals as the destroyers of the complex substances 

 made by plants. The union of molecules, synthesis, was 

 looked upon as the function of the vegetable kingdom. 

 Animals built into their tissues the products elaborated by 

 plants ; some of these products they shook to pieces for the 



