296 Biology in America 



food stuffs are absorbed mainly at least by the walls of the 

 intestine, whence they are carried by the body fluids (blood 

 and lymph) to the tissues of tlie body, where probably under 

 the influence of other ferments they are again built up into 

 complex substances, which compose the protoplasm of the 

 body cells.* Thus the kernel of the wheat, or the muscle of 

 the beef, is iu some mysterious way transformed into the 

 muscle and the nerves, the blood and bone of the animal 

 which consumes them. The various steps in the digestive 

 and absorptive processes are extremely complicated and their 

 character is not fully understood. The large number of prod- 

 ucts formed in tlie digestion of the proteid molecule form 

 one evidence of the complex nature of protoplasm. Leaving 

 out of consideration the simpler processes of digestion of 

 starch, sugar and fat, and dealing with proteid digestion 

 alone; passing over also the many and complicated stages in 

 the journey of the proteid molecule through the digestive 

 tract, we come to the end products of digestion, the amino- 

 acids or "l)uilding stones of proteid," as they have l)een 

 called. Tliese amino-aeids include a large number of sub- 

 stances, all built around the common nucleus of NHo. Witli 

 these as a basis the constructive ferments of the l)ody build 

 up its marvelously complex materials. 



A comparison of the animal body with a machine, the food 

 of the former corresponding to the fuel of the latter is only 

 partially exact, for in the machine, as we have seen, the fuel 

 is directly consumed to furnish energy, while in the animal 

 the change of food energy into work energy is effected in 

 part only through the medium of the body substance 

 itself. After the conversion of the digested food stuffs into 

 the protoplasm of the l)ody this must in turn be broken down 

 through the action of the oxydizing, or destructive ferments, 

 into a whole series of decomposition products, of gradually 

 decreasing complexity, the principal end results being carbon 

 dioxide and urea. 



Some of the food stuffs, notably those with the highest 

 energy content, the fats and carbohydrates, and to a less 

 extent the proteids also, may after digestion be directly 

 oxidized to furnish energy; or may in the case of fat and 

 glycogen be stored by the body as a reserve supply for future 

 need. Thus a hibernating animal, such as a bear, during the 

 summer lays up for himself a bountiful supply of fat upon 

 whieli to draw during the long winter's fast. This storage 

 of energy in the form of reserve food stuffs by the living 



*Sueh a brief statement as the above naturally overlooks the many 

 intermediate steps in this very complicated process. 



