146 NUTRITION OF FARM ANIMALS 



times katabolized must first become integral parts of the protoplasm. 

 In other words, it is probable that the cells have the power to katab- 

 olize substances present within them but not structurally a part of 

 them. 



204. Synthetic processes in the body. The foregoing conception 

 of metabolism implies that the body has power to carry out extensive 

 chemical syntheses, contrary to the idea still current that the course 

 of chemical change in the organic world is toward the building up of 

 complex compounds in the plant and their breaking down to simpler 

 ones in the animal. Synthethic chemical changes were long regarded 

 as peculiar to the vegetable kingdom, while the reactions in the ani- 

 mal body were supposed to be exclusively analytic. The first syn- 

 thetic action to be recognized in the animal was the formation of 

 hippuric acid from benzoic acid, discovered by Keller and Wohler in 

 1824, and which attracted wide attention. More recent physio- 

 logical investigations have shown that this is by no means an isolated 

 case, but that syntheses in great variety are executed in the animal 

 body. No such sharp distinction between animal and vegetable 

 organisms exists as was formerly supposed. The fundamental laws 

 of metabolism are the same for both and both execute synthetic as 

 well as analytic processes. It is only the special synthetic activity 

 of the chlorophyl in green plants which tends to obscure this funda- 

 mental likeness. The conception, then, that the digestive cleavages 

 supply to the body cells comparatively simple "building stones" 

 which are synthesized to produce the complex ingredients of cells 

 and tissues is quite in harmony with our general knowledge of the 

 nature of metabolism. 



205. Metabolism oxidative and analytic. Metabolism re- 

 garded as a whole may be characterized chemically as an oxi- 

 dation. Oxygen is introduced into the system through the 

 blood and reacts with the feed or tissue materials or with the 

 products of their breaking down, and the final excretory products 

 are either completely oxidized substances, like carbon dioxid 

 and water, or substances approaching this condition, like 

 urea, etc. 



From a slightly different point of view, metabolism as a whole 

 may be characterized as an analytic as opposed to a synthetic 

 process. The general tendency is toward the formation of 

 simpler molecules. For example, the molecule of dextrose or 

 levulose contains 24 atoms and those of the three most com- 

 mon fats, respectively, 155, 167 and 173 atoms, while the 

 molecules of carbon dioxid and water resulting from their metab- 



