NITROGENOUS METABOLISM 151 



tity of protein fed say 100 grams cannot contribute an 

 equal quantity to the protein supply of the body. When 

 protein of one kind undergoes complete hydrolysis and 

 protein of a new kind is to be made from the resulting 

 cleavage products, certain of the building-stones will be 

 needlessly plentiful, while others will be relatively scarce. 

 We have seen that if a single one of these structural units 

 is not furnished, there is complete failure to synthesize 

 the new compound. Similarly, if the second body is to 

 contain a large percentage of an amino-acid which is but 

 scantily represented in the first, the possible formation is 

 definitely limited. The principle is easily illustrated. 

 Suppose that in a club of 100 members there are 25 Demo- 

 crats. It is desired to elect for purposes of debate the 

 largest possible body, consisting of Democrats and Re- 

 publicans in equal numbers. Evidently, this body will 

 comprise 25 men of each party. There will be 50 men un- 

 related to the new organization. We may change the 

 comparison: A house is pulled down and another is to be 

 erected from the timbers. If the second house is of an 

 architecture entirely unlike that of the first, there will be 

 many unavailable pieces to discard and the new building 

 will be smaller than the old. It is not at all unlikely that 

 the misfit fragments of building material will go into the 

 cellar of the new house, later to be used as fuel. This is 

 just what the body does with the misfit amino-acids. So 

 far as they do not find place in the mosaic which is put 

 together they serve as producers of energy. 



Again, a better analogy suggests itself : The structure of 

 a molecule of food-protein, previously compared with that 

 of a watch or a house, may be likened to the type set up to 

 print a page. The letters, some of which occur frequently 

 and some rarely, stand for the amino-acids. The type is 

 allowed to fall apart, the symbol of digestion. It is to be 

 set up again to print different matter. If the language and 

 vocabulary are much as at first, it may be possible to com- 

 pose nearly a whole page before the lack of some letter 

 brings the proceeding to a standstill. But some shrinkage 



