10 INTRODUCTION 



tory while the animal is the analytical or de- 

 structive machine. The plant takes the small- 

 est parts and builds them up into highly com- 

 plex bodies, while the animal takes the com- 

 plex and splits them into pieces to be recon- 

 structed in its own body. In a general way 

 the above statement is true, but there are syn- 

 thetical processes going on normally in the 

 animal body and it is demonstrable that simple 

 proteins may be built into more complex mole- 

 cules in the animal body. Moreover, it is cer- 

 tainly true that in man with perfect digestion 

 practically all the nitrogen of the food is 

 absorbed in the form of amino acids. The 

 animal as well as the plant is a synthetical lab- 

 oratory, but the new material used by the 

 former is the finished product of the latter, 

 which is unravelled and then woven into a new 

 pattern which is different in each species of 

 animal. 



There are as many kinds of proteins as there 

 are kinds of living matter. Chemically pro- 

 teins are polymers of amino acids. The amino 

 acids demonstrated in proteins are only about 

 eighteen in number, but with these put to- 

 gether in an almost infinite variety of ways, we 

 get an unlimited variety of products, just as 

 with only twenty-six letters in the alphabet 



