388 POPULAR SCIENCE MONTHLY 



analysis, the breaking down of tissue material in the complicated proc- 

 esses of catabolism has been ascribed to oxidation. While this explana- 

 tion is in a measure true, the passing years have brought to light many 

 additional data which tend to show that simple oxidation is quite inade- 

 quate to account for the variety of transformations that pertain to 

 nutrition. Many other factors are involved that give to these processes 

 a totally different and at the same time broader scope than was formerly 

 thought of. The simple views of Lavoisier and the later theories of 

 Liebig do not suffice ; they fail to explain the numerous and complicated 

 reactions occurring during life. 



Something was lacking in our knowledge during these earlier years, 

 and that was an understanding of the role of the cell in nutrition. It 

 was difficult for the chemists of this period to let go of the tempting 

 hypothesis that oxidation in the animal body was akin to that of ordi- 

 nary combustion and their eyes were apparently closed to the many 

 inconsistencies that such a theory imposed. When, however, Virchow 

 developed his cellular hypothesis and it became clear that the living 

 cell was the morphological unit of the body, then it gradually dawned 

 on the physiological world that the cell was likewise the seat of the 

 many chemical transformations associated with nutrition. Oxidation 

 could not occur in the lungs, it did not take place in the blood, there 

 was no one particular spot where the fires of the body were located. 

 On the contrary, they occurred everywhere, in every living cell, and 

 all kinds of combustible or oxidizable material were burned. This con- 

 ception, in which the living cells might well be compared to miniature 

 laboratories, is now thoroughly justified by the facts at our disposal. 

 Still, it is not the cell as a physiological unit that is to be considered 

 as the cause of the varied decompositions that occur in the body. 

 Enzymes of various types appear in the foreground whenever we at- 

 tempt to unravel the nature of the processes associated with nutrition; 

 and this is equally true whether we are dealing with the changes inci- 

 dental to digestion or with those more subtle ones associated with the 

 processes of metabolism. In the living cells of the body there are many 

 agencies at command, enzymes or ferments of divergent forms endowed 

 with the power of inciting and carrying forward chemical changes of 

 differing degrees of magnitude, by means of which complex organic 

 matter is made to undergo alteration and decomposition. 



Turn for a time to the changes which the protein or albuminous 

 foods undergo in digestion. Here we have what was for a long period 

 considered as a simple process of transformation or polymerization, 

 brought about mainly through the agency of the two enzymes pepsin 

 and trypsin, of the gastric and pancreatic juice, respectively. Physi- 

 ologists for years believed they understood the purport of this process, 

 which was merely to transform the protein foods into soluble and more 



