analysis, the breaking down of tissue material in the complicated processes of catabolism has been ascribed to oxidation. While this explanation is in a measure true, the passing years have brought to light many additional data which tend to show that simple oxidation is quite inadequate 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 ordinary 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 conception, 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 attempt to unravel the nature of the processes associated with nutrition; and this is equally true whether we are dealing with the changes incidental 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. Physiologists for years believed they understood the purport of this process, which was merely to transform the protein foods into soluble and more