METABOLISM. 15 
dient of the food, as of the proteids, carbohydrates, or fats. Thus 
proteid metabolism signifies the chemical changes undergone by the 
proteids of the food in their conversion into the corresponding 
excretory products. In ordinary usage the chemical reactions 
undergone by the ash ingredients of the food are not included, the 
word metabolism being practically used to designate the chemical 
changes in the organic matter of food or tissue. 
MerTaxBo.ismM a Process or Oxipation.—The process of met- 
abolism as a whole is one of oxidation. While we must beware 
of being misled by analogy into regarding as a simple burning of 
food-materials that which is in reality a highly complex action of 
the living cells of the organism, still the final result is much the 
same in both cases. Starting with more or less complex organic 
substances and oxygen, we end either with the completely oxidized 
compounds carbon dioxide and water or with nitrogenous sub- 
stances like urea more highly oxidized than the protein from which 
they are derived. 
The oxidative character of the total metabolism is most simply 
illustrated by a comparison of the percentage of oxygen contained 
in the most prominent ingredients of the food, on the one hand, and 
in the chief excretory products, on the other hand, as in the,follow- 
ing statement: 
Percentage of Oxygen. 
In food: 
Protein (average)... .... 0. cece ee eee ee es 23.00 
PatsSiw i Hes Sees eee ee ee es seat ee 11.50 
DGXtPOSO: wired i Aa RRR eR ete oes 53.33 
In excreta: 
URC aie ae pawiee dae eal tes wee eee waceea 26.67 ‘ 
Carbon dioxide...........ee eee eee eees 72.72 
Wa ber idee Seen Ln gia SH ei asere ee 88.89 
Merapouism aN ANALYTIC Process.—From a slightly different 
point of view, metabolism may be described as an analytic process. 
The molecules of the food constituents are highly complex. The 
molecule of dextrose or levulose, the forms in which the carbo- 
hydrates are chiefly resorbed, contains 24 atoms; the molecules of 
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