16 PRINCIPLES OF ANIMAL NUTRITION. 
the three most common fats, respectively 155, 167, and 173 atoms. 
The molecular structure of the proteids has not yet been made out, 
but it is highly complex.* The molecules of the excretory prod- 
ucts, on the contrary, are comparatively simple, those of carbon 
dioxide and water containing but three atoms each, that of urea 
eight, and even that of hippuric acid but twenty-two. 
In metabolism, in other words, the complex molecules of the 
carbohydrates, fats, proteids, etc., which have been built up in the 
plant, by means of the energy contained in the sun’s rays, out of 
carbon dioxide, water, and nitric acid or ammonia, gradually break 
down again into simpler compounds, their atoms reuniting with 
the oxygen from which they were separated in the plant. 
Metaspotism A GRADUAL PRocrss.—The chemical changes in- 
cluded under the term metabolism take place gradually. As has 
already been indicated, metabolism is not a simple oxidation of 
nutrients, like the burning of fuel in a stove, but the nutrients enter, 
to a large extent at least, into the structure of the cells of which the 
various tissues are composed. Metabolism is really the sum of the 
chemical actions through which the nutrition and life of these cells 
is manifested. These actions, however, differ from tissue to tissue 
and from cell to cell, and even in the same cell from time to time, 
and the resulting metabolic products are correspondingly varied. 
Between the nutrients supplied to the cells by the blood and the 
final products of metabolism as excreted from the body there are 
innumerable intermediate products, a few of which we know but 
concerning most of which we are still ignorant. We know the first 
and last terms of the series and thus are able to measure, as it 
were, the algebraic sum of the changes, but of the single factors 
making up this sum. as well as of the specific tissues concerned in 
the changes, we are largely ignorant, although we know that they 
are numerous. 
ANABOLISM AND KaTasoLism.—While the process of metab- 
olism as a whole is one of analysis and oxidation, with liberation 
of energy, it must not be supposed that each single step in the 
process is of this nature. As has been already pointed out, the 
chemical activities of the tissues possess a dual character. By the 
* Osborne (Zeit. physiol. Chem., 38, 240) has recently obtained the 
number 14,500 as the approximate molecular weight of edestin. 
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