40 PRINCIPLES OF ANIMAL NUTRITION. 
Food Proteids and Body Proteids——What is especially to be 
noted in this connection is that the food proteids are not identical 
with the body proteids. This is especially true of the vegetable 
proteids in the food of the herbivora, and of the casein of milk, but is 
measurably true in all cases. A simple resorption of unaltered 
protein, therefore, would not serve the purposes of the organism. 
The food proteids must be changed to body proteids. This means, 
however, that the proportions of those molecular groupings which 
have just been spoken of must be changed—that is, the molecules 
of the food proteid must be so far broken down into their constituent 
molecular groupings as to permit of a rearrangement and repropor- 
tioning of the latter into molecules of body proteid. 
Such a partial breaking down of proteid material takes place in 
digestion, and indeed, as has been indicated above, it is the study of 
digestive proteolysis which has given us our general conception of 
the structure of the proteid molecule. The products of proteid 
digestion, then, as they are presented to the resorbent organs of the 
digestive tract, are no longer proteids, but the constituent molecular 
groupings out of which body proteids may be built up. 
Rebuilding of Proteids.—But while the proteids of the food are 
resorbed in the form of cleavage products, apparently largely as pep- 
tones, no trace of these bodies is found in the blood or in the lymph, 
nor even in the walls of the digestive canal. Still further, peptones 
when injected into the blood are treated by the organism as foreign 
substances and excreted as rapidly as possible, while if added in any 
considerable amount they act as poisons. The reconstruction of 
the proteid molecule from the fragments produced by the digestive 
process has been thought to take place in the epithelial cells of the 
intestines, the first product being probably serum albumen, so that 
we may say that the first step in proteid metabolism is anabolic. 
Recently, however, Okunew, working in Danilewsky’s labora- 
_ tory, has announced the discovery that the enzym of rennet (chy- 
mosin) has the power of synthesizing peptones to proteids, and 
Sawjalow * has published further studies on the same subject. 
The latter investigator finds the product to be a gelatinizing pro- 
teid which is identical whatever the original source of the peptones 
and which he calls plastein. He considers that this plastein is 
* Arch. ges. Physiol., 85, 171. 
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