METABOLISM OF PROTEINS 563 



more than this: they can prepare the ' building-stones ' themselves. 

 For even when the ingestion of phosphatides in the food is excluded, 

 or the intake is so small as to be negligible, the formation of phos- 

 phatides in the body goes on apparently without check. An instance 

 of this will be given on a future page in discussing experiments on 

 the relative value of different proteins for nutrition and growth. 

 A very striking observation has been recorded by McCollom, who 

 fed three hens on a diet almost free from fat. In about three and a 

 half months they laid fifty-seven eggs, containing over 9 per cent, of 

 phosphatides. Calculation showed that here, first of all, fats or their 

 components must have been constructed from carbo-hydrates. 

 Then the nitrogenous component of the phosphatides (cholin in the 

 case of lecithin, at least) must have been obtained from some source, 

 possibly from an amino-acid by the addition of methyl groups 



(CH 3 ), of which cholin, OH.H 2 C.H 2 C -NJ 3 (trimethyl-oxyethyl- 



\>H 3 

 ammonium hydroxide) contains three. 



SECTION I II. -(-MET ABOLISH OF PROTEINS./ 



( Blood-ProteinsJ-fThe two chief proteins of the plasma, serum- 

 globulin and serum-albumin,* must, as has been already pointed out, 

 be recruited from proteins absorbed from the intestine and for the most 

 part, at any rate, profoundly altered in its lumen and in their passage 

 through the epithelium which lines it.J Even when proteins are being 

 actively absorbed, the plasma, after the blood-proteins have been 

 separated, contains no substances which give the biuret reaction 

 (p. 444). So that the peptones, which can be demonstrated in the 

 intestinal contents, suffer great changes before or during their 

 absorption. The physiological reasons for this alteration are in a 

 measure known, and have already been alluded to in connection 

 with the digestion of proteins. No doubt the far-reaching decom- 

 position of the protein molecule may to some extent facilitate the 

 absorption of protein food. No doubt also it is imperative that 

 such comparatively slightly hydrolysed products as peptone, and 

 particularly proteose, should not appear in quantity in the blood, 

 for when injected they cause profound changes in that liquid, one 

 expression of which is the loss of its power of coagulation, and are 

 rapidly excreted by the kidneys, or separated out into the lymph. 

 J But the passage of the food from the stomach is so gradual an affair, 

 the quantity of digesting protein present at one time in any loop 

 of intestine is so small, and the rush of blood which irrigates the 



* It is probable that plasma contains a mixture of different albumins and 

 globulins. 



