ABSORPTION OF PROTEINS FROM THE INTESTINES 



393 



affirmed. In the preceding chapter the cleavage products of protein diges- 

 tion have been discussed. It was shown there that proteoses, peptones, 

 peptids, and the amino-acids are derived from the proteins as digestion 

 products. It has, in the past, been assumed that peptone represents the 

 form most freely absorbed. No peptone has, however, been isolated from 

 the blood or lymph on the vascular side of the epithelial membrane. The 

 present supposition is that the protein cleavage products are taken up by 

 the epithelium and synthesized into other and more complex forms before 



FIG. 283. A Small Portion of Medullary Substance from a Mesenteric Gland of the 

 Ox. d, d, Trabeculac; a, part of a cord of glandular substances from which all but a few of 

 the lymph corpuscles have been washed out to show its supporting meshwork of retiform 

 tissue and its capillary blood vessels (which have been injected and are dark in the figure) ; 

 6, 6, lymph sinus, of which the retiform tissue is represented only at c, c. X 300. (Kolliker.) 



being discharged into the blood; or that they are resynthetized into the charac- 

 teristic tissue proteins after absorption. The digestion cleavages not so 

 utilized are desamidized by the liver, and the ammonia so formed subse- 

 quently converted into and eliminated as urea. 



In animal foods, such as eggs, meat, etc., it is estimated that about 98 

 per cent, of the protein is absorbed; whereas in vegetable foods, where the pro- 

 tein is often protected from the action of the digestive enzymes, there may 

 be 10 to 15 per cent. loss. Analysis of the total lymph discharge of the 

 thoracic duct fails to show any increase of proteins during active digestion, 

 from which it is inferred that proteins pass by way of the liver. 



The non-nitrogenous residue is oxidized or temporarily stored as glycogen. 



From 12 to 15 per cent, of the protein remains in the food as it passes the 



