806 PHYSIOLOGY OF DIGESTION AND SECRETION. 



reaction.* Van Slyke and Meyer have shown that in the blood 

 of dogs amino-acids are constantly present in small amounts (3 to 

 5 mgms. of amino-acid nitrogen to 100 c.c. of blood), and that after 

 a meal of meat this concentration is definitely increased. In the 

 light of recent work, especially that of the authors last named, we 

 may suppose that the normal course of events is as follows: The 

 digested proteins are absorbed as amino-acids and distributed to 

 the tissues by the blood. The tissues select and store certain of 

 these bodies, and probably in each organ subsequent use is made of 

 them to build up new tissue or to repair the wastes of metabolism. 

 That is to say, there is probably no especial form of circulating 

 protein which serves as a pabulum for tissue-repair and growth, but 

 the amino-acids themselves constitute the form in which nitrogen 

 food is presented to the different tissues, just as dextrose consti- 

 tutes the circulatory form of carbohydrate food. Each tissue from 

 the amino-acids offered to it builds up its own form of protein, 

 and the amino-acids not used for this synthesis may be deaminized 

 and then employed for energy purposes. There is evidence that 

 in the liver especially many of the amino-acids arising from the 

 digested food undergo deaminization, the nitrogen being eliminated 

 as urea. During starvation the amino-acid content of the blood 

 is maintained, probably because the tissues themselves or some of 

 them undergo self-digestion or autolysis and thus furnish food 

 material for the active tissues. 



Examination of the contents of the small intestine at its junc- 

 tion with the large shows that under normal conditions most of the 

 protein has been absorbed before reaching this point. The process 

 is continued in the large intestine, modified somewhat by bacterial 

 action, and the amount that finally escapes absorption and appears 

 in the feces varies, in perfectly normal individuals, with the charac- 

 ter of the protein eaten. According to Munk,f the easily digestible 

 animal foods such as milk, eggs, and meat are absorbed to the 

 extent of 97 to 99 per cent., while with vegetable foods the utiliza- 

 tion is less complete. This difference is not due, however, to any 

 peculiarity of the vegetable proteins; it is probably an incidental 

 result of the presence of the indigestible cellulose found in our 

 vegetable foods. It is stated that from 17 to 30 per cent, of the 

 protein may be lost in the feces if the vegetable food is in such form 

 as not to be attacked readily by the digestive secretions. 



Digestion and Absorption in the Large Intestine. Observa- 

 tions upon the secretions of the large intestine have been made upon 



* Abel, Rowntree and Turner, "Journal of Pharmacology and Exp. Thera- 

 peutics," 5, 611, 1914. 



t See Munk, "Ergebnisse der Physiologic," vol. i., part i., 1902, article, 

 "Resorption," for literature and discussion. 



