846 PHYSIOLOGY 



to the tissues in any other form than that of amino-acids. The 

 experimental proof of this conclusion meets with many difficulties. 

 So great is the total volume of blood circulating through the vessels 

 of the alimentary canal during the hours that the observation is 

 taking place that a considerable amount of amino-acids could be 

 carried by this blood from the canal to the tissues without effecting 

 a change in the composition of the fluid which is within our errors 

 of analysis. The experimental investigation of this point has been 

 attempted by Kutscher and Seemann, and by Cathcart and Leathes. 

 The former observers analysed the portal blood during active protein 

 digestion after cutting out by ligature the circulation from all parts 

 of the body except the heart, lungs, liver, and alimentary canal. 

 They were unable to detect any increase in the amount of amino- 

 acids in the blood. Cathcart and Leathes investigated the content 

 of the blood in soluble, i.e. non-protein nitrogen, in normal animals 

 and in anaesthetised animals in whom large quantities of peptone 

 solution had been introduced into the small intestine, and found 

 a distinct increase in the latter case. In this experiment the rate 

 of absorption of the products of protein digestion was increased 

 much above its normal value, whereas in the experiments of 

 Kutscher and Seemann the rate of absorption in consequence of the 

 operative procedure was probably less than would obtain in the 

 normal animal after such a protein meal. The evidence therefore, 

 so far as it goes, is in favour of part, at any rate, of the protein being 

 carried by the blood to the tissues in the form of amino-acids. 



The negative results obtained by Kutscher and Seemann sug- 

 gested to these observers that possibly some process of integration 

 might occur in the mucous membrane of the alimentary canal itself, 

 and they were strengthened in this conclusion by the fact that although 

 no amino-acids could be extracted from the intestinal wall, they were 

 able, after treating the mucous membrane with acid, to extract 

 leucine, a fact suggesting that the leucine had been combined in some 

 ester-like compound with some other constituent of the mucous 

 membrane. Abderhalden is inclined to believe that there is an actual 

 regeneration of the protein in the mucous membrane, a formation 

 from the amino-acids of blood protein, either serum albumen or 

 serum globulin, and that this blood protein acts as a common protein 

 food for all the different cells of the body. It is difficult to bring 

 any experimental proof for this view. It is, however, practically 

 certain that a considerable proportion of the protein and of its crystal- 

 line products is broken up still further in the mucous membrane. 

 All observers who have investigated the point concur in the state- 

 ment that the portal blood contains more ammonia than does the 

 blood of the arterial system, and it seems probable that, as suggested 



