THE ABSORPTION OF THE FOOD-STUFFS 751 



in the muscles has suffered no appreciable fall. Since we have evidence 

 that the circulation of amino-acids through the liver gives rise in this organ 

 to the formation of urea, we must conclude that this organ is especially 

 responsible for the breakdown of the products of protein digestion which are 

 not directly required for replacing tissue waste. This breakdown must 

 involve a process of deamination. We may therefore conclude that the 

 amino-acids normally produced by a protein digestion are absorbed without 

 further change into the blood -stream. They then circulate throughout the 

 body, a certain proportion of them being built up in each tissue into the 

 proteins characteristic of that tissue in order to replace the waste caused by 

 wear and tear. The rest, probably the major part of the protein, is taken up 

 by the liver, where it undergoes deamination, the nitrogen moiety being 

 rapidly converted into urea and excreted by the kidneys, while the non- 

 nitrogenous moiety is carried to the working tissues to which it serves as a 

 ready and immediate source of energy. 



The fact that not only the blood but also the tissues contain amino-acids, 

 even after complete starvation for some days, shows that these substances 

 are intermediate steps not only in the synthesis but in the breaking down of 

 body proteins. Free amino-acids are thus the protein currency of the body, 

 just as glucose is the carbohydrate currency. In the fasting body we must 

 regard the processes of autolysis as the main source of the amino-acids found 

 in the tissues, and it is by autolysis that the proteins of the resting tissues are 

 made available in starvation for those whose continued working is essential 

 for the maintenance of life. The fact that high protein feeding does not 

 appreciably increase the amino-acid content of the tissues, shows that any 

 storage of nitrogen in the organism must take place, not in the form of 

 amino-acids, but as body protein. 



It was formerly thought that the deamination of amino-acids occurred on a large 

 scale in the wall of the alimentary canal, on the grounds that a larger amount of 

 ammonia was present in the portal blood than in the arterial blood. It seems, probable, 

 however, that the source of this excess of ammonia is to be found in intestinal bacterial 

 changes, and that the major portion of the amino-acids is actually absorbed unchanged. 

 The view of Abderhalden that the amino-acids are synthetised in the intestinal wall 

 to serum proteins, and absorbed in that form into the blood-stream, need here only 

 be mentioned, since it lacks experimental support. 



THE ACTUAL COURSE OF DIGESTION 



In a recent series of papers London describes the course of digestion of meals of 

 various characters in dogs which had been provided with fistulse in one of the following 

 places : (a) gastric fistula (into the fundus of the stomach) ; (6) pyloric fistula (on the 

 duodenal side of the pylorus) ; (c) duodenal fistula (about one foot below the pylorus) ; 

 (d) jejunal fistula (about the middle of the small intestine) ; (e) ileum fistula (just 

 above the caecum). 



We may take as an example the course of digestion of a meal composed of 200 grm. 

 of bread. This is eaten by the animal, mixed with the saliva 'and swallowed. On 

 arriving at the stomach it gives rise to the secretion of gastric juice. In a series 

 of special experiments London found that on the average 200 grm. of bread evoked 

 the secretion of 20 grm. of saliva, about 10 grm. of mucus from the coats of the stomach, 

 and about 315 grm. of gastric juice. The secretion of gastric juice is continuous during 



