DIGESTION OF THE PROTEINS 247 



the various tissues and particularly of the liver. (4) The "saturation- 

 limit" of the tissues for amino-acids. (5) The relative velocities of the 

 opposed processes of protein synthesis and degradation in the several 

 tissues of the body. Of these factors, two main groups may be recog- 

 nized. Absorption and deaminization on the one hand, determining 

 the abundance of nutrient material in the circulating medium, and the 

 excess or defect of the velocity of synthesis in comparison with that of 

 degradation in the tissues, on the other, determining the rapidity with 

 which the available nutrients are utilized. The former factors are 

 largely subject to environmental influence, for example that of the 

 abundance of the dietary. The latter factors are individually char- 

 acteristic of the organism, and in turn of the several tissues of which 

 the organism is composed. Two main groups of factors, therefore, 

 contribute to determine the nutrition, composition and growth of 

 organisms, an Environmental Group and an Internal Regulatory Group. 

 We shall see when we come to the consideration of the problem of 

 Growth (Chapter XX), that the diverse significance of these two 

 groups of factors may very clearly be recognized in the processes of 

 development. 



We have seen that the intestinal digestion of proteins leads to the 

 production of amino-acids, and that these are absorbed into the blood- 

 stream as such. A considerable degree of preliminary digestion of 

 proteins is, however, achieved by the Pepsin in the gastric juice, and 

 the question therefore arises as to whether any digestion-products of 

 proteins are absorbed from the stomach? 



This question may be answered in the negative. We have already 

 seen that under normal conditions neither carbohydrates nor fats 

 are absorbed from the stomach and, analogously, protein digestion- 

 products are not absorbed from the stomach. It is true that carbo- 

 hydrates may be absorbed from a li gated stomach and so, also, may 

 proteoses and peptones, but this constitutes a condition which is 

 nowise analogous to the conditions which pertain in normal digestion. 

 The non-absorption of protein digestion-products from the stomach 

 is in the first place guaranteed by the fact that the products of protein 

 hydrolysis by pepsin are proteoses and peptones, not amino-acids. It 

 would not be altogether safe, of course, to argue from the inability of 

 pepsin to digest peptones in vitro to a similar inability upon the part 

 of the stomach in situ, but the studies of London have shown that the 

 production of proteoses and peptones is, in actuality, the main result 

 of gastric digestion. This investigator has established in animals a 

 fistula opening into the intestine immediately below the pyloric sphinc- 

 ter of the stomach. From this fistula it is possible to collect samples 

 of the stomach contents the moment after the completion of gastric 

 digestion, and their ejection into the intestine. The samples not only 

 failed to contain any amino-acids, but the larger proportion of the 

 nitrogen was present in the form of Proteoses, and only a lesser proper- 



