288 PHYSIOLOGY CHAP. 



This evidence that the greater part of the soluble proteins can 

 be absorbed as such, or after slight changes, radically modifies our 

 ideas as to the importance of the peptonisation effected by the 

 gastric, and still more by the pancreatic, juice in the consumption 

 of alimentary protein. These juices obviously render soluble, 

 and therefore absorbable, the proteins introduced in the solid state, 

 and they also split up non-absorbable protein, and liberate the 

 utilisable protein-groups of the molecules : but soluble proteins can 

 be absorbed and utilised as such, without any previous hydrolytic 

 cleavage into proteoses and peptones. 



There is, however, no doubt that a considerable part of the 

 soluble proteins do undergo proteolytic cleavage previous to 

 absorption. When we reflect that part of the potential energy of 

 the natural proteins is wasted in their cleavage into proteoses and 

 peptones, the utility of this process is hard to understand. But it 

 should not be forgotten that proteoses and peptones (from their 

 physical and chemical properties), are more easily and promptly 

 absorbed by the intestinal epithelia than are natural or scarcely 

 altered proteins. In this lies the utility of scission and subdivision 

 of the latter, which is carried farther or arrested as required, 

 particularly in proportion as they are introduced in excessive or 

 scanty quantities, and as the proteolytic enzymes are abundant or 

 defective. 



The medical use of feeding debilitated persons and invalids 

 on proteoses or peptones may be justified by their ready absorp- 

 tion. But under physiological conditions, such a prescription for 

 utilising the alimentary principles is not only unnecessary, but 

 is neither economical nor advantageous in comparison with a diet 

 of natural proteins, as was clearly proved by Horton-Smith (1891). 



The fact that proteoses and peptones are capable of replacing 

 natural protein under all conditions of nutrition was clearly 

 demonstrated by the experiments of Maly and Plosz (1874). 

 The former fed a pigeon, the latter a dog, very satisfactorily for 

 some time upon a diet in which proteoses and peptones replaced 

 the natural proteins. The same successful results were obtained 

 in a striking manner by Zuntz (1885), Pollitzer (1885), Gerlach 

 (1891), and Pfeiffer (1885), the last of whom experimented on 

 himself for ten days in succession. 



These results showed the suggestion of some authors (Briicke, 

 Voit, and A. Fick in particular) to the effect that peptones, though 

 absorbable, cannot be assimilated by the tissues, and that natural 

 proteins alone can compensate for tissue waste, to be unfounded. 

 We may conclude that the protein stored up in the tissues comes 

 from two sources : from the dissolved proteins absorbed as such, 

 and from the products of their digestion, i.e. proteoses, peptones, 

 and amino-acids. It is difficult to determine how much of the 

 ingested protein is absorbed unchanged, and how much after 



