NITROGENOUS METABOLISM IN THE TISSUES. 899 



both of unorganised proteid and of non-proteid materials outside but in 

 contact with the molecules of bioplasm. Such a view, which is in a sense 

 intermediate between the extreme opinions advocated by Voit and Pfliiger 

 respectively, is consistent with all the known facts, and is more readily applic- 

 able to the phenomena, both of animal and vegetable metabolism, than the 

 exclusive acceptance of either of those opinions. 



Whether directly or indirectly, tissue proteid normally undergoes meta- 

 bolism to the extent of about 1 per cent, of its substance per diem (Voit). 



The proteids of the food are converted by digestion into albumoses 

 and peptones ; ultimately, probably, entirely into peptones. They are, 

 however, not absorbed as peptones, for no peptones are found in the 

 blood or chyle leaving the intestines. It is clear, therefore, that the 

 process of assimilation or the reconversion of peptones into proteids 

 must occur during their absorption, that is to say, in the substance of 

 the mucous membrane. It must not be forgotten, however, that a 

 certain amount of the proteid of food may possibly, as occurs in vitro, 

 be broken down beyond the stage of peptone into simpler nitrogenous 

 bodies, such as the amido-acids; and these, if their formation really 

 occurs to any extent in the intestinal tract, would be absorbed as such 

 into the portal blood and conveyed by it to the liver. Now we know 

 that the addition of amido-acids to the blood which is allowed to circu- 

 late through the liver, as well as their administration with the 

 food, causes an increase in the amount of urea in the blood after it has 

 passed through that organ, and an increased excretion of urea by 

 the kidneys. 1 From this it may be assumed that any amido-acids 

 absorbed are converted by a process of synthesis (possibly preceded by 

 a previous more complete breaking-down, into ammonia compounds) into 

 urea. If this process of formation of amido-acids occurs at all in natural 

 digestion, it is obviously a change by which the proteids of the food 

 would not be directly serviceable for the production of tissue ; and in 

 this sense such conversion of peptones into amido-acids may be looked 

 upon as a direct waste of proteid food. It is extremely improbable that 

 such a change occurs to any extent in the normal organism, nor has the 

 presence of these substances to any marked degree been determined in 

 the normal intestinal contents. Moreover, as Bunge remarks, there 

 is not sufficient carbon in the proteid molecule to permit of all the 

 nitrogen issuing as amido-acids. 2 In any case, these bodies must 

 probably be split up and oxidised into carbon dioxide and ammonia, and 

 from these urea become formed by synthesis in the liver. We may 

 therefore probably put aside as exceptional this mode of transformation 

 of proteid into urea, and consider only the change which is undergone 

 by the proteid which is actually assimilated. 



With regard to the agents in the mucous membrane which produce the 

 assimilation of proteids, that is to say the conversion of peptones into proteids, 

 there can be very little doubt that the columnar epithelium occupies the first 

 place. It is, however, difficult to prove that the change, which is one of 

 synthesis and dehydration, does actually occur in these cells. We have chiefly 

 analogy to guide us in coming to this conclusion. It can be definitely proved 

 that a synthesis of fat does occur in them ; and it is therefore probable that 



1 Schnltzen and Nencki, Ztschr. f. BioL, Miinchen, 1872, Bd. viii. S. 124 ; Salkowski, 

 Ztschr. f.physiol. Chem., Strassburg, 1879, Bd. iv. S. 100 ; v. Knieriem, Ztschr. f. Biol., 

 Munchen, 1874, Bd. x. S. 279. 



2 " Lectures," p. 320. 



