NITROGENOUS METABOLISM IN THE TISSUES. S99 
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 tbe extreme opinions advocated by Voit and Pfliiger 
respectively, is consistent with all the knoAvn 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 alburnoses 
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 Schultzen and Nencki, Ztschr. f. Biol., Munchen, 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. 
