9 oo METABOLISM. 
other syntheses which accompany assimilation, such as the formation of 
proteids from peptones, must also occur within them. Hof meister l has 
suggested that the leucocytes may also take an important part in determining 
the assimilation of the foodstuffs. They are present in great abundance in 
the intestinal mucous membrane, and especially those parts of that membrane 
where absorption proceeds most extensively ; and they are also, it has been 
shown, greatly increased in number during the process of absorption. It is, 
however, difficult to obtain affirmative evidence upon this point, and since 
leucocytes elsewhere do not possess this power, it is improbable that they are 
the agents for such conversion in the intestine. 
After assimilation the proteids are absorbed by the blood vessels of 
the intestinal mucous membrane. The evidence for this has been 
already given in the article on " Digestion and Absorption " (p. 433). 
If any proteids are taken up by the lacteals of the small intestine, they 
do not get into the thoracic duct, 2 but must be transferred to the blood 
vessels in passing through the mesenteric glands. At any rate we may 
assume that nearly the whole of the proteids are ultimately taken by the 
portal vein to the liver. The portal vein, therefore, contains the absorbed 
material derived from digestion and assimilation of proteid food ; it 
must have, therefore (besides the ordinary constituents of blood plasma), 
an additional amount of serum albumin or of serum globulin, obtained by 
the transformation of the peptones into these materials ; also extractives 
of the meat or other forms of proteid diet (which are absorbed equally 
by the blood vessels of the intestine), and in addition any products 
of further decomposition of peptones, such as the amido-acids, the 
possibility of the presence of which we have already discussed. But it 
must be borne in mind that the blood flow through the portal system is 
so large and rapid, that one could hardly expect these substances to be 
absorbed into it in such a proportion that it would be possible to detect 
by chemical means any appreciable difference of composition between 
the blood of the portal vein and that of the system generally, nor are 
there any satisfactory analyses directly showing such difference. Xever- 
theless there is a distinct physiological difference between the portal 
blood collected during absorption of food, and especially of proteid food, 
and the same blood collected during the intervals of digestion ; for it has 
been shown that in the former case such blood, on being passed through 
the liver, shows an increased amount of urea, whereas in the latter case 
such an increase is not noticed. It is certain, at any rate, that the 
products of absorption and assimilation of proteid foods are carried 
to the liver, and, having traced them to this organ, we have next to 
consider — (1) Whether they are stored at all within it; (2) whether 
they undergo any change in passing through it. 
Influence of the liver on proteid metabolism. — With regard to the 
possible storage of proteid in the liver, it is open to us to suppose 
that an excess of proteid material which is present in the portal blood 
as the result of the absorption of proteid food, might be temporarily 
taken up, at least in some measure, by the hepatic cells, and, after being 
1 Arch.f. exper. Path. n. Pharmakol., Leipzig, 1884-7, Bde. xix. S. 1 ; xx. S. 29 ; xxii. 
S. 306. 
2 Asher and Barbera found, however, in a dog a marked rise both in the amount of 
chyle and of the nitrogen of the chyle (estimated by Kjeldahl's method) during digestion 
of purely proteid food, given by a gastric fistula. The rise was greatest at the sixth hour 
after feeding, but there was a primary culmination at the second hour {Ccntralbl. /. 
Physiol., Leipzig u. Wien, 1S97, Bd. xi. S. 403). 
