434 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
on. This could obviously not be the case if any appreciable part of the 
proteid were absorbed by the lacteals. 
Again, if the thoracic duct be ligatured, and an hour after the opera- 
fcion the animal (dog) be given a rich meal of proteid food, absorption 
goes on in a normal manner. If the animal be killed after the lapse of 
about forty-eight hours, it will be found that all the proteid has been 
absorbed, while a corresponding amount of nitrogen has been eliminated 
in the urine. 1 
A similar proof has been given for carbohydrate absorption by the 
blood vessels. In this case the animal is fed with carbohydrate food 
instead of proteid ; and the amount of sugar in the lymph which flows 
from a fistula of the thoracic duct is estimated. The percentage of sugar 
lies between - 6 and 1*6 per thousand, and does not vary in the least 
with the state of digestion, this being the usual percentage of sugar 
found in lymph or blood serum. 2 
A direct proof has also been given of the absorption of sugar by the 
capillaries, as it has been shown that on injection of sugar into the 
intestine the percentage of sugar in the portal vein may rise as high 
as 4 per 1000, while in a fasting condition the amount of sugar 
contained in the blood of either portal or hepatic veins does not essentially 
differ from that in the blood of the remainder of the circulation. 2 
It may be taken, then, that, under normal conditions, all the soluble 
constituents which leave the epithelial cell are taken up by the capillaries. 
But when excessive absorption is taking place, as when large quantities 
of sugar in concentrated solution are injected into the intestine, this is 
not the case. Here the work of absorption becomes too great for the 
capillaries, a part of the dissolved foodstuff passes the region of their 
action and is absorbed by the lacteals, probably in a passive fashion. 
Conditions of absorption of carbohydrates.— There is no doubt 
that a considerable share of the carbohydrate food is taken up from 
the intestine by the absorbing cells as simple sugars (mainly as dex- 
trose and lsevulose), otherwise the reason of the ferment actions which 
have been previously described would be difficult to see. But we 
possess no experimental evidence to show that all the carbohydrate 
is absorbed in such a form. Indeed, it is probable that the absorbing 
cells are capable of taking up not only saccharoses, but even colloidal 
carbohydrates, such as dextrin and starch, and converting these into 
simple sugars before turning them into the blood stream. 
We have already seen, in discussing the digestion of starch and 
glycogen, that it is impossible, in experiments carried out in vitro, to 
further convert all the dextrin formed into maltose or other form of 
sugar. Sheridan Lea's 3 experiments show, indeed, that the rapidity of 
diastatic action is much increased by dialysis, and the quantity of dextrin 
left unchanged into maltose largely diminished. Lea argues from this 
result that, under the more favourable conditions for removal of digestion 
products existing in natural digestion, the conversion of dextrin into 
maltose may become complete. Contrary to this view, there is the 
experience of Musculus and Gruber, 4 that the unchanged dextrin remain- 
ing after a prolonged digestion of starch, with a diastatic ferment, is not 
1 Schmidt-Miilheim, Arch. f. Anat. u. Physiol., Leipzig, 1877, S. 549. 
2 Von Mering, ibid , 1877, S. 379. 
a Joum. Physiol., Cambridge and London, 1890, vol. xi. p. 226 ; see also pp. 321 and 394. 
4 Ztschr. f. physiol. Chem., Strassburg, 1878, Bd. ii. S. 177. 
