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- 

 tion 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 O6 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 leevulose), 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 G-ruber, 4 that the unchanged dextrin remain- 

 ing after a prolonged digestion of starch, with a diastatic ferment, is not 



1 Schmidt-Mlilheim, Arch. f. Anat. u. P/tysioL, Leipzig, 1877, S. 549. 



2 Von Mering, ibid., 1877, S. 379. 



" Journ. PhysioL, Cambridge and London, 1890, vol. xi. p. 226 ; see also pp. 321 and 394. 

 4 Ztsc/tr. /. physiol. Chem., Strassburg, 1878, Bd. ii. S. 177. 



