ABSORPTION FROM THE ALIMENTARY CANAL. 327 



taining sugar or starch does temporarily increase the quantity of sugar in 

 the portal blood. From this we may infer that such portions of the sugar 

 of the intestinal contents as are absorbed as sugar pass exclusively by the 

 portal vein. We may, however, here call attention to the difficulties attend- 

 ing an argument of this kind. In the first place the quantitative determi- 

 nation of a small amount of sugar in so complex a fluid as blood is attended 

 with great difficulties and uncertainties. In the second place a very large 

 quantity of blood is at any one moment streaming through the capillaries of 

 the alimentary canal ; and we may perhaps speak of the quantity which 

 passes through them during the whole period of digestion as being enormous. 

 Hence though each 100 c.c. in passing through the capillaries might take up 

 a quantity of sugar so small as to fall almost within the limit of errors of 

 observation, yet the whole quantity absorbed during the hours of digestion 

 might be considerable ; or to put it in another way, an error of observation, 

 unavoidable with our present means of analysis, on a sample of blood taken 

 from the portal vessels might lead to a wholly unwarranted conclusion that 

 sugar was or was not being absorbed. Making every allowance, however, for 

 these difficulties, the increase of sugar which has been observed in the portal 

 blood during digestion seems too great to permit of any other conclusion than 

 that sugar is really absorbed from the alimentary canal by the bloodvessels. 



W 7 hen, however, a large quantity of sugar dissolved in a large quantity of 

 water is present in the intestine, the sugar in the chyle is said to be increased. 

 In such a case the excess of water, as stated above, passes into the lacteals, 

 and in so doing appears to carry some of the sugar with it. 



In this connection it should be remembered that the sugar resulting from 

 digestion is for the most part maltose, while that in the portal blood is dex- 

 trose, the former being changed into the latter probably while passing through 

 the intestinal epithelium. 



262. Proteids. The difficulties attending the experimental determina- 

 tion of the path taken by proteids are greater even than in the case of sugar, 

 for the exact quantitative estimation of peptone in blood (and we are assuming 

 that proteids are mainly absorbed as peptone) is a task of the greatest diffi- 

 culty one compared with which that of estimating sugar appears almost 

 easy. Bearing this in mind we may state that all observers are agreed that 

 peptone is absent from chyle or at least that its presence cannot be satisfac- 

 torily proved. On the other hand, while some observers have succeeded in 

 finding peptone in the portal blood after food, but not during fasting, many 

 have failed to demonstrate the presence of peptone in the blood either of the 

 portal vein or of the vessels at large even after a meal containing large 

 quantities of proteids. Of course, as we argued in speaking of the absorp- 

 tion of sugar, the quantity of peptone passing into the portal blood at any 

 moment might be small, and yet a considerable quantity might so pass during 

 the hours of digestion. We may suppose, moreover, that that which does 

 pass is immediately converted, possibly by some ferment action, into one or 

 other of the natural proteids of the blood, or otherwise disposed of; and, 

 indeed, peptone injected carefully and slowly into a vein disappears from the 

 blood, though little or even none passes out by the kidney. And the view 

 that peptone is so changed, possibly in the very act of absorption, is sup- 

 ported not only by the statement that peptone may be found in the practi- 

 cally bloodless wall, that is, mucous membrane, of the intestine removed from 

 a dead animal even when it appears to be absent from the blood, but also 

 and especially by the following observation. If an artificial circulation of 

 blood be kept up in the mesenteric arteries supplying a loop of intestine re- 

 moved from the body, the loop may be kept alive for some considerable time. 

 During this survival a considerable quantity of peptone placed in the cavity 



