542 PATH TAKEN BY SUGAR. [BOOK n 



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 cc. in 

 passing through the capillaries might take up a quantity of sugar 

 so small as to fall almost within the limits 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 blood vessels. 



When 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. 



309. Proteids. The difficulties attending the experimental 

 determination 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 great difficulty, 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 satisfactorily 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 absorption 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 into 

 one or other of the natural proteids of the blood, or otherwise 

 disposed of. And the view that the peptone absorbed is so 

 changed, possibly in the very act of absorption, is supported not 

 only by the statement that peptone may be found in the practically 

 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 



