514 PATH TAKEN BY SUGAR [BOOK n. 



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 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 the greatest 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 imme- 

 diately 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 supported not only by the state- 

 ment that peptone may be found in the practically bloodless wall, 



