CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 415 



constant, neither being to any marked extent increased by even 

 amylaceous meals ; on the other hand, a meal containing 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 attending an argument of this kind. 

 In the first place the quantitative determination 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 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. 



In thus speaking of the sugar as passing into the portal 

 blood, it should be remembered, that while the greater part of 

 the sugar of a meal, that formed from starch, is maltose, the 

 sugar in the portal blood is dextrose ; either within the alimen- 

 tary or more probably in passing through the epithelium of 

 the wall of the alimentary canal, the maltose is changed into 

 dextrose. 



250. Proteids. The difficulties attending the experimental 



determination of the path taken by proteids are greater even 



than in the case of sugar ; since the quantitative determination 



of peptone in portal blood and chyle respectively, itself a task 



still more difficult than the quantitative determination of sugar 



i in the same fluids, gives us no clue. For we have evidence 



Uhat though peptone may be the form, or chief form, in which 



proteid material leaves the interior of the alimentary, it is not 



the form in which it reaches its destination be that destination 



