CARBOHYDRATES. 65 



can utilize up to fifty or sixty per cent of the pentosans contained in 

 vegetables. In these experiments a mixture of pentosans (araban, 

 xylan, methylpentosan) was fed to the animals, but recently Slowtzow * 

 has fed pure xylan to rabbits. In the excreta from 17.1 to 66.8 per cent 

 of the pentosans were found in an unchanged condition. The remainder 

 was undoubtedly utilized in the system after having been converted into 

 the simpler sugars (pentoses) . 



From the intestine on, the simple sugars (e.g., d-glucose) are quickly 

 absorbed, whether introduced into the system in this form, or obtained 

 from the destruction of more complicated sugars. There are two ways in 

 which the nutriment absorbed by the intestine can reach the general cir- 

 culation. In the first place it can enter directly by means of the blood-ves- 

 sels, in this case the branches of the portal vein. This is the path taken by 

 salts, carbohydrates, and proteins. In this way they reach the liver, there 

 to undergo certain important transformations, after which they are capable 

 of being introduced into the general circulation of the blood. The second 

 path is by way of the lymphatics, which conduct the absorbed sub- 

 stances, especially the fats, into the thoracic duct, from which they are led 

 into the Vena anonyma, and thus into the general circulation. 



The experiments showing that the absorption of carbohydrates as a 

 matter of fact takes place in the first manner will be discussed later when 

 we come to consider the absorption of fats. 



In order to get some idea of the process by means of which sugar is 

 transferred to the circulatory system, and in order to understand how 

 large amounts of carbohydrates, e.g. 500 grams, can be transferred in a 

 relatively short time to the blood-stream (especially into the portal vein) 

 without materially increasing the sugar content of the blood, we must 

 remember what an enormous surface for absorption is presented by the 

 extremely fine network of blood-capillaries. Absorption takes place 

 continuously hand in hand with the breaking down of the more com- 

 plicated carbohydrates into simpler sugars. Although the tiny molecules 

 of sugar are absorbed at thousands of places and pass into the blood, and 

 although they are immediately carried away, it would seem that there 

 must be an increase in the amount of sugar contained in the blood corre- 

 sponding to the amount absorbed. That this is not the case the normal 

 amount of sugar is 0.5 to 1.5 grams per liter and remains constant must 

 be due to the fact that sugar is removed from the blood to the same extent 

 that it is absorbed by it. This is, as a matter of fact, exactly what hap- 

 pens, and it is the liver which regulates the exchange of carbohydrates in 

 the whole animal system. It intercepts the absorbed sugar, and keeps the 

 sugar content in the blood constant. By means of the activity of the 



1 Z. physiol. Chem. 34, 181 (1901). See also Rudzinski : ibid. 40, 317 (1904). 



