790 PHYSIOLOGY 



system. The lymph from the thoracic duct contains no more sugar than 

 does the arterial blood taken at the same time, whereas several observers have 

 obtained an increased percentage of sugar in the portal blood during the 

 absorption of a big carbohydrate meal. 



Of the carbohydrates of the food, some, like starch, dextrin, glycogen, are 

 colloidal and indiffusible ; others, such as the disaccharides, cane sugar, milk 

 sugar, and maltose, are soluble and diffusible ; and the products of the action 

 of digestive ferments on these two classes, namely the monosaccharides, 

 mannose, fructose, glucose and galactose, are also soluble and diffusible. The 

 problem as to the mechanism involved in the passage of these substances 

 across the intestinal wall into the blood vessels has been already dealt with 

 in treating of the absorption of water and salts. The most striking fact is the 

 relative impermeability of the intestinal wall to the disaccharides as compared 

 with the monosaccharides. The intestinal wall is apparently able to take 

 up in any quantity only such sugars as can be utilised by the cells of the 

 organism. For this purpose the disaccharides are useless; cane sugar or 

 lactose introduced into the blood vessels or subcutaneously is excreted quan- 

 titatively in the urine and, as might be expected, does not increase in any 

 way the glycogen of the liver. When maltose is injected in the same manner, 

 a certain proportion of it is utilised owing to the fact that 'the blood and 

 fluids of the body contain a ferment, maltase, capable of converting the 

 disaccharide into the monosaccharide, glucose. The absorption of these 

 disaccharides occurs therefore much more slowly from the intestine than does 

 the absorption of monosaccharides, the process of absorption being always 

 preceded by and waiting for the process of hydrolysis. Thus huge doses 

 of cane sugar may be taken without causing the appearance of cane sugar 

 in the blood or urine. It has been found that sugar does not appear in the 

 urine until as much as 320 grm. of cane sugar have been ingested, whereas 

 any quantity of glucose over 100 gfm. may give rise to glycosuria. Lactose 

 is absorbed still more slowly and, in animals whose intestine is free from the 

 ferment lactase, is not absorbed; large doses of lactose in such animals 

 therefore give rise to diarrhoea. The behaviour of the intestinal wall to 

 the non-assimilable sugars of artificial origin has not yet been sufficiently 

 investigated. It would be interesting to inquire whether the rate of absorp- 

 tion of the different sugars is in any way determined by their stereomeric 

 configuration, whether, for instance, ^-glucose would be absorbed as rapidly 

 as the ordinary d-glucose. 



THE ABSORPTION OF PROTEINS 



In very few departments of physiology has there been so great a revo- 

 lution in our ideas as in that relating to protein absorption, especially as to 

 the form in which it is absorbed from the alimentary canal, and its fate after 

 absorption. As to the channel by which it obtains entry into the circulation, 

 practical agreement reigns that it is absorbed by the blood vessels. Almost 

 every physiologist who has occupied himself with the investigation of the 

 lymph flow from the thoracic duct has been impressed by the fact that ilio 



