838 PHYSIOLOGY 



divided form into the intercellular clefts and into the spaces of the 

 villus, whence by the contractions of the musculature of the villus 

 they are forced with the lymph transuding from the capillary blood- 

 vessels into the central lacteal, and thence along the mesenteric 

 lymphatics to the thoracic duct. This description would apply 

 to about 60 per cent, of the fat which is absorbed. It is probable 

 that all the fat which is absorbed is taken up in a dissolved condition, 

 but whether the remaining 40 per cent, enters the blood stream, 

 or is utilised and broken down in the tissues of the intestinal wall 

 itself, we have no means of judging. Under normal circumstances 

 the utilisation of fat is almost complete. By the time the intestinal 

 contents have arrived at the lower end of the ileum 95 per cent, of 

 the fat has been absorbed. Removal of the whole large intestine 

 was found by Vaughan Harley not to affect fat absorption. 



THE ABSORPTION OF CARBOHYDRATES 

 As a result of the action of the various digestive juices all the 

 carbohydrate constituents of the normal diet of man are reduced 

 to the state of monosaccharides. The absorption of these digestive 

 products may take place at any part of the alimentary canal, the 

 greatest part in the act of absorption being taken by the small 

 intestine. By the time that the food has arrived at the ileocsecal 

 valve practically the whole of the carbohydrate constituents of the 

 food have been absorbed. All experimenters are agreed that the 

 carbohydrates pass into the body by way of the vessels of the portal 

 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, monosaccharides, mannose, fructose, 

 glucose, 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 only able to take up in any quantity 

 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 



