714 PHYSIOLOGY OF DIGESTION AND SECRETION. 



in the urine, giving the phenomenon known as "alimentary glyco- 

 suria." The amount of any carbohydrate that can be eaten without 

 producing a condition of alimentary glycosuria is designated by 

 Hofmeister* as the assimilation limit of that carbohydrate. If taken 

 beyond this limit it forms a physiological excess, and some is lost 

 in the urine. The assimilation limit varies with a great many 

 conditions; but, so far as the different forms of carbohydrates are 

 concerned, it is lowest for the milk-sugar and highest for starch. The 

 simple sugars dialyze easily, and it would be natural to suppose that 

 they are absorbed into the blood by a simple process of diffusion. 

 Experimental facts, however, do not support this view entirely. It 

 is stated that the absorption of sugars does not vary directly as their 

 velocity of diffusion, and in this case, as with the other products of 

 digestion, it is necessary to assume that work is done by the wall of 

 the intestine itself, probably the epithelial cells. 



So far as the carbohydrates escape absorption as sugar they are 

 liable to undergo acid fermentation from the bacteria always present 

 in the intestine. As the result of this fermentation there may be 

 produced acetic acid, lactic acid, butyric acid, succinic acid, carbon 

 dioxid, alcohol, hydrogen, etc. This fermentation probably occurs 

 to some extent in the small intestines under normal conditions. 

 Macfadyen,f in the case already referred to, found that the contents 

 of the intestine at the ileocecal valve contained acid equivalent to 

 that of a 0.1 per cent, solution of acetic acid. Under less normal 

 conditions, such as excess of sugars in the diet or deficient absorp- 

 tion, the large production of acids may lead to irritation of the intes- 

 tines, diarrhea, etc. 



Absorption of Fats. Numerous theories have been held in 

 regard to the mode of absorption of fats. It has been supposed that 

 the emulsified (neutral) fat is ingested directly by the epithelial cells, 

 that the fat droplets enter between the epithelial cells in the so-called 

 cement substance, that the fat droplets are ingested by leucocytes 

 that lie between the epithelial cells, or lastly that the fat is first split 

 into fatty acid and glycerin and is absorbed by the epithelial cells in 

 these forms. The tendency of recent work is to favor this last view. 

 During digestion the epithelial cells contain fat droplets without 

 doubt, but it seems probable that these droplets are formed in situ 

 by a synthesis of the absorbed glycerin and fatty acids. The border 

 of the cell is said to be free from fat globules, a fact which would 

 indicate that the neutral fat is not mechanically ingested as oil drops. 

 But, granting that the fat is absorbed in solution, as fatty acids and 

 glycerin, the mechanism of absorption remains unexplained. It is 



* Hofmeister, "Archiv f. exper. Pathol. u. Pharmakol.," 25, 240, 1889, 

 and 26, 355, 1890. 



t Macfadyen, Nencki, and Sieber, loc. cit. 



