DIGESTION AND ABSORPTION IN THE INTESTINES. 803 



without producing alimentary glycosuria is designated by Hof- 

 meister* as the assimilation limit of that carbohydrate or, to use 

 another terminology, it establishes the degree of tolerance of the 

 animal for the particular carbohydrate employed. If taken 

 beyond this limit there is a physiological excess, and some sugar 

 is lost in the urine. The assimilation limit varies with a great 

 many conditions; but, so far as the different forms of carbo- 

 hydrates are concerned, it is lowest for the milk-sugar and high- 

 est for starch. That starch may be eaten in larger amounts 

 than sugar without raising the percentage of sugar in the sys- 

 temic blood above the normal level is in accord with w^hat we 

 know of the digestion of the two forms of carbohydrates. Dex- 

 trose requires no digestion, it is absorbed as such, while cane- 

 sugar needs only to be inverted. Starch, on the contrary, requires 

 the action of ptyalin or amylase and subsequent inversion by 

 maltase. Its absorption will, therefore, be much slower than that 

 of the sugars. In fact, it probably goes on for the period of four 

 or five hours, during which an ordinary meal is making its progress 

 from pylorus to ileocecal valve. During this period the entire 

 quantity of blood in the body is passed through the mesenteric 

 arteries over and over again, and it is probable that even in the 

 portal vein the quantity of sugar at any one moment rises but 

 little above the normal level, and this small excess is held back 

 by the liver cells, so that the systemic circulation is protected 

 from becoming hyperglycemic. 



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 



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

 and 26, 355, 1890. 



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



