ABSORPTION OF THE VARIOUS FOOD SUBSTANCES 435 



SECTION III. ABSORPTION OF THE VARIOUS FOOD SUBSTANCES. 



Absorption of Fat How the Fat gets into the Intestinal Epithelium. 

 It has been already mentioned that fat is split up in the intestine 

 into the corresponding alcohols (mostly glycerin) and fatty acids, 

 but it has been a subject of discussion whether it all undergoes this 

 change or only a portion of it. The common view has long been 

 that the greater part of the fat escapes decomposition, and, after 

 emulsification by the soaps formed from the liberated fatty acids, 

 is absorbed as neutral fat by the epithelial cells covering the villi. If 

 an animal is killed during digestion of a fatty meal, these cells are 

 found to contain globules of different sizes, which stain black with 

 osmic acid, are dissolved out by ether, leaving vacuoles in the cell 

 substance, and are therefore fat (Fig. 172). It has always been 

 difficult to explain how droplets of 

 emulsified fat could get into the 

 interior of the epithelial cells, al- 

 though, perhaps, no more difficult 

 than to explain the passage of living 

 tubercle bacilli from the contents of 

 the intestine into the chyle of the 

 thoracic duct a fact which has been 

 clearly demonstrated (Ravenel). The 

 fat is certainly contained within the 

 cells, and not between them. When 

 fat is found in the cement sub- 

 stance between the cells, it has been 

 mechanically squeezed out of them 

 by the shrinking of the villi in 

 preparation. This difficulty is obviated if we suppose that the 

 whole of the fat is split up in the intestine, the products being 

 absorbed in solution, the glycerin as such, and the fatty acids either 

 as soaps or in the free state, or partly free and partly saponified. 

 If this is the true theory and the evidence of its truth has of late 

 years been continually growing neutral fat must again be built 

 up in the epithelial cells from the absorbed glycerin and the fatty 

 acids or soaps. Now, it has been shown that when an animal is fed 

 with fatty acids they are not only absorbed, but appear as neutral 

 fats in the chyle of the thoracic duct, having combined with glycerin 

 in the intestinal wall; and the epithelial cells contain globules of 

 fat, just as they do when the animal is fed with neutral fat. Further, 

 it is known that fat-splitting goes on in the alimentary canal to a 

 much greater extent than would be necessary merely for the forma- 

 tion of a quantity of soap sufficient to emulsify the whole of the fat 

 in the food. Indeed, at certain stages of digestion most of the 



Fig. 172. Mucous Membrane of 

 Frog's Intestine during Absorp- 

 tion of Fat (Schafer). ep, epithe- 

 lial cells; str, striated border; 

 C, lymph corpuscles; I; lacteal. 



