S68 



ABSORPTION 



but in the last the process of absorption is not so clear. It is comparatively 

 easy to demonstrate the presence of microscopic globules of fat, both in the 

 intercellular substance and in the epithelial cells themselves. But it has 

 been constantly noticed that there is a clear zone along the free borders of 

 the cells. Fat drops exist in the adjacent digesting mass, and in the deeper 

 parts of the cells, but not in this border zone. Since the demonstration of 

 the reversible action of lipase, the view has been strengthened that in the 

 very act of absorption the emulsified fats are decomposed and passed through 

 the cell border only to be resynthesized in the cell protoplasm. This is of 

 course against the strictly mechanical view. The decreasing efficiency of 

 fats when the bile, which wets the mucous surface and dissolves the fatty 

 acids, is withheld from the intestine also supports this view. As absorption 

 progresses the size of the fat drops in the epithelial cells increases, a fact 



FIG. 285. Mucous Membrane of Frog's Intestine during Fat Absorption, ep, Epithelium; 

 sir, striated border; C, lymph-corpuscles; /, lacteal. (E. A. Schafer.) 



that is readily explained by supposing a continued synthesis and accumula- 

 tion of fat. 



The fat drops are ultimately discharged into the connective-tissue spaces 

 and finally pass into the lymph channels, the thoracic duct, and into the 

 blood of the subclavian vein. This is the course taken by the larger per- 

 centage of the fat. However, some of the fat is absorbed into the capillaries 

 of the villi and passes through the liver. The presence of fat drops in the 

 liver cells at certain times can be ascribed to storage of this absorbed fat. 



It is said that the more readily emulsified fats, those that melt readily at 

 the body temperature, are the more completely absorbed. The efficiency 

 of absorption is as high as 96 to 98 per cent for the oils, and decreases sharply 

 for such fats as the tallows. 



The large intestine is capable of absorbing fats, though not so readily 

 as the small intestine. 



Absorption of Minerals and Water in the Intestines. The salts 

 common in the foods are most of them readily soluble, dissociate quite com- 

 pletely in the dilute solutions, and diffuse and dialyze readily. Of the salts 

 of the foods, the sodium and potassium cations and chlorine anion are the 

 most readily dissociated and are most diffusible, while the calcium and 



