ABSOKPTION OF FATS AND INSOLUBLE SUBSTANCES. 463 



epithelium during the digestion of fat, contrasted with the 

 epithelium observed during the intervals of digestion ; showing 

 the cells, during absorption, filled with fatty granules. 1 The 

 hypothesis of Briicke, that the free ends of these cells have 

 no membrane, and that the fat enters to be passed out at 

 openings in the pointed ends, cannot be sustained ; for Kolli- 

 ker has demonstrated that the entire layer of epithelium 

 covering the villi is itself covered by a continuous mem- 

 brane. 2 



A more accurate knowledge of cell-action might enable 

 us to explain the mechanism of the passage of fat through 

 these structures in the intestine. In the general process of 

 nutrition, fatty granules are frequently deposited in various 

 tissues, cells, etc., by virtue of an inherent property common 

 to all living tissues, which enables them to select, as it were, 

 certain principles from the nutritive fluid. It is as difficult 

 to explain how fatty particles pass out through the walls of the 

 capillary blood-vessels and penetrate cells and other structures, 

 as it is to understand how the particles of chyle penetrate 

 the cells of the intestinal villi and enter the lacteals and 

 blood-vessels. The only purely physical facts which throw 

 any light upon this phenomenon are those of the passage of 

 emulsions through membranes moistened with alkaline 

 fluids. Excessively fine emulsions actually penetrate the 

 intestinal vessels and the structures which cover them ; and 

 the fluids contained in these vessels are always distinctly al- 

 kaline. How far the epithelial cells covering the villi are 

 concerned in this process is a question which cannot at 

 present be satisfactorily answered. 



It is true, as a general law, that insoluble substances, 

 with the exception of the fats, are never regularly absorbed, 

 no matter how finely they may be divided. The apparent 

 exceptions to this are, mercury in a state of minute sub- 

 division like an emulsion, and carbonaceous particles. In the 



1 D ALTON, Treatise on Human Physiology, Philadelphia, 1864, p. 172. 



2 KOLLIKER, IOC. Clt. 



