FROM THE SMALL INTESTINE 657 



Absorption of Fat. It would be simple to suppose that fat 

 was absorbed in some such, way as the following. Fat is first 

 digested into soap or a solution of fatty acids. This solution 

 diffuses into the epithelial cells, and is there converted by lipase 

 into neutral fat. This conversion would keep down the con- 

 centration of soap, &c., in the cell, and the absorption would 

 continue because of the difference of concentration within the cell 

 and in the gut. Lipase having a reversible action, converts the fat 

 in the cell into a soluble form, and this diffuses out of the cell 

 because of the difference in concentration of this soap within the 

 cell and in the subepithelial spaces. Here the soap is again 

 converted by lipase into fat, which passes into the lacteals. It 

 would be necessary to imagine that the epithelial cell has a one- 

 sided permeability for soap, otherwise it would begin to diffuse 

 back into the gut as well as into the subepithelial spaces. Even 

 such a rough working hypothesis as this necessitates several 

 considerable assumptions. It supposes that fat is digested into a 

 soluble substance and absorbed in solution ; this on the whole is 

 probable. It also assumes that this substance in solution will 

 diffuse into the epithelial cells. We have no direct evidence on 

 this point. The cells might be permeable to soaps, &c., in a 

 chemical way in accordance with Overton's views, or in a 

 mechanical way. As against this latter idea we have the ob- 

 servations of Moore and Parker. They found that solutions of 

 soaps in water did not diffuse through parchment paper ; that is, 

 they did not behave like crystalloids, because presumably they 

 existed as solution aggregates formed by the aggregation of many 

 molecules. Provided, however, that soap or some such substance 

 existing in a more or less mono-molecular form could diffuse into 

 the cell, it might well undergo aggregation within the cell, and in 

 this way keep up absorption ; for the absorbed substance would 

 no longer exist as such in the cell. This change of the absorbed 

 substance from a diffusible to a non-diffusible form might also 

 determine an osmotic absorption of water ; for Moore and Parker 

 have shown that soap solutions can exert an osmotic pressure 

 upon a parchment membrane to which they are impermeable. 

 The reversible action of lipase might be to produce first an 

 aggregation of molecules which proceeded until the dissolved 

 solution aggregates passed into the granular form, and then a 

 change in the reverse direction, the granules passing into colloidal 



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