MECHANISM OF ABSORPTION 



437 



Here it is seen that, from the markedly hypotonic solutions of the 

 first two observations, sodium chloride was absorbed, of course, along 

 with much water. But from the strongly hypertonic solution of the 

 last observation some water was also taken up, instead of water passing 

 into it from the blood. The suggested explanation of these and other 

 data yielded by the experiment is that an osmotic stream of water out 

 of the loop into the blood is established on introduction of the hypo- 

 tonic solutions, which raises their percentage of salt, while in the case of 

 the hypertonic solution a diffusion stream of sodium chloride is estab- 

 lished in the same direction, and its salt content falls. The volume of 

 the hypotonic solutions in the loop rapidly diminishes because the 

 osmotic current conspires with the normal ' physiological ' drift from 

 the lumen outwards. In this drift both salt and water are involved, 

 as if the cells were filters which maintained through expenditure of 

 their own energy a slope of hydrostatic pressure from free surface to 

 depth. The hypertonic solution diminishes only slowly in bulk, be- 

 cause the ' physiological ' current out of the lumen is opposed by the 

 osmotic stream of water into the lumen. Nevertheless, even the hyper- 

 tonic solution is gradually absorbed, because the pull of the cells the 

 suction, if it may be so expressed, of the cellular pump is powerful 

 enough to overcome the osmotic current and to force water up the 

 slope into the blood or into the tissue liquid, whose osmotic pressure is 

 not much more than half that of the solution. 



Permeability of the Intestinal Epithelium and Lipoid-Solubility 

 of Absorbed Substances. If the. cells of the intestinal mucosa are 

 to move materials out of the lumen of the gut, it is obvious that 

 these materials must first be able to enter the cells. The ease or 

 difficulty with which different substances are absorbed may there- 

 fore depend upon the degree in which the cells are permeable for 

 them. The question of the factors on which the permeability of 

 cells depends has already been discussed to some extent in the case 

 of the coloured blood-corpuscles. A famous theory attributes the 

 degree of permeability of erythrocytes and many other animal and 

 plant cells to given substances to the degree of the solubility of 

 these substances in the lipoids which are supposed to form the 

 essential constituents in the outer layer or envelope of cells (Overton) 

 Substances which are readily soluble in lipoids are supposed to gain 

 an easy entrance by going into solution in the envelope ; those which 

 are insoluble in lipoids are checked at the boundary. Attempts 

 have been made to apply this theory to the explanation of selective 



