278 PHYSIOLOGY OF ALIMENTATION. 



blue, water-soluble nigrosin, or benzoazurin are introduced 

 into the intestine the urine becomes colored, yet no deeply 

 stained granules are found in the protoplasm. 



These observations only show that the intestinal epithelium 

 is permeable to salts soluble in the lipoids; it does not as yet 

 prove that those insoluble in the lipoids are absorbed only 

 interepithelially. But this is rendered probable as soon as 

 fixing agents, such as ammonium molybdate, osmic acid, 

 corrosive sublimate, picric acid, ammonium picrate, tannic 

 acid, or gold chloride, are used to fix the pictures obtained 

 after simple introduction of a dye into the intestinal tract. 

 Under these circumstances it is found that if the fixing agent 

 is one soluble in the lipoids of the cell (such as osmic acid), 

 the granular pigmentation of the cell found after the use of 

 such a dye as methylene blue remains unchanged. If, how- 

 ever, a fixing agent insoluble in the cell lipoids (such as am- 

 monium molybdate, which is able to plasmolyze cells) is used, 

 the blue granules in the cell are seen to dissolve, to move 

 toward the periphery of the cell and to be precipitated here. 

 This is because the ammonium molybdate remains in the 

 intercellular spaces and precipitates the dye present here. 

 In this way the equilibrium between the dye without and 

 within the cells is destroyed. The stain in consequence begins 

 to move out of the cell toward its periphery, where it meets the 

 ammonium molybdate. What has been said of this fixing 

 agent holds for every one of the fixing agents capable of re- 

 acting with the stains employed and not soluble in the lipoids. 

 /with this is proven quite conclusively that salts insoluble 

 \ in the lipoids make their way from the intestine into the blood 

 \only through the interepithelial spaces. 1 



This is, perhaps, the best place to touch briefly upon the 

 absorption of such compounds as alcohol, urea, and certain 



1 It might seem from this that the ordinary salts are entirely in- 

 capable of entering the epithelial cells of the intestine or any other 

 cell containing lipoids. This is not true, but the means by which 

 they may or do enter cannot be dealt with here. 



