642 THE MECHANISM OF ABSORPTION 



phosphate, or citrate ; or to its cation alone, e.g. magnesium oxide ; 

 or to both its anion and cation, e.g. magnesium sulphate. Such 

 slowly absorbed salts constitute the saline purgatives which are 

 of importance in medicine and deserve consideration here. 



The Action of Saline Purgatives. We have seen that solutions 

 of some salts appear to be absorbed more slowly than others, 

 and that this conclusion is based chiefly on the fact that at the 

 end of the experiment more of a fluid isotonic with the serum 

 is found in the gut in some cases than in others. Hober, 

 Wallace, and Cushny have satisfied themselves by analysis that 

 in cases of slow absorption less of the salt has actually been 

 absorbed, and they have assumed that all the fluid remaining 

 in the gut is a solution of the corresponding unabsorbed salt. 

 The question whether this is true or not is a point of some 

 importance ; for the slowly absorbed salts constitute the great 

 class of saline purgatives, and the answer to the question in- 

 volves the explanation of their mode of action. Wallace and 

 Cushny believe that these salts produce a watery condition of 

 the fseces because they are absorbed with difficulty, and there- 

 fore keep back with them in the gut enough water to make 

 their solution isotonic with the serum. There is no doubt that 

 their purgative action can be altogether independent of osmosis ; 

 for they still act as purgatives when placed in the gut as 

 solutions isotonic with the serum. In order to prove this 

 Wallace and Cushny introduced into the stomach isotonic solu- 

 tions of various salines, and measured the fluid obtained within 

 an hour from an opening into the caecum. When 100 c.c. of a 

 NaCl solution was introduced into the stomach no fluid was 

 obtained from the fistula, but with the same quantity of a 

 solution of sodium sulphate or citrate in the stomach 75 c.c. were 

 obtained from the fistula. From this they concluded that whilst 

 all the NaCl was absorbed, 75 per cent, of the purgative solu- 

 tions was not. This conclusion is not now justified in the light 

 of MacCallum's experiments. He found that much more of 

 the saline purgative had to be administered, and that it took 

 longer to act when given by the mouth than when given sub- 

 cutaneously, and that their intravenous injection required a 

 still smaller dose and acted much more rapidly. These results 

 suggested that the salts can act after absorption ; and he proved 

 that this was so by showing that these solutions set up local 



