THE MECHANISM OF ABSORPTION 621 



wholly abnormal fluid, such, as 0*9 per cent, sodium chloride 

 solution, we may ignore perhaps the effect of the cells upon 

 the salt, but we cannot ignore the possible effect of the salt on 

 the cells. It is certainly a euphemism to call such a solution 

 normal saline ; it may be isotonic with the blood, but we know 

 that sodium chloride is by no means an indifferent salt to tissues. 

 And in regard to the effect of ions upon the functional activities 

 of cells, we know that different ions have different effects upon 

 a given cell, and also that the same ion may affect tissues in 

 different ways. In fact, no mixture of salt solutions can be 

 made normal for all tissues. In researches dealing with in- 

 testinal absorption, as with many other physiological subjects, 

 the experimental conditions are often so abnormal that, even 

 if some mechanism used in response is clearly brought to light, 

 it cannot be safely assumed that the same mechanism is in 

 constant use in the normal body. No part of the body can 

 have had a more varied education than the alimentary canal, 

 but it is clear that the only strictly physiological experiment 

 on absorption would be one studying the absorption of the 

 normal products of digestion in concentrations which do occur 

 in the alimentary canal. But here we are met by the serious 

 difficulty that between the gut lumen and the subepithelial 

 capillary or lymphatic, the final products of digestion undergo 

 change by ferments, such as lipase, maltose invertin, and 

 erepsin. The ferments very likely act both within and without 

 the very cell whose properties in absorption we wish to in- 

 vestigate. These 1 considerations make the use of solutions 

 of soap, maltose, or peptone objectionable. Dextrose has 

 suggested itself, and being a non-conductor it undergoes no 

 ionisation ; but it is not by any means certain that dextrose is 

 a normal constituent in the alimentary canal, at any rate in 

 large quantity. These ferments, when compared with living 

 cells, are extremely resistant to the action of chemical poisons, 

 &c., and it may be stated safely that, long before the ferments 

 were destroyed, the living cells would be killed. In the matter 

 of what concentration can be used with safety we are pro- 

 foundly in the dark. For, we have no means of knowing at 

 what point the intestinal epithelium ceases to function normally. 

 Experiment has gradually evolved a criterion by which we 

 can say with certainty that the cells are no longer normal ; but 



