PHYSIOLOGICAL ABSORPTION. ^7 



across the dead gastric mucosa ; Susini that the epithelium 

 of the bladder interfered with the diffusion of potassium 

 ferrocyanide to the water in which it was suspended, and 

 Cazeneuve and Livon showed the same thing as regards 

 urea. Fleischer, too, demonstrated that sodic salicylate 

 diffused far more slowly through a living than a dead loop 

 of intestine. 



Further than this, it has long been known that diffusive 

 currents may occur with greater ease in one direction than 

 the other through certain animal membranes. As long ago 

 as 1825, Lebkuchner showed that solution of potassium 

 ferrocyanide diffused more readily from within outwards 

 than in the reverse direction through the intestine of cat, 

 rabbit, and fox, and Matteucci and Cima found similar 

 differences of osmotic transfer of water across the skins of 

 the frog and eel, and certain gastric mucosa, according to 

 the direction. 



Such observations showed that if we are to follow the 

 process of absorption of solutions from the intestine, we 

 must proceed by direct experiment only, and not by deduc- 

 tion from results gained with dead membranes. Experi- 

 ments have now definitely shown that in addition to the 

 osmotic process, we have also phenomena that cannot be 

 explained except upon the assumption that the living cells 

 themselves exert a " triebkraft " during absorption. We 

 have admitted such an assumption for the act of secretion 

 since the days when Ludwig showed that the secretory 

 pressure exceeds that of the blood, and for absorption, 

 which is practically a reversed secretion, we must now 

 make the same admission. 



The data concerning osmose with which we are con- 

 cerned are as follows : — 



1. If the solutions on the two sides of a porous mem- 

 brane have equal osmotic pressures, such solutions undergo 

 no change of volume. 



2. If solutions of unequal osmotic pressure are separated 

 by a porous membrane, water passes from the side where 

 there is a less to that where there is a greater osmotic 

 pressure. 



