OSMOTIC PRESSURE 13 



which does not readily diffuse, and sodium chloride, which diffuses 

 quickly; the osmotic flow will take place from the sodium-chloride solu- 

 tion to the cane sugar even when the sodium-chloride solution is stronger 

 than the sugar. In such a case, water molecules will pass from the fluid 

 having the higher osmotic pressure (Nad) toward a fluid in which 

 this is lower (sugar). 



Furthermore, the simple law r s of osmosis may be upset by an attrac- 

 tive influence of the membrane toward certain substances [due to their 

 becoming dissolved or adsorbed in it (see. page 65)] but not toward 

 others. Many membranes of this nature are known to the chemist 

 (e. g., rubber membranes in contact with gases, pyridine solutions, etc.), 

 and it is probable that such a property of selective solubility may play 

 a not unimportant role in the transference of substances across animal 

 membranes (Kahlenberg 5 ). 



These few conditions which may modify the direction of the osmotic 

 flow, are indicated here to show how involved such problems are, and 

 how careful we must be not to assume that, because a substance is trans- 

 ferred through a living membrane contrary to the simpler laws of os- 

 mosia and diffusion, it must involve the expenditure of forces different 

 from those operating in dead membranes. 



Another force comes into operation under certain conditions namely, 

 that of filtration. This is a purely mechanical process, in which mole- 

 cules are forced through the pores of a filter (i. e., membrane) by dif- 

 ferences in pressure on its two sides. 



We are now in a position to consider in how far the above physical 

 forces explain certain physiological problems. 



1. Is the absorption, into the Hood and lymph circulating in the intes- 

 tinal walls, of substances in solution in the intestinal contents entirely 

 dependent upon the processes of filtration, diffusion and osmosis f The 

 absorption of weak solutions of highly diffusible substances is probably 

 very largely a matter of osmosis and diffusion, and water passes quickly 

 into the blood because of osmotic attraction, but that other forces ordi- 

 narily come into play is very clearly established by the following ob- 

 servations. If a piece of intestine is isolated from the rest by placing 

 two ligatures on it, and the isolated loop filled either with a solution con- 

 taining the same saline constituents in similar proportions as in blood 

 serum, or better still, with some of the same animal's blood serum, it 

 will be found after some time that all of the solution becomes absorbed 

 into the blood; the contents of the loop are therefore absorbed into the 

 blood, even though the osmotic pressures of the dissolved substances are 

 the same on both sides of the membrane (Weymouth Reid 6 ). 



The intestinal membrane seems to possess towards readily diffusible 



