150 



PHYSIOLOGY 



peritoneal cavity, as well as from the intestine, salt may be taken up 

 from fluids containing a smaller percentage of this substance than 

 does the blood plasma, and they regard this absorption as pointing 

 indubitably to an active intervention of living cells in the process. 

 This argument requires examination. Let us suppose the two vessels A 

 and B (Fig. 25) to be separated by a membrane which offers free passage 

 to water and a difficult passage to salts. Let A contain 0-5 per cent, salt 

 solution and B a solution isotonic with a 1 per cent. NaCl, but con- 

 taining only 0*65 per cent, of this salt, the rest of its osmotic tension 

 being due to other dissolved substances. If the membrane were 

 absolutely * semi-permeable,' water would pass from A to B until 

 the two fluids were isotonic, i.e. until A contained 1 per cent. NaCl 

 (we may regard volume B as infinitely great to simplify the argument). 



m 



B 



FIG. 25. 



If, however, the membrane permitted passage of the dissolved sub- 

 stances, the course of events might be as follows : At first water would 

 pass out of A, and salt would diffuse in until the percentage of NaCl 

 in A was equal to that in B. There would now be an equal partial 

 pressure of NaCl on the two sides of the membrane, but the total 

 osmotic pressure of B would still be higher than A. Water would 

 therefore still continue to pass from A to B more rapidly than the other 

 ingredients of B could pass into A. As soon, however, as more water 

 passed out from A, the percentage of NaCl in A would be raised above 

 that in B. The extent to which this occurs will depend on the imper- 

 meability of the membrane. As the NaCl in A reaches a certain 

 concentration it will pass over into B, and this will go on until equili- 

 brium is established between A and B. Extending this argument to 

 the conditions obtaining in the living body, we may conclude that 

 neither the raising of the percentage of a salt in any fluid above that 

 of the same salt in the plasma, nor the passage of a salt from a hypo- 

 tonic fluid into the blood plasma, can afford in itself any proof of an 

 active intervention of cells in the process. 



In the case of the pleura, for example, we seem to have a membrane which 

 is very imperfectly semi -permeable. It is permeable to salts, but presents 

 rather more resistance to their passage than to the passage of water. Hence 

 on injecting 0-5 per cent. NaCl solution into the pleural cavity, water passes 



