24 GENERAL PROPERTIES OF LIVING TISSUES 



In a solution of this concentration the red blood- 

 corpuscles of mammals are in osmotic equi- 

 librium. 1 The isotonic (isosmotic or normal) 



1 In this calculation it is assumed that the osmotic pressure 

 is" proportional to the concentration. This is not strictly cor- 

 rect. When serum is diluted with an equal volume of water, 

 the osmotic pressure of the resulting liquid is not merely half 

 so large as before, but is somewhat greater than one half the 

 original pressure. For when the water is added, a part of the 

 molecules not yet dissociated are split into ions and each ion 

 has an osmotic pressure equal to that of an undissociated mole- 

 cule. If the serum upon dilution with water followed the 

 same dissociation curve as the sodium chloride solution of 0.92 

 per cent, the above method of calculation would be strictly 

 correct. This is, however, not the case ; on the contrary, the 

 dissociation curves separate from each other. But this devia- 

 tion, in the case of the dilutions considered here, lies within 

 the limit of errors of observation, probably because the osmotic 

 pressure of the serum is due chiefly to sodium chloride (Ham- 

 burger, p. 186). 



The above calculation also leaves out of account the fact 

 that the blood-corpuscles are permeable for anions. It is evi- 

 dent that the exchange of ions between blood-corpuscles and 

 surrounding liquid will differ according as this liquid is dilute 

 serum or pure sodium chloride solution. For the partial ten- 

 sion of the ions in the two solutions is not the same. The 

 partial pressure of the 01' ions in the 0.6 per cent sodium 

 chloride solution is greater than in the serum diluted with 60 

 per cent water; while in the serum there is a pressure of 

 CO"' ions, lacking in the sodium chloride solution. Moreover, 

 the blood-corpuscles placed in these liquids acquire a different 

 composition. The osmotic pressure of the blood-corpuscles is 

 increased by imniersion in sodium chloride solution. 



Dissociation and permeability act with opposite sign, balanc- 

 ing each other. Th.it the blood-corpuscle method is practically 



