12 GENERAL AND PHYSICO-CHEMICAL. 



by HEIDENHAIN as being dependent upon their power of abstracting 

 water from the tissues. 



If we admit that the cells normally contain only small amounts of 

 sugar and amino-acids at any one time, then, if these substances are being 

 continuously burned within the cells, new quantities must constantly 

 be taken up and in this way gradually large quantities of the mentioned 

 substance would be taken up and burned. If the combustion is arrested 

 no new quantities are taken up. The fact that certain substances 

 are only taken up in small quantities at a time does not prove that they 

 are not burned within the cells. 



According to MOORE and ROAF 1 the salts exist in the blood cor- 

 puscles in the form of " adsorpates;" these are adsorbed by the solid 

 constituents of the blood corpuscles. As we will see further on (page 

 27) an adsorbing substance can only take up a limited amount of another 

 substance. If, after the saturation limit is reached, more of the adsorbed 

 substance is added then practically no more is taken up. In this way 

 we can explain why the blood corpuscles only take up very little of the 

 salts added. The slight ability of the sugars and amino-acids to be 

 taken up can perhaps be explained in a similar manner. 



Osmotic Pressure of Animal Fluids. As is apparent from the 

 above, a substance exerts upon living cells an entirely different influence, 

 depending upon whether the substance is able to pass into the cell or 

 not, and whether the substance which does not pass in has the ability 

 of attracting water or not. Therefore that part of the osmotic pres- 

 sure of body fluids which is caused by bodies not passing in is called 

 the effective osmotic pressure. In this manner therefore the salts of the 

 alkalies and alkaline earths and the sugars act. As sugar, as well as the 

 bodies which according to the just mentioned experiments are readily 

 taken up by the cells, occurs under ordinary conditions only in very 

 small amounts in the blood, and also as the proteins are practically with- 

 out influence upon the osmotic pressure, the normal osmotic pressure of 

 the blood is chiefly due to the salts. As the depression of the freezing- 

 point is almost the only method used for animal fluids, therefore ordinarily 

 the freezing-point depression (A) is given as a measure of the osmotic 

 pressure. For mammalian blood A is constant with the exception of slight 

 variations due to the food and perhaps also to other 'circumstances. It 

 is 0.56 , 2 which corresponds to a 0.90 per cent NaCl solution and to an 

 osmotic pressure of about 6J atmospheres. In lower animals A may 

 be slightly lower, for example, in the frog A = 0.46. In invertebrate 

 sea animals the body fluid is equal to the osmotic pressure of the sur- 



1 Bioch. Journ., 3, 55 (1908). 



2 Hamburger, Osmotischer Durck u. lonenlehre, 1, 456. 



