OSMOTIC PRESSURE. 35 



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

 the effective osmotic pressure. These bodies include HEIDENHAIN'S lym- 

 phagogues of the second order (Chapter VII). Their action, according 

 to HEIDENHAIN, depends upon their ability to abstract water from the 

 tissues. To this group also belongs the salts of the alkalies and alkaline 

 earths and the sugars. 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 without 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 , 1 which corresponds to a 0.90 per cent NaCl solution and to an 

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

 be slightly lower, for example, in the frog J = 0.46. In inver- 

 tebrate sea animals the body fluid is equal to the osmotic pressure of the 

 surrounding sea water (^ = 2.3) and varies with the quantity of salt 

 in the water (BOTTAZZI 2 ) . In lower fishes (Selachii) the osmotic pressure 

 of the blood is equal to the surrounding medium, and in higher fishes 

 (Teleostomi) lower (J=1.0) (BOTTAZZI). 



In sea fishes as well as fresh-water fishes, for example, the eel, a lower 

 osmotic pressure (^=0.41) is found when kept in fresh water than 

 when kept in sea water (J = 0.55) 3 . In lower sea animals the osmotic 

 pressure is equal to the surrounding medium, while higher animals are 

 independent of the surroundings. HOBER calls attention to this condi- 

 tion and points out the analogy with the body heat of the various 

 animals. 4 



If we pass to other body fluids we must mention that the lymph 

 shows a somewhat higher osmotic pressure than the blood, and this is 

 due to the lymph taking up from the tissues metabolic products 

 having a low molecular weight. 5 Milk and bile have the same 

 osmotic pressure as the blood, 6 while saliva has a lower pressure. 7 



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



2 Archives ital. de Biol., 28, 61, 1897. 

 3 Dekhuisen, Arch, neerland, 10, 121, 1905; Quinton, Compt. rend. soc. biol., 57, 



470, 513, 1904. 



4 Physik. Chem. d. Zelle u. Gewebe, 2. AufL, 1906, 33. 



5 Leathes, Journ. of PhysioL, 19, 1, 1895. 



6 Dresser, Arch. f. exp. Path. u. Pharm., 29, 303, 1892. 



7 Nolf, Traveaux du lab. de phys. de Liege, 6, 225, 1901. 



