PROPERTIES CONFERRED BY CONSTITUENTS 261 



Lowering of 

 Species. freezing-point. 



Man . . 0.526 



Ox 0.585 



Horse . . 0.564 



Pig 0.615 



Rabbit . 0.592 



Dog :...... 0.571 



Cat. . - fc . .- -. . . . ^ . . . 0.638 



Sheep 0.619 



Echidna hystrix 0.600 



From this table two remarkable facts are apparent: In the first 

 place that the osmotic pressure of the blood of species of mammalia so 

 diverse as man, herbivora, carnivora and the monotremes is extra- 

 ordinarily constant, and in the second place that it has the magnitude 

 of no less than some eight atmospheres, corresponding to the pressure 

 exerted by a one-third molecular solution of a non-ionized substance 

 such as sugar or urea, or a one-sixth molecular solution of sodium 

 chloride. - 



The osmotic pressure of the blood-serum, as evaluated from the 

 lowering of the freezing-point, rises slightly, but unmistakably, after 

 the absorption of the products of digestion derived from the meal. 

 The Lymph has usually a higher osmotic pressure than the blood, a 

 fact which is attributed to the extraction of products of metabolic 

 activity from the tissues somewhat more rapidly than they can be 

 discharged from the lymph into the blood. Milk and Bile have the 

 same osmotic pressure as blood, Saliva a lower pressure. Urine is 

 in general much more concentrated in diffusible constituents than the 

 blood or tissue-fluids and therefore displays a much greater lowering 

 of the freezing-point, usually between 1.3 and 2.3. 



The blood-sera of Birds possess an osmotic pressure very similar to 

 that of mammalian blood -sera. It is a curious fact, however, that 

 the Eggs of birds have a distinctly lower osmotic pressure than that 

 of the blood-serum of the birds that lay them, or of the blood-serum 

 of the embryos that develop within them. This is strikingly shown 

 by the following estimations of Atkins. 



Lowering of 1 

 Species. freezing-point. 



Fowl-blood . .- . 0.607 



Fowl-egg 0.454 



Duck-blood 0.574 



Duck-egg 0.452 



Goose-blood 0.552 



Goose-egg '. . 0.420 



During Incubation of the egg the osmotic pressure of its contents 

 increases until it approximates to that of the blood. Since in so many 

 anatomical particulars the Ontogeny of the individual represents an 

 abbreviated outline of the Phylogeny of the species, Atkins has sug- 

 gested that the low osmotic pressure of the egg-contents may indicate 

 descent of the birds from ancestors in which the blood-serum was more 

 dilute than it is in the birds of the present epoch. Since the birds 



