160 SCIENCE PROGRESS. 



from B to A, but as soon as the Na CI concentration on 

 the two sides has become the same by diffusion the power 

 of x to attract water from the other side will make itself 

 felt, and this attraction will be proportional to the osmotic 

 pressure of x. 



We have an example of such a process in the absorption 

 of salt solutions from the connective tissues by the blood- 

 vessels, as well as in the absorption of the normal tissue 

 lymph. The capillaries of the connective tissues of the 

 limbs and peripheral parts of the body are almost imperme- 

 able to proteids. In consequence of this impermeability the 

 fluid which is transuded from the capillaries under pressure 

 contains very little proteid, whereas it contains exactly the 

 same proportion of salts as does the blood plasma. It seems 

 probable therefore that the proteid left in solution in the 

 capillaries must exert a certain osmotic attraction on the 

 salt solution outside the capillaries. It is easy to 

 measure this attractive force. If blood serum be placed in 

 a small thistle funnel, on the open end of which is stretched 

 a layer of membrane soaked in gelatine, and the inverted 

 funnel be immersed into salt solution which is isotonic or 

 even hypertonic as compared with the serum, measured by 

 the freezing-point, within the next two to four days fluid 

 will pass into the funnel and rise up in its capillary stem to 

 a considerable height. I have found that the osmotic 

 pressure of the non-diffusible constituents of blood serum 

 measured in this way amounts to between 30 mm. and 40 mm. 

 Hg. Now although this osmotic pressure is so small, it is of an 

 order of magnitude comparable with that of the hydrostatic 

 pressure in the capillaries. This fact is of importance in 

 that, whereas the capillary pressure determines transudation 

 from the vessels, the effective osmotic pressure of the 

 serum (proteids ?) determines absorption by the blood- 

 vessels. Moreover the osmotic attraction of the serum for 

 the extravascular fluid will be proportional to the force 

 expended in the production of this extravascular fluid, so 

 that at any given time there must be a balance be- 

 tween the hydrostatic pressure in the capillaries and the 

 production or absorption of fluid from the extravascular 



