136 PHYSIOLOGY 



salt in any fluid above that of the same salt in the plasma, nor the passage 

 of a salt from a hypotonic fluid into the blood plasma, can afford in itself 

 any proof of an active intervention of cells in the process. 



In the case of the pleura, for example, we seem to have a membrane which is very 

 imperfectly semi-permeable. It is permeable to salts, but presents rather more 

 resistance to their passage than to the passage of water. Hence on injecting 0-5 per 

 cent. NaCl solution into the pleural cavity, water passes from the pleural fluid into 

 the blood, until the percentage of sodium chloride in the fluid is raised perceptibly 

 above that in the blood plasma. The limit of the resistance of the pleural membrane 

 to the passage of salt is, however, soon reached, and then salt passes from pleural fluid 

 into blood ; but in every case this passage is from a region of higher to a region of 

 lower partial pressure. Hence at a certain stage of the experiment we find a higher 

 percentage of salt in the pleura than in the blood-vessels, although the total amount 

 of salt in the pleural fluid is less than that originally put in, or, in other words, salt has 

 been absorbed. 



We have already seen that the effective osmotic pressure of a substance, 

 i.e. its power of attracting water across a membrane, varies inversely as its 

 diffusibility, or as the permeability of the membrane to it. What, then, will 

 be the effect if on one side of the membrane we place some substance in 

 solution to which the membrane is impermeable ? 



We will suppose that A and B both contain 1 per cent. NaCl, but that 

 B contains in addition some substance x to which the membrane is im- 

 permeable. Since the osmotic pressure of B is higher, by the partial pressure 

 of x, than that of A, fluid will pass from A to B by osmosis. But the conse- 

 quence of this passage of water will be to concentrate the NaCl in A, so that 

 the partial pressure of this salt in A is greater than in B. NaCl will therefore 

 diffuse from A to B, with the result that the former difference of total 

 osmotic pressure will be re-established. Hence there will be a continual 

 passage of both water and salt from A to B, until B has absorbed the whole 

 of A. This result will be only delayed if the osmotic pressure of A is at first 

 higher than B, in consequence of a greater concentration of NaCl in A. 

 There may be at first a flow of fluid from B to A, but as soon as the NaCl 

 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 attrac- 

 tion will be proportional to the osmotic pressure of x. We shall have 

 occasion to discuss a specific instance of this case when dealing with the 

 mechanism of absorption of fluid by the blood-vessels from the connective 

 tissue spaces. 



A more familiar example is afforded by the process known as dialysis. 

 Many animal membranes, all of which are colloidal in character, and others 

 such as vegetable parchment, while freely permeable to salts, are impermeable 

 to dissolved colloids. If, therefore, a fluid containing both colloids and 

 crystalloids in solution, e.g. blood-serum, be enclosed in a tube of vegetable 

 parchment, which is hung up in a large bulk of distilled water (Fig. 26), all 

 the salts diffuse out, and if this be frequently changed, we obtain finally a 

 fluid within the dialyser free from salts and other crystalloid substances, but 

 containing the whole of the colloidal proteins originally present. 



