326 LYMPH AND LYMPHATIC GLANDS [CH. XXII. 



The osmotic pressure of solutions may also be compared by observing their 

 effect on red blood corpuscles, or on vegetable cells such as those in Tradescantia. 

 If the solution is hypertonic, i.e., has a greater osmotic pressure than the cell 

 contents, the protoplasm shrinks, and loses water, or if red corpuscles are used, 

 they become crenated ; if the solution is hypotonic, i.e., has a less osmotic pressure 

 than the material within the cell-wall, no shrinking of the protoplasm in the 

 vegetable cell takes place ; and if red corpuscles are used they swell and liberate 

 their pigment, hotonic solutions, like physiological salt solution, produce neither 

 of these effects, because they have the same molecular concentration and osmotic 

 pressure as the material within the cell-wall. 



Physiological Applications. It will at once be seen how important all these 

 considerations are from the physiological standpoint. In the body we have aqueous 

 solutions of various substances separated from one another by membranes. Thus 

 we have the endothelial walls of the capillaries separating the blood from the lymph ; 

 we have the epithelial walls of the kidney tubules separating the blood and lymph 

 from the urine ; we have similar epithelium in all secreting glands ; and we have 

 the wall of the alimentary canal separating the digested food from the blood-vessels 

 and lacteals. In such important problems, then, as lymph-formation, the forma- 

 tion of urine and other excretions and secretions, and absorption of food, we have 

 to take into account the laws which regulate the movements both of water and of 

 substances which are held in solution by the water. In the body osmosis is not the 

 only force at work, but we have also' to consider filtration, that is, the forcible 

 passage of materials through membranes, due to differences of mechanical pressure. 

 Further complicating these two processes we have to take into account another 

 force, namely, the secretory or selective activity of the living cells of which the 

 membranes in question are composed. This is sometimes called by the name vital 

 action, which is an unsatisfactory and unscientific expression. The laws which 

 regulate filtration, inhibition, and osmosis are fairly well known and can be experi- 

 mentally verified. But we have undoubtedly some other force, or some other mani- 

 festation of force, in the case of living membranes. It probably is some physical 

 or chemical property of living matter which has not yet been brought into line with 

 the known chemical and physical forces which operate in the inorganic world. We 

 cannot deny its existence, for it sometimes operates so as to neutralise the known 

 forces of osmosis and filtration. 



The more one studies the question of lymph-formation, the more convinced one 

 becomes that mere osmosis and filtration will not explain it entirely. The basis of 

 the action is no doubt physical, but the living cells do not behave like the dead 

 membranes of a dialyser ; they have a selective action, picking out some substances 

 and passing them through to the lymph, while they reject others. 



The question of gaseous interchanges in the lungs is another of a similar 

 kind. Some maintain that all can be explained by the laws of diffusion of gases ; 

 others assert that the action is wholly vital. Probably those are most correct 

 who admit a certain amount of truth in both views ; the main facts are explicable 

 on a physical basis, but there are also some puzzling data that show that the 

 pulmonary epithelium is able to exercise some other force as well which inter- 

 feres to some extent with the known physical process. Take again the case of 

 absorption. The object of digestion is to render the food soluble and diffusible ; it 

 can hardly be supposed that this is useless ; the readily diffusible substances will 

 pass more easily through into the blood and lymph : but still, as Waymouth Reid 

 has shown, if the living epithelium of the intestine is removed, absorption comes 

 very nearly to a standstill, although from the purely physical standpoint removal of 

 the thick columnar epithelium would increase the facilities for osmosis and filtration. 



The osmotic pressure exerted by crystalloids is very considerable, but their 

 ready diffusibility limits their influence on the flow of water in the body. Thus if a 

 strong solution of salt is injected into the blood, the first effect will be the setting 

 up of an osmotic stream from the tissues to the blood. The salt, however, would 

 soon diffuse out into the tissues, and would now exert osmotic pressure in the 

 opposite direction. Moreover, both effects will be but temporary, because excess of 

 salt is soon got rid of by the excreting organs. 



Osmotic Pressure of Proteids, It has been generally assumed that proteids, 



