CH. XXIV.] OSMOTIC PKESSUKE 327 



If the solution is hypertonic* i.e.* has a greater osmotic pressure than the cell con- 

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

 become crenated ; if the solution is hypotonic, i.e., has a smaller 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, such as 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 (or adsorption), and osmosis are fairly well known and 

 can be experimentally verified. But we have undoubtedly some other force, or 

 some other manifestation 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 

 membrane 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 have asserted that the action is wholly or partly vital. We shall, however, 

 find that recent accurate work has shown that the main facts are explicable on a 

 physical basis. 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 Proteins. It has been generally assumed that proteins, 

 the most abundant and important constituents of the blood, exert little or no 

 osmotic pressure. Starling, however, has claimed that they have a small osmotic 

 pressure ; if this is so, it is of importance, for proteins, unlike salt, do not diffuse 

 readily, and their effect therefore remains as an almost permanent factor in the 



