APPENDIX 207 



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. Fiurther complicating these two processes we have to 

 take into account another force, namely, the secretory or selective activity of 

 the liA-ing cells of which the membranes in question are composed. This is 

 sometimes called by the name vital action, which is an rmsatisfactory and 

 unscientific expression. The term is merely a confession of ignorance ; 

 what we cannot understand we may call vital, but it is no explanation of any 

 matter to say we cannot understand it. The laws which regulate filtration, 

 imbibition, and osmosis are fairly v/ell known and can be experimentally 

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

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

 physical or chemical property of living matter which has not yet been 

 brought into line wdth the knowTi chemical and physical forces which 

 operate in the inorganic world. We cannot deny its existence, for it some- 

 times operates so as to neutralise the known forces of osmosis and filtration. 

 It is, however, contrary to scientific usage and a bar to aU progress La 

 knowledge to sit quietly down and merely label it as %-ital ; we must look 

 forward to the time when workers will succeed in unravelhng its so-called 

 mysteries, and show what it really consists in. 



The more one studies the question of Ij-mph-formation, the more con- 

 vinced 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 has been another 

 battlefield of a similar kind. Some maintain that all can be explained 

 by the laws of difiiision 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 interferes 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 difiusible ; it can hardly be sup- 

 posed that this is useless ; the readily difiusible substances will pass more 

 easily through into the blood and lymph : but stUl, as Waymouth Eeid has 

 shown, if the living epithelium of the uatestine is removed, absorption comes 

 very nearly to a standstill, although from the pvirely 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 diflfusibUity 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 



