THE ACTIVITIES OF THE ORGANISM 95 



tion, and assimilation. We have applied it to the case 

 of fat-digestion, but very much the same general 

 scheme might also apply to many other processes in 

 the body. Obviously it enables us to describe these 

 processes in terms of physico-chemical reactions, but 

 we cannot fail to see that ultimately we are compelled 

 to assume the existence of reactions which were not 

 included in the original conception — the activation 

 of the enzyme at the proper moment by the kinase, 

 the operation of the anti-enzyme, and the passage of 

 the enzyme into the zymoid. Just why these things 

 happen as they do we do not know, yet the whole 

 problem becomes shifted on to these reactions. 



In the same way we apply the purely physical 

 processes of the osmosis and diffusion of liquids to the 

 circulation of substances in the animal body. The 

 nature of these processes will probably be familiar 

 to the reader, nevertheless it may be useful to remind 

 him that by diffusion we understand the passage of a 

 liquid, containing some substance in solution, through 

 a membrane ; and by osmosis the passage of a solvent 

 (but not of the substance dissolved in it) through a 

 " semi-permeable membrane." The molecules of the 

 solvent (water, for instance) pass through the membrane 

 (the wall of a capillary, or lymphatic vessel), but the 

 molecules of the substance (salt, for instance) dis- 

 solved in the solvent do not pass. Let us suppose that 

 a strong solution of common salt in water is injected 

 into the blood stream : what happens is that osmosis 

 takes place, the water in the surrounding lymph spaces 

 passing into the blood stream because the concen- 

 tration of salt there is greater than it is in the lymph. 

 While this is happening, the capillary walls are acting 

 as semi-permeable membranes, allowing the molecules 

 of water to pass through but not the molecules of salt. 



