PHYSIOLOGICAL 395 



great importance in physiology, and is reflected in studies of the 

 effect of these salts on the contraction of muscle, the beating of 

 cilia, and so on. 



In the field of permeability the recent work of Chambers is very 

 important; it seems that sodium attacks the cell membrane, making 

 it too permeable, while calcium salts have the opposite effect, and 

 prevent either of the salts from entering at all. But if these salts 

 are injected into the cell, by means of the micro-dissection apparatus, 

 so that the cell membrane does not interfere, the result shows that 

 calcium is the more deadly to the internal protoplasm. From with- 

 out, however, it is unable to exert this action. 



To return to the theories of the surface film: the theory of the 

 lipoid membrane was followed by a theory of a protein membrane 

 (for proteins have a great power of forming surface films). This again 

 was followed by an ingenious theory of Clowes (1916), who showed 

 that a film of lipoid containing droplets of watery fluid as an emul- 

 sion might very readily change to an emulsion of droplets of lipoid 

 in a continuous watery fluid. In the first case substances soluble in 

 water would pass with great difficulty; in the second case more 

 readily; and it appears that the antagonistic effects of sodium and 

 calcium on such a film are to sway the balance from one state to 

 the other, so that sodium increases permeability by making the 

 watery phase continuous, with only droplets of lipoid, while calcium 

 reversed this effect. In many other ways, Clowes's hypothesis has 

 been found to agree with the facts. 



The essential facts are that certain gases, especially oxygen and 

 carbon dioxide, certain organic substances soluble in fat-solvents, 

 and water itself, enter cells with considerable readiness, while salts 

 or other substances enter so much more slowly that for a time at 

 least the membrane is almost semi-permeable and comparable to 

 the artificial copper-ferrocyanide membrane, and osmotic pressure 

 may be observed. On the other hand colloidal particles, which are 

 of more than molecular size, do not pass through the film at all; 

 where they do enter cells, it is by the engulfing or swallowing process 

 called phagocytosis, and this applies also to solid insoluble granules. 

 Lastly, it must be noted that the surface of the cell is far from being 

 an unalterable datum ; it may be affected in various ways ; thus its 

 permeability may be increased by diverse factors, by the inter- 

 penetration of external substances, and by injuries due to internal 

 intruding organisms. 



BUILDING UP AND BREAKING DOWN— Characteristic of 

 the living organism, illustrated, for instance, in the familiar con- 

 traction of muscle, is the simplification of large compounds into 

 lesser ones, a process often accompanied by oxidation, but not 

 always. The hexose-phosphate of the muscle-cell gives rise to the 



