192 PROTOPLASMIC ACTION AND NERVOUS ACTION 



lipoid and the aqueous phases of protoplasm; in general, 

 the greater the relative lipoid- solubility, the larger the 

 proportion of the anaesthetic compound which is in 

 solution in the Kpoids when the partition-equilibrium is 

 reached. Hence when a compound has a very high 

 Hpoid-solubihty it may exert narcotic action in extremely 

 dilute solution; phenanthrene, for example, was found 

 by Overton to narcotize tadpoles in dilutions of one part 

 in 1,500,000 of water. 



The general conception known as the ''Overton- 

 Meyer theory of narcosis" may be defined as follows. 

 The solubility of narcotizing compounds in the cell- 

 hpoids forms the basis of their narcotic and presumably 

 other pharmacological properties. By dissolving in the 

 lipoids, such compounds alter the physical properties 

 of these essential components of the protoplasmic 

 system, and hence all properties of the system, especially 

 irritability, which are dependent on the state of the 

 Hpoids. Since simple solution without chemical combi- 

 nation is the basis of this effect, the latter is readily 

 reversed by allowing the compounds to diffuse away. 



The general conclusion that selective solubility is 

 the essential basis of narcotic action does not, however, 

 necessarily follow, since the same reasoning would 

 apply to other physical effects which are reversible under 

 similar conditions; e.g., effects dependent on surface- 

 activity, involving a lowering of surface-tension at the 

 protoplasmic phase-boundaries and a concentration of 

 the narcotizing compound at these surfaces. Probably, 

 however, the case is not one of alternatives; if lipoids 

 are present in the system, any substances which are 

 soluble in these components must inevitably dissolve 



