72 THE ORIGIN OF THE NERVOUS SYSTEM 



as it occurs in the highly specialized tissues, it will serve 

 as a point of departure for interpretation of the facts in 

 the following discussion. It may be pointed out, how- 

 ever, that the conception of the gradients and their 

 significance in the organism do not depend upon this or 

 any other particular theory of excitation but upon the 

 observed facts. 



The starting-point of Lillie's theory is the generally 

 accepted fact of electrical polarization of the plasma 

 membrane, the existence of which is well established by 

 the work of numerous investigators. This polarization 

 is usually regarded as determined by the fact that the 

 membrane in the unexcited condition is normally per- 

 meable to ions of one sign, and impermeable, or less 

 permeable, to ions of opposite sign. The facts indicate 

 that the unexcited plasma membrane acts as if per- 

 meable or reversible to certain cations and impermeable 

 or less permeable to certain anions which are present in 

 higher concentration in the cell than in the surrounding 

 medium. The evidence indicates further 



that the change in permeability associated with stimulation is 

 not a direct effect due to merely physical changes in the proto- 

 plasmic surface layer, but is the consequence of a chemical reaction 

 which alters the character of the surface film and temporarily 

 deprives it of its normal semipermeable and electromotor prop- 

 erties, and that this chemical process may in a highly irritable 

 tissue like nerve be initiated by any slight local decrease in polari- 

 zation, provided t^e change is sufficiently rapid [R. S. Lillie, 

 1914, p. 443]. 



In various other papers (e.g., R. S. Lillie, 1917, pp. 80- 

 82; 1918, p. 3; 19 1 9, p. 458) Lillie points out that the 

 chemical change involved in the process is a metabolic 

 reaction, probably oxidative in nature. Whether the 



