PHYSIOLOGICAL GRADIENTS. 179 



theory of excitation and transmission 1 in terms of the plasma 

 membrane, its electrical depolarization in excitation with increase 

 in permeability and resulting electric current. The membrane 

 changes in excitation are conceived by Lillie as involving a chem- 

 ical reaction, probably oxidative, and the electric current result- 

 ing from the depolarization at any point is regarded as the agent 

 of transmission, since it tends to produce depolarization at other 

 points within a certain range and so induces excitation at those 

 points. 



Lillie's theory seems to me to account very satisfactorily for 

 a wide range of observed facts, particularly in the more highly 

 specialized excitations and transmissions, but the question may 

 be raised whether the depolarization of the membrane is always 

 the primary factor in excitation or the chief source of the elec- 

 tric current. A region of rapid oxidation gives rise to an elec- 

 tric current similar to that which Lillie regards as arising from 

 the depolarization of the membrane. Doubtless in protoplasm 

 membrane changes and oxidations are not independent, but while 

 the highly specialized process in the nerve may be very largely 

 or wholly a membrane process, it seems probable that in the prim- 

 itive excitation processes the oxidations play a more important, 

 perhaps the chief part, and that the excitation is not necessarily 

 limited to the plasma membrane. In his later papers Lillie ap- 

 parently recognizes and admits this probability. 



As Lillie points out, the strength of the current produced at a 

 point of excitation decreases with increasing distance from that 

 point, because of resistance, and it can act as a stimulus only 

 within a certain distance. In other words, this agent of trans- 

 mission undergoes a decrement or represents a gradient from 

 the point of original excitation, and except in highly specialized 

 tissues which react according to the " all or none " law, the cur- 

 rent must determine decreasing degrees or amounts of change 

 and so of excitation at increasing distances from the point of 

 original excitation. Even though these new excitations give rise 

 to sufficient current to excite points beyond the range of the orig- 

 inal current the transmission process will show a decrement and 



i R. S. Lillie, 'oga, b, c, 'n, '13, 'H, '15. 'i 6 * 7 , b, '17, '18, 'iQ! R- S. Lillie 

 and E. X. Johnston. 'TO. 



