74 THE ORIGIN OF THE NERVOUS SYSTEM 



electric current arising in relation to the excitation of a 

 given region a affords the condition necessary to bring 

 about excitation at b, c, etc., i.e., at all points within a 

 certain distance from a where the strength of the current 

 is sufficient to bring about the changes determining 

 excitation. In short, according to this theory of excita- 

 tion and transmission, the electrical changes involved in 

 the excitation of any point a, however that excitation 

 be produced, determine the transmission of excitation 

 from a to b, c, etc. The important point for present 

 purposes is that in consequence of the increasing resist- 

 ance with increasing distance from the point a, a gra- 

 dient in electrical condition of the plasma membrane 

 results from the excitation at a, the (external) electro- 

 negativity decreasing from b to some point at which 

 the resistance becomes so great that the electrical effect 

 of the excitation at a is zero. If Lillie's conception is 

 correct every local excitation gives rise for the time 

 being to such a gradient. 



According to Lillie, then, the electrical change pro- 

 duced by excitation at a becomes the "stimulus" which 

 brings about excitation at b, c, d, etc., at all points, 

 that is, within the limit of its physiological effective- 

 ness. If it is true, as the facts indicate (see pp. 80-81), 

 that in protoplasm in general, where specialized con- 

 ducting paths do not exist, a relation between the energy 

 or the intensity of excitation and the energy or the 

 intensity of stimulus exists, it follows that when excita- 

 tion occurs at a given region a of such protoplasm a 

 gradient in the degree, energy, or intensity of excita- 

 tion, i.e., in its ability to act as stimulus for other points, 

 must result in consequence of the electrical conditions, 



