82 THE ORIGIN OF THE NERVOUS SYSTEM 



the conditions arising at the point a of original excitation, 

 but points 6, c, etc., become factors in extending the 

 range. Unless the degree of excitation at the various 

 points b, c, d, etc., within the range of the current pro- 

 duced at a is the same and therefore independent of the 

 strength of the current at these points, the excitation 

 will undergo a decrement in the course of transmission 

 and will finally reach a limit, and the steepness of the 

 decrement will differ according to the relation between 

 stimulus and excitation in the protoplasm concerned. 

 The facts indicate that between the primitive gradient 

 in which transmission depends entirely upon conditions 

 at the point of original excitation and the condition of 

 transmission without decrement various intermediate 

 conditions exist, even in the same organism at different 

 stages of development and perhaps in some cases with 

 different degrees of excitation (see p. 225). 



THE QUESTION OF REVERSIBILITY 



When we take into account all the facts it seems 

 necessary to conclude, as I have already suggested, that 

 excitation in a broad sense is essentially nothing else than 

 an acceleration of the fundamental activities of life, and 

 particularly or primarily those concerned with energy 

 liberation. Excitability or irritability is a fundamental 

 characteristic of living protoplasm and life is apparently 

 made up of a series of excitations and their effects. The 

 processes may differ as the mechanisms differ, but the 

 fact remains that excitation means a rise in the level of 

 living. The physiological or metabolic gradients which, 

 as I have endeavored to show, constitute the funda- 

 mental features of axiate pattern, are essentially gradients 



