208 THE ORIGIN OF THE NERVOUS SYSTEM 



ontogeny but of evolution, and as such is of course 

 associated with changes in the hereditary mechanism 

 of excitation, however we may conceive such changes 

 to be brought about. 



The ascending phase, the front of the excitation 

 wave or impulse, is itself essentially a physiological 

 gradient which, instead of being stationary, advances 

 over the protoplasm. The descending phase of the 

 wave represents the fact that this gradient is readily 

 and rapidly reversible, at least to a large extent. The 

 more rapid the change from the unexcited condition to 

 the completely excited condition, the steeper the gra- 

 dient, and the steeper it is the more effective it becomes 

 in inducing excitation at other points. With the 

 increase in conductivity, i.e., the increase in the effective- 

 ness of the excitation at any point in bringing about 

 excitation at another point, the decrement becomes less 

 and less steep, until in the vertebrate nerve fiber a con- 

 dition is attained in which the "all-or-none law" holds, 

 and conduction supposedly occurs without a decrement 

 (see pp. 81, 223). In such protoplasm a certain minimal 

 degree of stimulation sets off, so to speak, the whole 

 mechanism of excitation and a maximal excitation results 

 automatically, just as a tremendous explosion may 

 result from the reaction initiated by a minute spark. 

 Concerning the actual physicochemical basis of these 

 changes in the character of excitation in protoplasm we 

 know little, but concerning the occurrence of the change 

 there can be no doubt. 



In this development of excitation rhythmicity fre- 

 quently appears, that is, instead of the continuous 

 existence of a state of excitation and its transmission or 



