228 THE ORIGIN OF THE NERVOUS SYSTEM 



its effect as a stimulus on following points produces an 

 even weaker excitation. A relatively steep decrement 

 therefore appears in the cooled or anesthetized region, 

 but if this is not so long that the excitation becomes 

 completely ineffective the impulse regains its normal 

 characteristics when it emerges from the experimentally 

 altered region. 



These and various other characteristics of nervous 

 conduction suggest that the evolution of the excitatory 

 process has consisted in part in the modification of the 

 primitive mechanism in the direction of high intensity, 

 but such modification evidently also involves high 

 susceptibility. Inhibiting agents and conditions first 

 eliminate, reversibly or irreversibly, these highly sus- 

 ceptible features of the mechanism, but affect the more 

 primitive factors more slowly. At certain stages of 

 such action, therefore, highly specialized paths, such as 

 the axon, appear to lose some of their specialization, and 

 their behavior resembles that of more primitive paths. 



CONDUCTION IN THE NEURON AND THE PROBLEM 

 OF THE SYNAPSE 



In its functional relations as a part of the nervous 

 system the neuron conducts impulses from the tips of 

 the dendrites to the tips of the axons, but if an axon 

 is artificially stimulated at some point, the electrical 

 change, the negative variation, which accompanies the 

 passage of a nervous impulse, runs in both directions 

 from the point of stimulation. If this negative variation 

 is a characteristic feature of the nervous impulse, as 

 there is every reason to believe, the axon is evidently 

 capable of conducting impulses in both directions, but, 

 so far as we know, the impulse which passes toward the 



