574 SIGNIFICANCE OF THE NERVOUS SYSTEM 



along the fiber path. The repetition of impulses, therefore, leads to 

 the formation of open paths, and herein hes the cause of facihtation 

 which in turn gives rise to the formation of habits constituting the 

 neural basis of memory. 



Inhibition. — It must be clear that afferent impulses can produce 

 their characteristic reactions only if the neurons over which they 

 pass, do not simultaneously conduct other impulses. Stated differ- 

 ently, the primary impulse must have a perfectly clear path before it, 

 otherwise a conflict will arise between them which must finally lead 

 to the obUteration of one or the other of these impulses. Only the 

 more effective of them will succeed in eliciting a reaction. It is com- 

 monly believed that this inhibition and elimination of impulses occurs 

 in the centers and not in the fiber paths, because the function of the 

 central cells consumes time and energy so that they cannot do more 

 than attend to a single activity. 



Inhibitory phenomena are explained in two ways, namely, by as- 

 suming a paralysis of the assimilative^ or a paralysis of the dissimila- 

 tive processes. In accordance with the latter hypothesis, which seems 

 to be the more acceptable, all nervous processes are considered as 

 excitations of dissimilative changes and hence, inhibitions must 

 result whenever this dissimilation following upon the reception of an 

 impulse, is stopped. Clearly, therefore, the chief factor in inhibition 

 seems to be an attenuation of the refractory condition of the nerve 

 cell towards secondary impulses. ^ 



CHAPTER XLVII 

 THE FUNCTIONAL UNIT OF THE NERVOUS SYSTEM 



The Reflex Concept. — In the same manner as the complex masses 

 of nervous tissue may be reduced to a single unit, designated as the 

 neuron, so may all nervous actions be reduced to a simple action, 

 known as a reflex. In the same way as the neuron forms the build- 

 ing stone of the nervous system, so does the reflex constitute the func- 

 tional basis of all nervous processes. To be sure, there are many 

 organisms in existence which are not in possession of a nervous system 

 nor even of nervous elements, but which nevertheless react in a manner 

 that, relatively speaking, cannot be said to be inferior to the power 

 of reaction of the higher forms. But as these forms are absolutely 

 devoid of nervous tissue, their actions cannot be said to be reflex in 



iGaskell, Jour, of Physiol., vii, 1885; also: Meltzer, New York Med. Jour., 

 1899. 



2 Verworn, Archiv fiir Physiol., Suppl., 1900, and Zeitschr. fiir allg. Physiol., 

 vi, 1907. 



