ORIGIN OF NEURON PATTERN 195 



from each other by the development of neuroglia and 

 intercellular colloidal substance containing water and 

 salts. The dendrites which show definite orientation 

 grow toward certain axons or axon tracts of other neurons 

 and establish synaptic contacts with them. Assuming 

 that these tracts are regions of high developmental or 

 functional nervous activity, electrical conditions are 

 those of the second type of polarization (Fig. 59) in 

 which the cell n and the electronegative surface a are 

 not in continuity, but separated by a conducting 

 medium. Under these conditions the dendrites must 

 grow toward the electronegative surface of the axon 

 tract. It is quite unnecessary to assume, as Kappers 

 has done, that the dendrites react differently to the 

 electric current from the axon. The physiological effect 

 of polarization is the same in both axon and dendrite, 

 but the two outgrowths develop, so to speak, in the 

 two different halves of the electrical circuit, in which the 

 currents flow in opposite directions. 



The apparently selective or specific relations between 

 the axons of certain neurons and the dendrites of others 

 may then be determined by the developmental or func- 

 tional activity of the one and its effect on the direction 

 of growth of the other. Different dendrites of a par- 

 ticular neuron may enter different electrical fields and 

 so establish connections with different neurons, and it 

 is conceivable that some dendrites may remain nutritive 

 organs. 



The dendrite is evidently a less highly specialized 

 axis than the axon, and its frequent branching suggests 

 a low degree of dominance during growth. It reacts 

 chiefly with reference to local conditions rather than to 



