50 THE NERVOUS SYSTEM 



Bartelmez (1915) has shown that an axon and dendrite, entering into the forma- 

 tion of a synapse, are each surrounded by a distinct plasma membrane and 

 that there is no direct protoplasmic continuity. It has been maintained by 

 Bethe and others that at such points of contact the neurofibrils pass without 

 interruption from one neuron to another, but this has been denied by Cajal. 

 The relation between two neurons at a synapse appears to be one of contact, 

 but not of continuity of substance. 



Nerve impulses pass across the synapse in one direction only, i. e., from the 

 axon to the adjacent cell body or dendrite. As a corollary of this it is obvious 

 that impulses must travel within the neuron from dendrites to perikaryon and 

 then out along the axon, as indicated by the arrow in Fig. 30. This is known 



Fig. 29. Basket cell from the cerebellar cortex of the white rat. The Purkinje cells are indicated 



in stipple. Golgi method. (Cajal.) 



as the law of dynamic polarity. The polarity is, however, not dependent upon 

 anything within the neuron itself, but upon something in the nature of the 

 synaptic interval which permits the impulses to travel across it in one direc- 

 tion only. There are many lines of evidence which indicate that when once 

 activated a nerve-fiber conducts equally well in either direction. When a motor 

 fiber bifurcates, sending a branch to each of two separate muscles, stimulation 

 of one branch will cause an impulse to ascend to the point of bifurcation, and 

 then descend along the other branch to its motor ending (Fig. 30). This can 

 often be demonstrated in regenerated nerves (Feiss, 1912). The phenomena 

 of antidromic conduction and the axon reflex (Bayliss, 1915) are also explained 

 by the assumption that impulses are able to travel along a nerve-fiber in either 

 direction. 



