54 INTRODUCTION TO NEUROLOGY 



In the synaptic systems, as found in all highly differentiated 

 nervous centers, the majority of neurologists teach that at the 

 synapse the two neurons involved are simply in contact and that 

 the nervous impulse passes from one to the other across a very 

 short gap in the conducting substance. Others believe that 

 they have demonstrated very delicate protoplasmic threads 

 which bridge this gap, thus establishing continuity of the con- 

 ducting substance across the synapse. Good histological prepara- 

 tions show, however, in some of the most intimate synapses 

 known where the axon ends directly on the cell body of the sec- 

 ond neuron that there is a distinct cellular membrane around 

 the terminals of the fibers of the first order and a second cellu- 

 lar membrane enveloping the body of the neuron of the second 

 order, so that continuity of the ordinary protoplasm of the 

 neurons here seems to be quite impossible, so far as our present 

 technic is adequate to decide the question. 1 



The following important points regarding the synapse seem 

 to be established: 



1. Unimpeded protoplasmic continuity across the synapse has 

 not been clearly established, and in some cases there is clearly 

 a membranous barrier interposed between the two neurons. 

 But the exact nature of this barrier is unknown and it by no 

 means follows that the synaptic membrane is an inert substance. 

 It may be composed of living substance of a different nature 

 from that of the other protoplasm of the neurons. 



2. The transmission of the nervous impulse across the synapse 

 involves a delay greater than that found in the nerve-fiber or 

 the cell body. This suggests that there is some sort of an ob- 

 struction here which does not occur elsewhere in the reflex arc 

 (see p. 98). 



3. The synapse is more susceptible to certain toxic substances, 

 such as nicotin, than is any other part of the reflex arc. 



4. Though a nerve-fiber seems to be capable of transmitting 

 an impulse in either direction, the nervous impulse can pass the 

 synapse only in one direction, viz., the direction of normal dis- 

 charge from the axon of one neuron to the dendrite of another. 



1 For an illustration of such a synapse see BARTLEMEZ, G. W., Mauthner's 

 Cell and the Nucleus Motorius Tegmenti, Jour. Comp. Neur., vol. xxv, 

 1915, Figs. 11, 12, and 13, pp. 126-1 2S. 



