208 THE ELEMENTARY NERVOUS SYSTEM 



rones to excite the corresponding dorsal elements. The 

 course that is open to transmission in one direction is 

 not open in the opposite direction. This state occurs ap- 

 parently wherever one neurone joins another and, as 

 it is known not to be characteristic of transmission within 

 a single neurone, it is assumed to be peculiar to the region 

 in which the impulse is transmitted from one neurone to 

 another. The special mechanism resident in this situa- 

 tion and valve-like in its action is known as the synapse. 

 It limits polarization in that it allows nervous transmis- 

 sion to occur only in one of the two possible directions. 

 It is perhaps the most general and definite criterion of 

 a central nervous organ. It would be an extremely im- 

 portant step if an easy test for the presence of synapses 

 could be established. Anatomically this is probably quite 

 impossible because of the very minute size of the struc- 

 tures concerned. Physiologically it might be attained, 

 and already an interesting suggestion in this direction 

 has been made by Moore (1917). It has long been known 

 that strychnine greatly heightens the reflex excitability 

 of many animals and it has been commonly assumed that 

 this action is due to the reduction, under the influence of 

 this drug, of the synaptic resistances. Hence strychnine 

 and other drugs having related effects may be used as a 

 test for the presence of synapses. From this standpoint 

 Moore's results are of extreme interest, for he has found 

 that the drug has no effect on the neuromuscular re- 

 sponses of coalenterates, a slight one on echinoderms, a 

 much greater one on crustaceans and mollusks, a series 

 that leads up to the well-known condition in vertebrates 

 and suggests in its continuity that the effects are depen- 

 dent upon the appearance and degrees of differentiation 

 of the synapse. 



