THE NERVOUS SYSTEM OF MAN 1^9 



Stimulus from one neuron on to the next. There 

 is a clear conviction at present on this question, 

 namely, that each neuron is an independent unit 

 and its connection with other neurons is by contact 

 only. This means that there is a break in the 

 continuity of the nerve pathway. An axon may 

 end through numerous minute terminal branches 

 on a second neuron or in contact with the cell body 

 of such a neuron. The point in a nervous path- 

 way where the stimulus reaches such a structural 

 break, — where it actually passes over into the next 

 neuron is called the synapse. Little is scientifi- 

 cally known about what happens in synapsis. 

 The more synapses in a given route, the longer it 

 takes for a stimulus to reach its destination. The 

 reflex excitability of animals with a synaptic 

 nervous system is greatly increased by strychnin 

 while this drug has no effect on Coelenterates 

 which do not have the synaptic system. In numer- 

 ous parts of the spinal cord and brain more than 

 one neuron receives the traveling stimulus. Thus 

 a single sensory stimulus may at a definite place be 

 distributed and redistributed until it reaches the 

 entire nervous system. Just the opposite structur- 

 al plan also exists by means of which several dif- 

 ferent stimuli are finally routed into one common 

 center thus bringing about a summation of stimuli. 

 Science is just beginning to realize the nature of 



