138 THE HUMAN BODY 



which communicates in turn with the dendrites of an association 

 neuron. Thus each sensory neuron has possible connection with 

 various association neurons located in different parts of the cen- 

 tral nervous system. The association neurons likewise are richly 

 branched, each branch terminating in a synaptic connection with 

 another neuron, and this in turn may be an association neuron, 

 or may be a motor neuron. In the second place the dendrites of 

 all association and motor neurons doubtless have synaptic con- 

 nection with end arborizations of numerous neurons, sensory or 

 association as the case may be. Thus a sensory neuron has a 

 wide choice of paths over which to send its impulses ; and a motor 

 neuron may receive impulses from a great variety of sources. 



Irreversible Conduction. In all this maze of connections and 

 interconnections within the central nervous system, how is it 

 that the impulse coming in at a sensory neuron always comes 

 out finally at a motor neuron instead of becoming switched 

 sometimes to another sensory neuron? The orderly progress of 

 impulses is insured by a very simple arrangement, namely, that 

 impulses can pass freely across a synapse from end arborization 

 to dendrites but can never pass in the reverse direction, from 

 dendrites to end arborization. When a sensory neuron delivers 

 its impulse to an association neuron the impulse doubtless spreads 

 to all parts of the latter. It can leave it, however, only by way 

 of its end arborizations, and these communicate only with the 

 dendrites of motor neurons or of other association neurons. The 

 final outcome is bound to be a motor neuron since all association 

 neurons lead ultimately to them. Sensory neurons never receive 

 impulses from other neurons because they have no dendrites 

 within the central nervous system by which impulses might be 

 received. The portion of a sensory neuron which corresponds to 

 the dendrites of a motor neuron is the long axon-like process 

 communicating with the receptor. 



Graded Synaptic Resistance. Another question which nat- 

 urally arises when one considers the innumerable courses which 

 an impulse may take within the central nervous system is what 

 determines the course it actually does take? Why, for instance, 

 when my eye is threatened do I wink instead of opening my 

 mouth, or why do I sometimes wink and sometimes dodge? A 

 complete answer to this question cannot be made in the present 



