GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 159 



process previously described (p. 136). The cell-body of the sensory 

 neuron always lies outside the central nervous system in a dorsal 

 root ganglion; its axon extends thence by way of the dorsal root of 

 the spinal nerve into the spinal cord (or brain) and enters the dorsal 

 column of white matter (p. 144). 



Within the central nervous system is located the cell-body of 

 the motor neuron which forms the outgoing part of the path. This 

 will be found in the ventral horn of gray matter of the spinal cord. 

 The simple reflex arc is formed by one of the branches of the 

 sensory axon which penetrates the gray matter and whose end 

 arborization makes synaptic connection with the dendrites of the 

 motor neuron. 



Reflex Arcs Not Rigidly Fixed Paths. Although a given sen- 

 sory stimulus usually arouses the same sort of reflex response 

 every time it is applied, this does not mean that the reflex path 

 followed in such a case is the only one into which that sensory 

 neuron leads. Very different reflex responses may originate in 

 the same receptor. A good illustration of this is furnished by 

 certain reflexes through the eye. If I see that a small floating 

 particle threatens my eye I am apt to wink; if a flying insect ap- 

 proaches I am more likely to turn my head to one side; if the 

 threatening object is a swiftly thrown baseball I will probably 

 bring the hands before the face, or perhaps dodge to one side. 

 All these actions are performed mechanically and are therefore 

 true simple reflexes. The originating sensory impulses travel in 

 each case over the same sensory neurons, those of the optic nerves. 

 It is evident, then, that impulses coming in over the sensory 

 neurons of the optic nerve do not have to pass over to any partic- 

 ular motor neuron, such as the one which leads to the muscle 

 of winking, but may follow any one of various courses, finally 

 terminating in muscles far distant from the eye. In fact, and this 

 is one of the most important things to remember about the nervous 

 system, there is such an extraordinary richness of connection 

 among the various neurons within the central nervous system that 

 any sensory neuron may be brought into communication with any 

 motor neuron. 



This richness of connection is afforded anatomically through 

 two rather simple arrangements. In the first place the axons of 

 sensory neurons after entering the central nervous system continue 



