GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 137 



being interposed in every case between the sensory and the motor 

 neuron. 



The cell-body of the sensory neuron always lies outside the 

 central nervous system in a dorsal root ganglion or a homologous 

 ganglion of a cranial nerve; its axon extends thence into the cen- 

 tral nervous system, and finally enters gray matter where it comes 

 into synaptic connection with the dendrites of an association 

 neuron. This makes connection in turn with a motor neuron 

 whose cell-body lies in the ventral horn of gray matter of the 

 cord, or in a corresponding gray region in the brain (Fig. 65). 



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 sec 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 impulse travels 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 par- 

 ticular 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 con- 

 nection among the various neurons within the central nervous 

 system that any sensory neuron may be brought into commu- 

 nication 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 spinal cord continue along it 

 for some distance, giving off branches, called collaterals, at va- 

 rious levels. Each collateral terminates in an end arborization 



