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THE HUMAN BODY 



along it for some distance, giving off branches, called collaterals, at 

 various levels. In the spinal cord the dorsal white columns con- 

 tain these axons. They extend toward the brain, but each gives 

 off a branch which extends a short distance down the cord in the 

 opposite direction. Each collateral terminates in an end arboriza- 

 tion which communicates in turn with the dendrites of another 

 neuron, either motor or association. Thus each sensory neuron, 

 besides its connection with one or more motor neurons, has con- 

 nection with various association neurons located in different parts 

 of the central nervous system. The association neurons likewise 

 are richly branched, each branch terminating in a synaptic con- 



FIG. 64. Diagram to illustrate how a single sensory neuron may communicate 

 with several motor neurons, and a single motor neuron with several sensory neurons. 



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

 neuron, or may be a motor neuron. In the second place the den- 

 drites of all association and motor neurons doubtless have synaptic 

 connection 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 

 (Fig. 64). 



