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CENTRAL NERVOUS SYSTEM 



fibers pass from one ganglion to the next in trunks known as intergan- 

 glionic connectives. 



In order that a muscular response may result from a stimulus applied 

 to the skin of a worm, a disturbance must pass through the following 

 structures: (1) the receptor with its afferent fiber, (2) a motor neuron 

 with its efferent fiber, to (3) the muscle (or other effector). This group 

 of structures is called a reflex arc. It is the simplest anatomical arrange- 

 ment in the central nervous system, capable of bringing about a signi- 



Fig. 205. Arborization of collaterals from the posterior root fibers around the cells of the 

 posterior horn. A, ascending fiber in posterior columns; B, collaterals; C, cells of posterior hor/i; 

 E, synapses. (From Ramon y Cajal.) 



ficant motor response. The activity produced by such a mechanism is 

 called a reflex. 



The reflex arc as we have described it lies frequently within a single seg- 

 ment of the worm. Consequently it can provide each segment with a 

 great degree of autonomy, so that many of its activities are undisturbed 

 by the removal of other parts of the animal. No provision is made by 

 such an arrangement for correlating the activities af adjacent segments. 

 The ganglia of worms contain, as a matter of fact, an additional type 

 of neuron which makes the correlation of the activities of different seg- 

 ments possible. These are neurons which lie entirely within the central 

 nervous system. Their axons lie in the interganglionic connectives and 



