REFLEX ACTION 



239 



are carried from the center to the effector cells is known as the efferent 

 path. 



279. Reflex Acts. — In a typical and ideally simple reflex act a receptor 

 neuron situated in the hypodermis is stimulated from without (Fig. 142), 

 the stimulus being received directly by the cell body or by a short den- 

 drite. An afferent impulse follows the axon of this cell to a synapse in a 

 central ganglion where it is passed to the dendrite of an adjustor neuron. 

 This sends out an efferent impulse along its axon to a nuiscle cell, the 

 effector, causing contraction and resulting in a movement appropriate 



This entire mechanism is 



to the stimulation which originated the act 

 Acijustor 



Efferent nerve 

 fiber 



(j/anf fibers 



Ventral ganglion 



Synapse 



Afferent 

 nerve fiber 



Body 

 wall 



Effectors 



Ventral longitudinal 

 muscles 



Fig. 142. — Diagram illustrating reflex action in an earthworm. 



Sici. Mon., vol. 75, after Retzius.) 



Receptor 



{From Parker, in Popular 



called a reflex arc. Actually such a simple reflex does not occur, since 

 in any action several receptors are stimulated at the same time and several 

 effectors participate in the action. Also not one but several adjustors 

 are usually involved, and they form chains of neurons through which 

 conduction takes place. Since each of these adjustors is in communica- 

 tion with several others, spreading or radiation of the impulse also occurs. 



In the earthworm not only may several adjustors in the same ganglion 

 be involved in a reflex act, but it is also possible for impulses to pass from 

 one ganglion to another. This transmission of an impulse from one 

 metamere to another is due to adjustors the axons of which are known 

 as association flbers. The association fibers are contained in three tracts 

 known as giant fibers — though they are really bundles of fibers — which 

 lie in the dorsal part of the ventral nerve cord. These fibers put the 

 cell of which they are a part into communication with cells in other ganglia 

 in front of the ganglion in which this cell is located or behind it. In this 

 way very strong stimuli may affect the entire body of the earthworm, 

 causing it to act as a whole. 



280. Anterior Ganglia. — The two anterior pairs of ganglia of an earth- 

 worm are larger than the others, and to the first of these, or the supra- 



