CHAPTER IV 



THE REFLEX CIRCUITS 



THE cellular unit of the nervous system, as we have seen, is 

 the neuron. Neurons, however, never function independently, 

 but only when joined together in chains whose connections are 

 correlated with the functions which they serve. Accordingly, 

 the most important unit of the nervous system, from the phys- 

 iological standpoint, is not the neuron, but ttae reflex circuit, a 

 chain of neurons consisting of a receptor or sensory organ, a cor- 

 relating center or adjuster, and an effector or organ of response, 

 together with afferent and efferent nerve-fibers which serve as 

 conductors between the center and the receptor and effector 

 respectively (see p. 25). In a reflex circuit the parts must be 

 so connected that upon stimulation of the receptive end-organ a 

 useful or adaptive response follows, such, for instance, as the 

 immediate jerking away of the hand upon accidentally touching 

 a hot stove. 



A reflex act, as this term is usually defined by the physiologists, 

 is an invariable mechanically determined adaptive response to 

 the stimulation of a sense organ, involving the use of a center of 

 correlation and the conductors necessary to connect this center 

 with the appropriate receptor and effector apparatus. The act 

 is not voluntarily performed, though one may become aware of 

 the reaction during or after its performance. 



The term "reflex" is often popularly very loosely applied, but 

 as generally used by physiologists it involves the rather complex 

 nervous function above described. If an electric shock is ap- 

 plied directly to a muscle or to the motor nerve which innervates 

 that muscle, the muscle will contract, but this direct contraction 

 is not a reflex act. Many acquired movements have become so 

 habitual as to be performed quite automatically, such as the 

 play of the fingers of an expert pianist or typist; but these 



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