576 



SIGNIFICANCE OF THE NERVOUS SYSTEM 



tion of the will. This definition implies that the impulse generated 

 in the sense-organ, must be conveyed to a center before it can be 

 transferred to the corresponding motor end-organ (Fig. 285). In 

 its simplest form, therefore, the nervous circuit which is necessary 

 for the mediation of a reflex, must consist of two neurons, one of which 

 serves the purpose of conveying the impulse from the sense-organ {R) 

 to the center, and the other, from the center (C) to the motor organ 

 (Er). The first neuron forms the sensory path {A) and the second, 

 the motor path (E) of this reflex arc or circuit. The terms afferent and 

 centripetal are frequently applied to the ingoing path and the terms 

 efferent and centrifugal to the outgoing path. Furthermore, the sen- 



FiG. 285. Fig. 286. 



Fig. 285. — Diagram Illustrating the Construction of the Reflex Circuit, 

 R, Receptor; A, sensory path; C, center; E, motor path; Er, effector. 



Fig. 286. — Diagram Illustrating the Construction of the Reaction Circuit 

 (Volitional Response). 



R, Receptor; A, primary sensory path; C, reflex center; A', secondary sensory 

 path; V, higher center; E', primary motor path; E, secondary motor path, Er, effector. 



sory side of the reflex arc is often designated as the analyzer, while the 

 sensory end-organ is called the receptor and the motor end-organ the 

 effector. Stated in detail, therefore, a reflex circuit is composed of a 

 receptor, a sensory path, a center, a motor path and an effector. 



The circuit required for a volitional reaction, differs from the reflex 

 circuit only in the number of neurons which are necessary to convey 

 the impulse into the cerebrum where the psychic faculties (volition) 

 are situated. The impulse is first conveyed from the receptor to the 

 lower (reflex) center and from here by a secondary afferent neuron 

 to the higher center involving volition. Upon its being transferred 

 to the efferent side of the reaction arc, the impulse first attains the 

 lower center and later on the effector. 



The Rudimentary Nervous System is a Reflex System. — The neuron, 

 as has been emphasized above, is the structural unit of the nervous 



