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INTRODUCTION TO NEUROLOGY 



dilemma. Either no reaction at all results, or there is a sort of 

 physiological resolution (sometimes called physiological choice), 

 one motor pathway being taken to the exclusion of the other. 

 Which path will be chosen in a given case may be determined by 



Fig. 18. Diagrams representing the relations of neurons in five types of 

 reflex arcs: A, Simple reflex arc; B, chain reflex; C, a complex system illus- 

 trating allied and antagonistic reflexes and physiological resolution; D, a 

 complex system illustrating allied and antagonistic reflexes with a final 

 common path; E, a complex system illustrating the mechanism of physio- 

 logical association. A, A, association neurons; C, C', C", Cl, and C2, centers 

 (adjusters); E, E', E", El, and E2, effectors; FCP, final common path; 

 R, R', R", Rl, and R2, receptors. 



the physiological state of the organs. If, for instance, one motor 

 system, E2, is greatly fatigued and the other rested, the thresh- 

 old of E2 will be raised and the motor discharge will pass to El . 

 Figure 18, D illustrates the converse case, where two receptors 



