CHAPTER II. 

 REFLEX ACTION. 



"Pure reflexes are admirably adapted to certain ends. They are 

 reactions which have long proved advantageous in the phylum of 

 which the existent individual is a representative embodiment. 

 Perfected during the course of ages, they have during that course 

 attained a stability, a certainty, and an ease of performance beside 

 which the stability and facility of the most ingrained habit acquired 

 during an individual life is presumably small." SHERRINGTON, 

 The Integrative Action of the Nervous System. 



The term reflex action is used in a wide and somewhat 

 indefinite sense, but it is applied in general to responses of the 

 organism which are evoked directly upon the application of 

 a stimulus. In a typical reflex we have a sensory or afferent 

 impulse set up in some sense organ and proceeding toward 

 a ganglion or larger nerve center. From the latter an efferent 

 impulse passes outward along a motor nerve, causing a 

 muscular contraction. Frequently the impulse is reflected 

 back near its point of origin, more or less after the fashion 

 in which light may be reflected back to its source from a 

 mirror and the reaction occurs in the stimulated part. It is 

 from analogy with the reflection of light that the term re- 

 flex action is chosen, although the analogy, it must be con- 

 fessed, is rather loose, and in some respects misleading. 

 When one involuntarily winks the eyes after they are 

 suddenly illuminated by a glare of light, impulses are set 

 up in the retina which proceed along the optic nerve to the 

 brain. Here they traverse certain established connections 

 among the fibers, by means of which they become directed 

 outward along the motor nerves to the muscles of the eye- 



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