CHAPTER VII. 

 REFLEX ACTIONS, 



Definition and Historical. By a reflex action we mean the 

 involuntary production of activity in some peripheral tissue through 

 the efferent nerve fibers connected with it in consequence of a 

 stimulation of afferent nerve fibers. The conversion of the sensory 

 or afferent impulse into a motor or efferent impulse is effected in 

 the nerve centers, and may be totally unconscious as well as invol- 

 untary, for instance, the emptying of the gall-bladder during 

 digestion, or it may be accompanied by consciousness of the act, 

 as, for example, in the winking reflex when the eye is touched. 

 The application of the term reflex to such acts seems to have been 

 made first by Descartes* (1649), on the analogy of the reflection 

 of light, the sensory effect in these cases being reflected back, so 

 to speak, as a motor effect. The attention of the early physiologists 

 was directed to these involuntary movements and many instances 

 were collected, both in man and the lower animals. Their invol- 

 untary character was emphasized by the discovery that similar 

 movements are given by decapitated animals, frogs, eels, etc. 



Some of the earlier physiologists thought that the reflex might 

 occur in the anastomoses of the nerve trunks, but a convincing 

 proof that the central nervous system is the place of reflection 

 was given by Whytt (1751). He showed that in a decapitated frog 

 the reflex movements are abolished if the spinal cord is destroyed. 

 Modern interest in the subject was excited by the numerous works 

 of Marshall Hall (1832-57), who contributed a number of new 

 facts with regard to such acts, and formulated a view, not now 

 accepted, that these reflexes are mediated by a special set of fibers 

 the excitomotor fibers. 



In describing reflexes the older physiologists had in mind only 

 reflex movements, but at the present time we recognize that the 

 reflex act may affect not only the muscles, voluntary, involuntary, 

 and cardiac, but also the glands. We have to deal with reflex 

 secretions as well as reflex movements. 



The Reflex Arc. It is implied in the definition of a reflex 

 that both sensory and motor paths are concerned in the act. Ac- 



* See Eckhard, " Geschichte der Entwickelung der Lehre von den Reflex- 

 erscheinungen," "Beitriige zur Anatomic u. Physiologic," Giessen, 1881, vol. 

 ix. 



134 



