CHAPTER VII. 

 REFLEX ACTIONS. 



Definition and Historical. By a reflex action we mean the 

 involuntary production of activity in some peripheral tissue in 

 consequence of a stimulation of afferent nerve fibers. The conver- 

 sion of the sensory or afferent impulse into a motor or efferent 

 impulse is effected in the nerve centers, and may be totally uncon- 

 scious as well as involuntary, for instance, the emptying of the 

 gall-bladder during digestion, or it may be accompanied by con- 

 sciousness 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 ani- 

 mals. Their involuntary character was emphasized by the dis- 

 covery 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 or 

 turning-point was given by Whytt (1751). He showed that in a de- 

 capitated 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 spe- 

 cial 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," " Beitrage zur Anatomie u. Physiologic/' Giessen, 1881, vol 

 ix. 



140 



