

REFLECTION OF IMPRESSION. 487 



extent and direction of such communications, also, pheno- 

 mena corresponding to those of transference and diffusion 

 to sensitive nerves, are observed in the phenomena of 

 reflection. For, as in transference, the reflection may take 

 place from a certain limited set of sensitive nerves to a 

 corresponding and related set of motor nerves ; as when in 

 consequence of the impression of light on the retina, the 

 iris contracts, but no other muscle moves. Or, as in diffu- 

 sion or radiation, the reflection may bring widely-extended 

 muscles into action : as when an irritation in the larynx 

 brings all the muscles engaged in expiration into coincident 

 movement. 



It will be necessary, hereafter, to consider in detail so 

 many of the instances of the reflecting power of the several 

 nervous centres, that it may be sufficient here to mention 

 only the most general rules of reflex action : 



1 . For the manifestation of every reflex muscular action, 

 three things are necessary; (i), one or more perfect centri- 

 petal nerve-fibres, to convey an impression ; (2), a nervous 

 centre to which this impression may be conveyed, and by 

 which it ma} be reflected; (3), one or more centrifugal 

 nerve-fibres, upon which this impression may be reflected, 

 and by which it may be conducted to the contracting tissue. 

 In the absence of any one of these three conditions, a proper 

 reflex movement could not take place ; and whenever im- 

 pressions made by external stimuli on sensitive nerves give 

 rise to motions, these are never the result of the direct 

 reaction of the sensitive and motor fibres of the nerves on 

 each other ; in all such cases the impression is conveyed by 

 the sensitive fibres to a nervous centre, and is therein com- 

 municated to the motor fibres. 



2. All reflex actions are essentially involuntary, and may 

 be accomplished independently of the will, though most of 

 them admit of being modified, controlled, or prevented by 

 a voluntary effort. 



3. Reflex actions performed in health have, for the most 



