REFLECTION OF IMPRESSIONS. 489 



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 may 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 when- 

 ever impressions 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 communicated 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. Eeflex actions performed in health have, for the most 

 part, a distinct purpose, and are adapted to secure some 

 end desirable for the well-being of the body; but, in 

 disease, many of them are irregular and purposeless. As 

 an illustration of the first point, may be mentioned 

 movements of the digestive canal, the respiratory move- 



