REFLECTION OF IMPRESSION. 385 



The diffusion or radiation of impressions is shown when an 

 impression received at a nervous centre is diffused to many 

 other fibres in the same centre, and produces sensations extend- 

 ing far beyond or in an indefinite area around the part from 

 which the primary impression was derived. Hence, as in the 

 former cases, result various kinds of what have been denomi- 

 nated sympathetic sensations. Sometimes such sensations are 

 referred to almost every part of the body : as in the shock and 

 tingling of the skin produced by some startling noise. Some- 

 times only the parts immediately surrounding the point first 

 irritated participate in the effects of the irritation ; thus, the 

 aching of a tooth may be accompanied by pain in the adjoin- 

 ing teeth, and in all the surrounding parts of the face; the ex- 

 planation of such a case being, that the irritation conveyed to 

 the brain by the nerve-fibres of the diseased tooth is radiated 

 to the central ends of adjoining fibres, and that the mind per- 

 ceives this secondary impression as if it were derived from the 

 peripheral ends of the fibres. Thus, also, the pain of a calculus 

 in the ureter is diffused far and wide. 



All the preceding examples represent impressions communi- 

 cated from one sensitive fibre to others of the same kind ; or 

 from fibres of special sense to those of common sensation. A 

 similar communication of impressions from sensitive to motor 

 fibres, constitutes reflection of impressions, displays the impor- 

 tant functions common to all nervous centres as reflectors, and 

 produces reflex movements. In the extent and direction of such 

 communications, also, phenomena corresponding to those of 

 transference and diffusion to sensitive nerves, are observed in 

 the phenomena of reflection. For, as in transference, the re- 

 flection 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 move- 

 ment. 



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 : (1), one or more perfect centrip- 

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

 centre to which this impression may be conveyed, and by 



