488 THE NERYOUS SYSTEM. 



exciting sneezing. In all these cases, the primary impres- 

 sion may be conducted as well as transferred ; and in all 

 it is transferred to a certain set of nerves which generally 

 appear to be in some purposive relation with the nerves 

 first impressed. 



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 

 extending 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 denominated sympathetic sensations. Some- 

 times such sensations are referred to almost every part 

 of the body : as in the shock and tingling of the skin pro- 

 duced by some startling noise. Sometimes only the parts 

 immediately surrounding the point first irritated partici- 

 pate in the effects of the irritation ; thus, the aching of a 

 tooth may be accompanied by pain in the adjoining teeth, 

 and in all the surrounding parts of the face ; the explana- 

 tion 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 perceives 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 com- 

 municated 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 important functions common to all nervous 

 centres as reflectors, and produces reflex movements. In the 

 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 



