486 THE XERVOTJS SYSTEM. 



of tickling in the glottis, which best, or almost alone, ex- 

 cites the act of coughing. Or, again, when the sun's light 

 falls strongly on the eye, a tickling may be felt in the nose, 

 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 ap- 

 pear 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 sensa- 

 tions 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 



