SPECIFIC NERVE INFLUENCE. 123 



sorium by a specific sensation in them, but the fact of common 

 or electrical irritations of the nerve trunks causing the sen- 

 sation usually dependent on the natural peripheral irritation is 

 against that. The optic nerves under the action of common 

 irritants cause the sensation of light, but neither they nor the 

 fibres of the retina itself are susceptible to the stimulus of 

 light, except when they are connected with the rods and cones. 

 The specificity of the nerve influence does not thus lie in any 

 specific kind of excitement of the mere nerve cords, but must 

 be referred to the central organs to which it is conducted. 

 The mental organs, which are excited by the stimulus conveyed 

 through the nerves of special sense, are so excited because, from 

 the special constitution of their protoplasm, they are only com- 

 petent to produce that determinate sensation. "The same 

 stimulus when.it affects different mental organs, will be inter- 

 preted according to their several specific energies" (Ranke, 

 p. 693). If, however, we are wholly to deny the transmission 

 of specific irritation by the nerves several pathological and thera- 

 peutical theories must be much modified, and the specific effects 

 of poisons must be referred solely to absorption, except in so 

 far as a mere plus and minus of action is spoken of in organs 

 connected by nervous sympathy. But we must not forget 

 that, even if the vis nervosa is a single physical force, it may 

 be "capable of an almost infinite number of variations or 

 gradations/ 7 like the shades of colour or tones of sound ; and 

 without such differences how can we explain the differences of 

 impression of sweet and bitter conveyed by the same nerve, 

 for example, or any specific impression at all conveyed by 

 them 1 so that Fletcher's theory of the conveyance of specific 

 irritations by his respiratory system of nerves of organic 

 sympathy (corresponding to the central vaso-motor system now 

 spoken of) may still be tenable. He considered that the 

 stimulus of poisons, besides being diffused by absorption into 

 the blood, might be conveyed rapidly by means of the common 

 centre in the spinal cord through all the nerves of organic 

 sympathy, and only take effect on parts which were susceptible 

 of its influence. 



