on the Agency of Galvanism. 83 



of caloric*, and are independent of the sensorial power, are 

 permanent f. 



The greatest difficulty, perhaps, to be encountered in defend- 

 ing the identity of the nervous influence and galvanism, arises 

 from the former having been very generally regarded as part of 

 the sensorial power. If the nervous power is once admitted to 

 be merely a chemical agent, the question assumes a very differ- 

 ent character. It has not been unusual to regard the sensorium 

 as pervading, by means of the nerves, the whole system, and to 

 suppose that we actually feel in the parts to which we refer our 

 sensations. But a moment's reflection shews the inaccuracy of 

 such a position. It is evident, that either this must be the case, 

 or that there must be some centrical part to which the power of 

 sensation is confined, and to which all impressions causing 

 it are conveyed, a knowledge of the relative position of the 

 part impressed being the result of experience. By disproving 

 either of these positions, therefore, the other is established. 

 A'^arious ways present themselves of disproving the first. I shall 

 confine myself to one, because it appears to be conclusive. 

 When we complain of the toes of a limb which has been ampu- 

 tated, the cause of sensation cannot be in the toes. Here we 

 see irritation of the stump of the nerve producing the same 

 effect on the sensorium, which used to arise from irritation cf 

 its extremities. Many similar instances might be mentioned. 

 We cannot shew why the immediate cause of sensation should 

 so exist in these cases, and not in all. The sentient organ, there- 

 fore, is confined to a centre, to which all impressions, causing 

 sensation are conveyed. It is thus that infants cannot distin- 

 guish the parts of the body impressed. This power is derived 

 from experience, but, as the experience from which we derive it 

 commences witii our existence, and it is of no importance to us 

 by what means we acquire it, it is a necessary consequence that 

 we Bhould wholly overlook the means of its acquisition ; as, for 

 example, we judge of distance by the eye, without thinking of the 

 means by wiiici) we have been enabled to do so, and at first c\\n 

 hardly believe that they are such, as on inquiry we find them to be. 



* See Part 11. Chapter .9, on tlu' ii^o ol llie Ganglions, 

 t lb. 



G 2 



