54 PHYSIOLOGY. 



pend upon a peculiar modification of the ordinary fermenta- 

 tion ; but we observe the same singular effects of qualities, 

 where there is not the least room to think of any fermentation 

 being concerned. Another question upon the subject is this : 

 Supposing that the substance acts upon the nerves, and that 

 the effects arise from the singular constitution of these, 

 do they depend upon the constitution of the extremities, or 

 upon some circumstance in the common sensorium ? And a 

 third question would be, in what the peculiar condition of 

 the extremities or of the sensorium consists ? with regard to 

 which last, I can only say that we know nothing of the matter. 

 I have said that we know of no necessary connexion between im- 

 pression and sensation, and that, for aught we know, the sub- 

 stances that act upon our smell might have been made to give 

 the idea of colour ; and it will be difficult to say when the con- 

 dition is in the extremities or in the sensorium ; we can only 

 say that one is probable in one case and another in another 

 case.' 1 



" Among the symptoms referred to the functions of sense, we 

 mention here the more general sensations of pain, pruritus, rest- 

 lessness, and uneasiness : 



" PAIN. In XLII. I stated that those strong impressions 

 which do not destroy the organ, give the general sensation of 

 pain rather than the sensation of any particular object ; and in 

 XLII I. I explained that sensations are strong or weak in pro- 

 portion to the force of impressions and the degree of sensibility 

 taken together, and that the sensibility is not to be measured by 

 any standard, but is relative to the preceding state of the nerve. 

 Now, when pain arises in the internal parts of the body, where 

 the body giving impression is not evident, we must inquire in- 

 to the nature of these internal impressions : it would seem that 

 we can judge of them only by analogy drawn from external im- 

 pressions ; these are reduced to two heads the mechanical and 

 the chemical, to which, accordingly, we shall, in the first place, 

 refer all painful sensations. 



" 1. The mechanical impressions are very various, but may 

 all be comprised under the three heads of extension, comprcs- 



