HE AD ACM. 539 



" Where we cannot observe a distention of vessels, we next 

 suspect a chemical impression of acrimony, and this also un- 

 doubtedly takes place in some cases ; but the evidence is not 

 always clear ; and we have often concluded its existence merely 

 from the presence of pain. Indeed, since the chemical patho- 

 logy was received, this has been the most common supposition, 

 and acrimony has been suspected everywhere, even in the nerv- 

 ous fluid itself. But without discussing that, we can say it is 

 doubtful, so long as other causes can be assigned with equal prob- 

 ability. 



" One such is inflammatory congestion, another is spasm. 

 The latter is a well known affection of muscles producing pain, 

 but we have no clear notion of the nature of the affection. We 

 know in general, that it is an affection of muscular fibres inde- 

 pendent of any change in the motion or quality of the fluids, 

 and hence consider it as a distinct head from the other two ; but 

 we are uncertain what internal parts it affects, and whether it af- 

 fects the blood-vessels ; and, therefore, when pain appears in 

 membranes not provided with other muscular fibres than those 

 of the vessels, it is a question whether the pain is inflammatory, 

 chemical, or spasmodic or if there are still other kinds of pain. 



" We believe that there are, arising particularly from an in- 

 crease of motion in the nerves of sensation only. 



" We think we have an example of this in the pain of the 

 teeth produced by sounds ; in the glottis from stimulus of the 

 ear ; in the glans penis from the neck of the bladder ; in rheuma- 

 tism ; in the tooth ach, and probably in the headach. With re- 

 gard to all these instances there may be a doubt about the man- 

 ner of communication ; but such a doubt does not prevent the 

 conclusion, that in them the pain arises from motions in the 

 nerves of sensation. 



" There may be a question too, why the pain does not ap- 

 pear in the place of the impression, rather than in other parts. 

 Sometimes this may be referred to greater sensibility, as in the 

 case of the glottis, or glans penis ; but sometimes also to an inter- 

 ruption of motion in a nerve, as in the case of rheumatism and 

 of toothach. These last doubts seem to confirm the general 

 conclusion with regard to sensory nerves. 



" Here then is another case of pain, the nervous, different 



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