56 PHYSIOLOGY. 



cause they are connected with qualities which we distinguish as 

 such. A tendency to a solution of continuity might be sup- 

 posed to be the cause of such pain, as we perceive that the sub- 

 stances which most powerfully produce it, such as caustic alka- 

 lies, concentrated acids, and a few others, have the power of 

 dissolving and destroying the texture of our solids ; but I am 

 very doubtful how far this will extend ; for there are certainly 

 substances which may excite motions in our sentient parts suffi- 

 cient to give pain, but without any tendency to destroy the solid 

 texture : thus, a very concentrated solution of common salt gives 

 considerable pain, but I think no chemist has discovered that it 

 has a tendency to destroy the solid substance of the body. I 

 would say, therefore, that even caustics are painful, not because 

 they dissolve the part, but because they excite those motions 

 in the sentient extremities upon which pain depends. Heat, 

 also, is painful or pleasing, according to the preceding temper- 

 ature of the body, and in a degree which does not, in any mea- 

 sure, approach to a solution of continuity ; and the pain from 

 cold certainly cannot be so explained, as this has the contrary 

 effect of condensing the substances to which it is applied. 



" There is a cause of pain, which, though it probably may be 

 referred to a mechanical impression, deserves a particular consi- 

 deration, viz. the pain arising from Spasm, or from a contraction, 

 considerable in degree and in force, of the muscular fibre. 

 Wherein this contraction, difficultly admitting of relaxation, con- 

 sists, we cannot pretend to explain ; but I would observe that it 

 is a direct cause of pain in itself, even in a less degree than 

 that which amounts to spasm, in as far as it takes off the flexi- 

 bility of the muscular fibres, and puts them into the state in 

 which they are in chronic rheumatism, and thereby renders them 

 more sensible to the slighter degrees of distention, and hence 

 more liable to the sources of pain. Both the strictly spasmodic 

 state therefore, and the rheumatic contraction, may be combined 

 with causes of distention, and modify the pain arising from them. 

 This is evident in the case of colic, nephritic, and biliary pains. 

 In colic the pain arises from spasmodic contractions ; but these 

 have the .effect of including a certain quantity of air, in conse- 

 quence of which the distention becomes considerable, and the 

 constriction from spasm becomes more painful. In nephritic 



