PHYSIOLOGY. 55 



sion, and division of continuity. The last two occur, I believe, 

 as causes of internal pain ; but they are much rarer occur- 

 rences than extension, the chief instance of which is the disten- 

 tion of the arterial system. This may produce pain by stretch- 

 ing or pressing upon nerves placed in the neighbourhood ; but 

 more probably the pain depends upon the distention of the 

 muscular fibres of the arteries themselves. Thus we observe, 

 that membranes, which do not seem sensible to pricking or 

 cutting, are still sensible to this kind of distention ; and in 

 chronic rheumatism, where there is not the same distention of 

 nerves as in inflammation and in acute rheumatism, there 

 is a difficulty in explaining the violent pain, except by a 

 state of the muscular fibres which does not easily admit of 

 extension. 



" We observe different modes of pain, and distinguish be- 

 tween the pressing, tearing, pricking, cutting pain, Sec., which, 

 when external, are manifestly different, according to the mode 

 of impression. Now, the like feelings, from internal causes, are 

 supposed to arise from the same modes of impulse ; but these I 

 find it difficult to suppose ; and I think it much more probable 

 that the chief internal impulse is the distention of the vessels, 

 even by fluids moving in them, or the stretching of the neigh- 

 bouring nerves by this distention. These, by their difference of 

 force, manner of repetition, and extent, may produce sensations 

 resembling those from the different modes of external impulse. 

 For our sensations are very arbitrary : the sense of touch from 

 external impression, indeed, gives us an idea of the mode of im- 

 pulse ; but, in many other sensations, it is agreed that we can 

 perceive no necessary connexion between the mode of impression 

 and the sensation ; and we must, therefore, not expect to be 

 able to explain how far any sensation depends on any particular 

 mode of impulse. Thus, we are at a loss to explain the sensa- 

 tions of acrimony, dryness, Sec. in the fauces producing thirst, 

 and the sensation of hunger in the stomach, either mechanically 

 or chemically. The same applies to many other sensations, and 

 to the causes of pain arising from internal causes. 



" 2. Chemical impressions. In many cases we can perceive 

 no motion of a body producing the pain : we are contented to 

 comprehend such impressions under the name of chemical, be- 



