198 LECTURE VIII. 



ment, alone sufficient to cause the exudation of fibrine, 

 even when no obstruction arises in the circulation. Such 

 obstruction is not therefore in any way needed in order 

 that the production of fibrine may commence at any 

 given point. On the contrary, we see that the cause of 

 the greatest differences in the nature of exudations is to 

 be found in the special constitution of the irritated parts. 

 On the simple application of an irritating substance to 

 the surface of the skin, there arises, when the irritation, 

 whether chemical or mechanical in its nature, is only 

 slight in degree, a vesicle, a serous exudation. If the 

 irritation is more violent, a liquid exudes, which in the 

 vesicle appears quite fluid, but coagulates after its eva- 

 cuation. If the fluid from a blister raised by a cantha- 

 rides-plaster be received into a watch-glass and exposed 

 to the air, a coagulum forms, showing that there is 

 fibrinogenous substance in the fluid. But we sometimes 

 meet with conditions of the body, in which an external 

 stimulus is sufficient for the production of blisters con- 

 taining a fluid which directly coagulates. I had, last 

 winter, a patient in my wards, whose feet had remained 

 ji a state of anaesthesia ever since they had been frozen, 

 and I employed as a remedy, amongst other things, 

 local baths containing aqua regia. After a certain num- 

 ber of these baths, blisters, which varied in diameter up 

 to two inches, and were found, when opened, to be filled 

 with large, jelly-like masses of coagulum, formed upon 

 every occasion on the anaesthetic part of the soles of the 

 feet. In other persons probably ordinary blisters would 

 have formed, containing a fluid, which would not have 

 coagulated until after its evacuation. Such a difference 

 manifestly depends upon a difference, not in the com- 

 position of the blood, but in the disposition of the part 

 affected. The difference between that form of pleurisy, 

 which from its very commencement furnishes coagulable 



