COUNTEI{-Ii:rJTANTS — PATHOLOGY OF CIRCULATION. 277 



Action of Counter-irritants. — The application of an irritant, 

 whether mechanical, chemical, or thermal, injures the tissues 

 of the part to which it is applied ; and what better means of 

 removing the injury and restoring health could bo imagined 

 than a copious supply of blood, and the removal of every 

 hindrance to its free flow which contraction of the vessels might 

 present ? 



Experiments are still wanting to decide how far the vascular 

 dilatation will extend in the neighbourhood of the irritated part 

 when more or less powerful irritants are applied, or which the 

 vessels are (if any) that especially contract, when certain others 

 dilate ; so that at present, when we apply a mustard plaster to 

 the chest to relieve bronchitis, we are unable to say with 

 certainty whether tiie relief is due to a more full flow of blood 

 through the vessels of the bronchi, or to contraction of their 

 lumen diminishing congestion, or (though this is unlikely) to 

 some unknown action independent of the vessels altogether. 

 The experiments of Sinitzin, however, (detailed by a recent 

 writer in the British Medical Journal, 1871, p. 535) which 

 show that ulcers of the cornea,, eyelids and lips, occurring after 

 division of the fifth nerve, rapidly heal when dilatation of the 

 vessels of these parts is produced by extirpation of the superior 

 cervical ganglion, render it in the highest degree probable that 

 it is to the increased flow of blood that healiiig is due. As 

 a general rule, too, the vascular dilatation seems to extend more 

 widely the stronger the irritant applied ; and we may thus see 

 liGW a strong irritant, or one applied over a large extent of 

 surface, may prove beneficial in a deep-seated inflammation 

 when a weak one or one applied to a small surface has no 

 e fleet. 



For convenience of reference, I have put together the causes 

 of alteration in the blood-pressure in the following table 

 (p. 278). 



Application to Patlwlogy. — The brief sketch of the circulation 

 which I have given, will enable you to understand and appre- 

 ciate the meaning of the changes produced in our circulation by 

 any drug, and to explain the facts we may meet with in the 

 course of an investigation. I may remind you that the altera- 



