COUNTER-IRRITANTS 665 



Counter-irritants. 



A counter-irritant is an irritant which acts counter, or 

 against an existing irritation, result of irritation, or pain. 

 In applying a "twitch" to a horse, we are ioflicting an irri- 

 tation to relieve some other source of irritation elsewhere. 

 It is taken for granted that the damage and pain caused by 

 the artificial irritant are not so severe as those already ex- 

 isting. The amount of injury produced by an irritant 

 depends upon the nature of the material, its strength, the 

 duration of its action, the mode of application, and the part 

 to which it is applied. We may consider the effects oc- 

 casioned by a mild and increasing action following the 

 continued use of a single agent, or representing the 

 action of materials of different degrees of potency. There 

 first appears redness of the skin, accompanied by some 

 burning or pain (rubefacient action), and if the irritation 

 progresses there is a. serous exudate poured out into the 

 mucous layer of the skin. This leads to swelling and 

 oedema. Greater irritation causes more exudation of serum 

 and an elevation of the epidermis in a circumscribed area or 

 areas, and the formation of blebs or blisters (vesication). 

 If the irritation ceases at this point, the blisters break open 

 and their contents dry on the surface, covering the parts 

 with a thick scab. Synchronous with vesication we observe 

 a similar process attacking the hair follicles. The hairs are 

 loosened and fall, but as the papillae are usually unaffected, 

 the growth of hair is soon renewed. The recovery of hair 

 is facilitated by the application of grease to the part. Cer- 

 tain agents cause circumscribed inflammation of the skin 

 with the formation of pustules (pustulants), as croton oil, 

 and these create necrosis of the hair papillae and, therefore, 

 permanent loss of hair. If the irritant is severe, suppura- 

 tion follows vesication ; or, if an ordinary irritant is ap- 

 plied with violent friction, is covered with a bandage, or 

 placed over an already inflamed part, the same result 



