Counterirritation is a valuable means of accomplishing that which is sometimes induced by leeches--namely, attracting blood to the surface for the relief of deep congestion. In fact it comprises more than this, since there is some deep influence exerted through the medium of the nervous system; it not only equalizes the circulation, but tranquillizes a disturbed innervation. The milder and more domestic means include the use of the so-called rubefacients--hot water, mustard, and turpentine. These are of little use in surgical conditions which call for counterirritation; their use should be controlled with caution lest mere counterirritation be converted into actual burning. By the use of _vesicants_ a blister is produced, _i. e._, an effusion of serum and lymph beneath the superficial and outside of the deeper layer of the skin. Mustard and cantharis are the principal vesicants in common use. The former may be used in full strength, in which case it is active, or it may be reduced with wheat flour or linseed meal. To bring out the full strength of mustard, hot water should not be used in its preparation, as it renders it almost valueless. A mustard paste or plaster should be watched at intervals, and it should be removed when the desired effect has been obtained--at all events, when the surface to which it has been applied is covered with vesicles. Cantharis, or Spanish fly, is used either in the form of the cantharidal cerate or mixed with collodion, the latter being the neatest and most pleasant preparation. Several layers are painted on the surface where its effect is desired. This is then protected, and vesication will be found to have been produced within an hour or two, except where the skin is most resistant. The stronger chemicals, like ammonia, chloroform, strong iodine, and nitrate of silver solutions, will be found to be active blistering agents, but should be used with caution. Two other methods of irritation were at one time in favor--namely, the _seton_ and the _issue_. The former consisted of a bundle of threads or a wick, drawn into a large needle with a lancet-shaped point; the skin was picked up into a fold, the needle made to traverse it, and the wick was thus drawn through and cut off, so as to be left in place. The issue was made by drawing a blister with a powerful agent, and then preventing it from healing by the use of an irritating foreign body. These procedures have been abandoned by the medical profession, but are still in vogue among veterinary surgeons.