the older surgeons, in order to evacuate the matter and give relief to the patient; or else to continue the application of the caustic for an inordinate space of time, which is a practice altogether unnecessary, extremely cruel, and productive of much irritation, constitutional as well as local. A too common result of abscess, when inertly treated, is the formation of a _Sinus_; that is, a canal, the circumference of which is condensed by deposited lymph, and which furnishes a discharge of unhealthy purulent matter, frequently thin and gleety. Several sinuses frequently unite, and evacuate their contents by one opening. Previously to treating a sinus, its extent must be carefully examined by the probe; this requires considerable caution, for the full extent of the canal may not be discovered, in consequence of its tortuous course, or from its diverging into collateral branches; or the probe, by being used too forcibly, may pass into parts altogether unconnected with the morbid cavity. Thus, in exploring a sinus at the lower part of the leg, or in the foot, the probe may be pushed to a considerable extent beneath the tendinous sheaths of the muscles, and induce the surgeon to adopt treatment unnecessarily severe. In the treatment, we may at first employ setons, injections, and graduated pressure, as formerly explained; and if these fail, the canal must be freely laid open by the knife—a mode of practice much more effectual; then there is formed a cavity similar to that of a recent abscess, and to be treated accordingly. Incision is most frequently necessary when the sinus exists in adipose substance, in tendinous structure, in parts possessed of little vitality, and in patients of a sluggish and enfeebled constitution. In sinus, as well as in chronic abscess, the potass is of essential service; a stick of it may be introduced into the canal, and if the sinus is superficial, the integuments may be divided by this caustic as effectually as by the knife. The indolent and callous surface of the sore is thereby destroyed, and the effects are similar to those which have been already mentioned, when speaking of the use of potass in abscess; in fact, by its application the sinus is transformed into an acute and open abscess. Foreign bodies, such as diseased and dead portions of bones, must be early removed; for it is to be remembered that these are much more frequently the cause than the consequence of suppuration. The healing of an abscess which has been opened closely resembles the process of union by the second intention in a flesh wound; granulations arise, attended by the secretion of pus, the cavity gradually contracts; the surrounding effusion is absorbed along with a portion of the adipose matter; and on the granulations reaching the surface, new skin is formed, and the parts coalesce.