An excellent suggestion is to stain all fistulous tracks with methylene-blue; the blue trail after doing this may be followed, no matter how irregular its course (Fergusson). If the color is mixed with a little hydrogen dioxide, and this forced into a sinus mouth or a fistulous opening, it will carry the dye to all parts of the cavity. This may be used even in dealing with fecal fistulas or those extending deeply into the interior of the body or among the viscera. Fistulas of congenital origin and those which connect two normal cavities of the human body are usually due to a cause which has ceased to act. Consequently we should endeavor solely to atone for the result. The direction and the course of a sinus may be learned by the use of a probe curved to suit and manipulated by a gentle hand, force never being required. Or sometimes, when the silver instrument fails to pass, a flexible bougie or catheter may be introduced. The character of the passage can be judged for the most part by the appearance of the discharges. With sinuses of recent origin leading down to recent suppurative foci it may be sufficient to enlarge the opening and to wash the cavity thoroughly. If a particle of gauze, tube, or sponge has been left therein, its removal is necessary to secure prompt healing. In cases of long standing antiseptic and stimulating substances should be injected or the interior should be cauterized with strong solutions of zinc chloride or silver nitrate, or with these melted upon the end of a probe. The _chronic_ sinus, as well as the chronic rectal fistula, is usually an expression of _local tuberculous disease_. Accordingly these passages may be found lined with the same dense, fungating membrane which lines a cold abscess cavity--the membrane, protective in its purpose, to which I have given the name _pyophylactic_. Whenever such tissue and such membrane are met with they should both be extirpated thoroughly, since in this way only can absolute eradication of the tuberculous infection be relied upon. After such complete excision--which means usually laying open the entire sinus--the parts may be brought together with sutures (this, at least, is usually possible about the rectum) to secure primary union; otherwise, the whole sinus, as well as the cavity to which it has led, must heal by the granulating process, both being kept packed with gauze or some other desirable foreign body acting as an irritant, thereby provoking more rapid formation of granulation tissue. When it is necessary thus to pack a cavity, or when it is desired to keep its upper exit open lest it heal before the lower part, ordinary white beeswax, as suggested by Gunn, makes a serviceable material. This can be molded in hot water to fit the cavity; can be tunnelled or bored for drainage; can be diminished in size as the cavity heals, and is absolutely non-absorbent.