Fistula (Lat., a pipe), an ulcer in the form of a narrow canal, more or less deep and sinuous, lined by a pale false mucous membrane, indolent and indisposed to heal, kept up by some local pathological condition of the soft parts or bones, or by the presence of some foreign irritating body, and leading or not to a suppurating cavity. There may be a single external or internal opening, or there may be a communication between the skin and the mucous, serous, or synovial cavity. Some writers restrict the term fistula to such of the above lesions as take their origin from some natural cavity or excretory duct, while those communicating with abscesses and caused by foreign bodies or disease of the bones are called fistulous ulcers or sinuses; but the distinction is of little importance, as the pathological conditions and the principles of treatment are the same. Fistulas arise when abscesses are not thoroughly healed from the bottom, when any irritating substance (as a ligature or a piece of dead bone) remains in the tissues, or after wounds of excretory ducts. If superficial and of recent origin, fistulas may heal of themselves; but if deep-seated or chronic, they generally require surgical interference.

They are usually rather tedious and annoying than dangerous; but when large, deep, with several openings and profuse discharge, they may produce hectic fever and fatal exhaustion. The principles of treatment are: to remove any irritating cause, as a piece of dead bone or foreign body; to prevent the accumulation of matter, by counter openings, if necessary, and by properly directed compression; and to excite adhesive inflammation by pressure, stimulating injections and applications, setons, caustic, and, as a last resort, incision of the fistula, that the soft parts may have an opportunity of healing from the very bottom of the wound; the constitution should also be strengthened by nourishing diet and tonic medicines. The most common varieties are the anal, lachrymal, salivary, and urinary fistulas.-Anal fistula is situated by the side of the sphincter ani muscle, and is difficult to heal both on account of the constant muscular contractions and the passage of faecal matter into it. There may be an opening into the bowel internally and externally, either or both; according to Brodie, this affection always begins by an ulceration on the side of the rectum into which the faecal matter escapes, causing abscess and consequent fistula; but in some cases there is no opening into the bowel, the sinus reaching only to its outer coat; this affection is frequently a painful complication of consumption.

The simple and efficient remedy for this fistula is division of the walls from the internal opening to the skin, so as to prevent muscular contractions; after this operation the introduction of lint allows the wound to heal by granulation from the bottom. This affection is considerably more common in males than in females.-Lachrymal fistula is situated at the inner corner of the eye, and communicates with the lachrymal sac; it begins by an obstruction of the nasal duct, followed by inflammation, abscess, and fistulous opening. Besides the usual remedies for acute and chronic inflammation, the obstructed duct may be restored by the introduction of a metallic or elastic style. In a similar manner the duct of Steno may be obstructed, so that the saliva dribbles out on the cheek instead of passing into the mouth; the remedy is to establish the passage from the fistula to the mouth by puncture and the introduction of silk or flexible wire, and then paring and uniting the edges of the external opening.-In urinary fistula there is an opening from the perineum into the urethra, through which the urine dribbles wholly or in part; it is generally caused by urinary abscess and extravasation into the soft parts.

For its relief all strictures should be dilated, the urethra brought to a healthy condition, and the fistula stimulated to contract and granulate by external applications. Sometimes there is a communication between the urethra and the rectum. But the most disgusting and difficult to remedy are the vesico-vaginal and recto-vaginal fistulas, in the former of which the bladder, and in the latter the rectum communicates with the vagina; both of these affections are the consequences of the laceration and sloughing after tedious labor; the most successful method of treatment is by paring the edges of the fistula and uniting them by sutures.-A fistula may communicate with any of the abdominal viscera, or with any part of the body, on the surface or deep-seated, which may be diseased from abscess, dead bone, or the presence of a foreign substance.