While it is true that a chronic pyogenic abscess is sometimes met with--for example, in the breast and in the marrow of long bones--in the great majority of instances the formation of a chronic or cold abscess is the result of the action of the tubercle bacillus. It is therefore more convenient to study this form of suppuration with tuberculosis (p. 139). SINUS AND FISTULA #Sinus.#--A sinus is a track leading from a focus of suppuration to a cutaneous or mucous surface. It usually represents the path by which the discharge escapes from an abscess cavity that has been prevented from closing completely, either from mechanical causes or from the persistent formation of discharge which must find an exit. A sinus is lined by granulation tissue, and when it is of long standing the opening may be dragged below the level of the surrounding skin by contraction of the scar tissue around it. As a sinus will persist until the obstacle to closure of the original abscess is removed, it is necessary that this should be sought for. It may be a foreign body, such as a piece of dead bone, an infected ligature, or a bullet, acting mechanically or by keeping up discharge, and if the body is removed the sinus usually heals. The presence of a foreign body is often suggested by a mass of redundant granulations at the mouth of the sinus. If a sinus passes through a muscle, the repeated contractions tend to prevent healing until the muscle is kept at rest by a splint, or put out of action by division of its fibres. The sinuses associated with empyema are prevented from healing by the rigidity of the chest wall, and will only close after an operation which admits of the cavity being obliterated. In any case it is necessary to disinfect the track, and, it may be, to remove the unhealthy granulations lining it, by means of the sharp spoon, or to excise it bodily. To encourage healing from the bottom the cavity should be packed with bismuth or iodoform gauze. The healing of long and tortuous sinuses is often hastened by the injection of Beck's bismuth paste (p. 145). If disfigurement is likely to follow from cicatricial contraction--for example, in a sinus over the lower jaw associated with a carious tooth--the sinus should be excised and the raw surfaces approximated with stitches. The _tuberculous sinus_ is described under Tuberculosis. A #fistula# is an abnormal canal passing from a mucous surface to the skin or to another mucous surface. Fistulæ resulting from suppuration usually occur near the natural openings of mucous canals--for example, on the cheek, as a salivary fistula; beside the inner angle of the eye, as a lacrymal fistula; near the ear, as a mastoid fistula; or close to the anus, as a fistula-in-ano. Intestinal fistulæ are sometimes met with in the abdominal wall after strangulated hernia, operations for appendicitis, tuberculous peritonitis, and other conditions. In the perineum, fistulæ frequently complicate stricture of the urethra.