Fistula. A narrow track communicating externally with the skin and internally with a mucous surface is called a fistula. Such a condition of things may arise from error of development, as in the branchial fistula? of the neck, or may result from disease or injury. The most common form of fistula is fistula in ono. It owes its origin to the formation of an ischio-rectal abscess which bursts internally into the lower bowel and externally upon the skin. The unnatural track of communication thus formed is very prone to remain open, giving rise to chronic discharge of offensive matter from the cutaneous opening. Other forms of fistula are lachrymal fistula, where there is a track of communication between the lachrymal sac and the skin of the face, salivary fistula, biliary fistula, and the like. The cure of fistula usually resolves itself into a question of operative treatment.