These are terms applied to more or less _tubular channels abnormally connecting various parts of the body, or connecting some cavity with the surface of the body in a way anatomically quite abnormal_. Or they may be regarded as _tubular ulcers_, or ulcerated tunnels, connecting as above. A more exact distinction between the two terms would imply that a _sinus_ connects the surface with some deeper portion where a cavity is not normally present--_i. e._, with a focus of disease--whereas a _fistula_ properly refers to a tubular passage connecting natural or preëxisting cavities in an abnormal manner. Thus we speak of buccal, rectal, vesicovaginal fistulas, etc., whereas a passage leading down to an old abscess or to a focus of disease in bone, for instance, is properly referred to as a sinus. It is possible for the margins of a fistula to become more or less cicatrized and cease to be ulcerous, whereas the entire track of a sinus is practically a continuous ulcer, only tubular in arrangement. =Causes.= A. =Congenital.=--There are numerous points about the body where, as the result of arrest of development or failure to grow, fistulous passages which are comprised within the normal fetal arrangements, but which should close later, either before or at birth, fail to do so. Thus we have congenital fistulas of the neck, persistent urachus, persistent omphalomesenteric duct, etc. These are in no sense primarily connected with diseased conditions, but may become so secondarily. B. =Pre-existing Abscess with Unhealed Channel of Escape=--_e. g._, rectal, fecal, and other fistulas and sinuses which connect with tuberculous foci in any part of the body. C. =Previous Traumatic or other Destruction of Normal Tissues=--_e. g._, vesicovaginal fistulas due to tissue death from pressure, buccal fistulas from gangrene of the cheek, as in noma. D. =Foreign Bodies=--bullets, ligatures, etc.--which prove irritating or infectious enough to prevent absolute healing. More or less tortuous sinuses will generally be found leading down to the irritating material. E. =The Presence of Necrosed or Necrotic Material=--_e. g._, a sequestrum in bone, which is usually evidenced by the presence of one or more sinuses. =Treatment.=--If the determining cause is still acting, the treatment is to remove the cause. Consequently, when the sinus leads down to diseased bone or other dead or dying tissue, the complete evacuation of the cavity is necessary before the sinus may heal. If the cause is a foreign body, its removal should be at once insisted upon.