These are narrow, more or less elongated wounds, opening on to the surface by a small orifice.

Sinuses usually communicate with an abscess, and are the channels by which pus makes its escape.

Where injury occurs to muscles, bones, or other structures, resulting in suppuration, the prospect of healing after the matter has been evacuated will greatly depend upon the extent to which motion can be restricted. Where the part is in- continued action, as occurs in the region of the poll, the pus has a tendency to burrow among the muscles, and to keep up the irritation and prevent healing; it is on this account that what is known as "poll evil" proves so abiding and intractable an ailment.

Sinus invariably follows the long confinement of matter when formed in the feet as the result of a prick; or it may arise from the lodgment of some foreign substance, as a splinter of wood, in the flesh; or from the too early closing of a wound before the deeper parts have healed; or from the presence of a piece of dead bone in the process of sloughing, or; which has sloughed as the consequence of injury or disease.

The treatment of a sinus will, of course, depend upon the cause by which it was produced and is made to continue.

Where movement is the offending factor, means must be adopted to stop the action of the muscles as far as possible, and bring the part into a state of rest.

Confined pus must be evacuated either by laying the canal freely open along its entire length, or by making a counter-opening at the most depending part, and allowing free drainage. All partitions in the cavity to which the sinus may lead should be divided, and pockets made to communicate, to facilitate the escape of pus. Where foreign bodies exist, they must be removed before reparation can be hoped for.

The sinus, whether laid open or under-drained, should be dressed once or twice, at an interval of two days, with undiluted carbolic acid or a solution of chloride of zinc of the strength of 1 in 40, to be followed by the daily application of iodoform and a covering of gauze and antiseptic wool. Some practitioners plug old, obstinate sinuses with cotton-wool dressed with perchloride of mercury and arsenic, and allow the dressing to remain until the sinus is healed.

Sinuses of recent formation may be found to yield to strong boracic ointment and boracic lint as a dressing.

Fistula is an unnatural canal or wound with an opening at one extremity on the skin, and at the other into one or another of the various cavities or conduits of the body; or it may extend between two cavities, as between the bladder and the vagina - vesico-vaginal fistula.

Fistula of the Parotid Duct.

Fig. 424. - Fistula of the Parotid Duct.

Thus a wound in the wall of the abdomen may communicate with some part of the intestine, or one in the cheek may have connection with the parotid duct (fig. 424), through which faeculent matter in the one case and saliva in the other may escape. It is the constant passage of these solids and fluids through the canal, and the impossibility of rendering them aseptic, which makes fistulous wounds so troublesome to heal.

Treatment in these cases must be directed towards arresting the outflow of the discharge and directing it into and along its proper channel. Sealing over the outer orifice with collodion is in some cases sufficient to bring about healing in fistula of parotid duct. In others the surface of the fistula must be brought into a healthy condition by antiseptic treatment to induce healing, or, if possible, laid freely open and scraped, and treated as an ordinary wound. In some chronic cases the orifice has been caused to close by touching it lightly with the actual cautery.