As a result of inflammation extending from without inward, a gradual narrowing of the milk duct may occur from thickening and narrowing of its lining membrane. This may be limited to a small area near the lower end, or it may extend through the whole length of the teat. The stream of milk becomes finer and finer until it finally ceases altogether, and a firm cord is felt running through the teat. If the constriction is only at the outlet, the teat may be seized and distended by pressing the milk down into it from above, and an incision may be made with a sharp penknife in two directions at right angles to each other and directly in the original opening. The knife should be first cleansed in boiling water. The opening may be kept from closing by a dumb-bell shaped bougie of gutta-percha (Pl. XXIV, fig. 5) or by the spring dilator. If the obstruction is more extended it may be perforated by L¸thi's perforating sound. (Pl. XXIV, fig. 1A and 1B.) This is a steel wire with a ring at one end, and at the other is screwed on to the wire a conical cap with sharp cutting edges at the base, which scrapes away the thickened masses of cells as it is drawn back. This may be passed again and again to enlarge the passages sufficiently, and then the passage may be kept open by wearing a long, dumb-bell bougie, a thick piece of carbolized catgut, or a spring dilator. If the passage can not be sufficiently opened with the sound it may be incised by the hidden bistoury. (Pl. XXIV, fig. 2.) This is a knife lying alongside a flattened protector with smooth, rounded edges, but which can be projected to any required distance by a lever on the handle. The incisions are made in four directions, as deep as may be necessary, and the walls then can be held apart by the spring dilator until they heal. In case the constriction and thickening of the canal extend the whole length of the teat, it is practically beyond remedy, as the gland is usually involved so as to render it useless. CLOSURE OF THE MILK DUCT BY A MEMBRANE. In this form the duct of the teat is closed by the constriction of its lining membrane at one point, usually without thickening. The closure usually takes place while the cow is dry; otherwise its progress is gradual, and for a time the milk may still be pressed through slowly. In such case, if left at rest, the lower part of the teat fills up and the milk flows in a full stream at the first pressure, but after this it will not fill up again without sufficient time for it to filter through. This is to be cut open by the hidden bistoury (Pl. XXIV, fig. 2), which may be first passed through the opening of the membrane, if such exists. If not it may be bored through, or it may be pressed up against the membrane at one side of the teat and opened toward the center, so as to cut its way through. Incisions should be made in at least two opposite directions, and the edges then may be held apart by wearing the spring dilator until healing has been completed.