Never spare the knife. Think well before you touch that tool; but, having it in hand, assure yourself its edge is sharp, and never do at two cuts that which might have been accomplished in one.
Always slit up a sinus where such a proceeding is possible. When the sinus is too long, supposing the pipe to take an internal direction, as from the withers to the chest, insert a seton with the guarded seton needle, a representation of which is given below.
THE GUARDED SETON NEEDLE.
The blade of this instrument is generally about two feet long. Before using it, the cutting head is always retracted by pulling back the nut at the extremity, and securing it in its place by means of the screw situated on the middle of the handle. The blade then reposes upon a blunt companion, and may with impunity be inserted down any sinus or false canal. Having reached the bottom of the pipe, and all important vessels being passed, the screw is loosened, and the projecting end of the blade at the extremity of the handle is struck forcibly, when the sharp point is driven forward, and this pierces the flesh.
THE SETON NEEDLE PROTRUDED, AND SECURED WITHIN THE HANDLE BY MEANS OF A SCREW.
Behind the cutting head there is a free space. Through that opening a long piece of tape is threaded, and the instrument is withdrawn, pulling the tape into the sinus, in which it remains. A knot is made at either end of the tape; thus a seton is with safety placed in situations where the depth to be penetrated would defy ordinary measures, and the vessels to be passed would render such measures more than doubly hazardous.
The use of a seton is to act as a drain, or to stimulate an unhealthy canal—to provoke a sinus to secrete healthy pus, instead of a thin and often a foul discharge—and thus to cause the diseased pipe to heal or to become obliterated.
When operating, always make your first incision through the skin rather too large than in the least too small; remember, the division from within outward occasions much less pain than the separation, made after the ordinary fashion, from without inward.