Great care must be taken in every case to saw slowly, to remove the sawdust, and examine the track of the saw by a probe or quill, lest one part should be cut through quicker than another. The last turns of the instrument must specially be cautious ones. When the disk of bone does not at once come away in the trephine, the elevator or the special forceps for the purpose will easily remove it. If the abscess, extravasation, or exostosis be then discovered and removed, all that remains is to remove any sawdust or loose pieces of bone, and possibly to smooth off any sharp edges of the orifice by an instrument called the lenticular. This is very seldom required, and now hardly ever used. 2. _In cases of depressed or punctured fracture_ the trephine is occasionally required (when symptoms of compression are present) for the purpose of enabling the depressed portion to be elevated. It is unsafe to apply it to the depressed or fractured bone, lest the additional pressure of the instrument should cause wound of the dura mater or brain. It is generally applied on some projecting corner of sound bone under which the depressed portion is locked, and hence it is rarely necessary to remove a complete circular portion. In fact very many cases of such displacement may be remedied more easily by a pair of strong bone-forceps, or a Hey's saw, applied to remove the projecting portion of sound bone. The same precautions must be used as in the operation already described, and the sawing must be done even more cautiously, as it is rarely more than a semicircle that requires cutting. In former days trephining was a much more frequent operation than it is now, and apparently more successful. The reason of the greater apparent success can easily be found in the fact that it was performed in many cases merely as a precautionary measure against dreaded inflammation of the brain, which probably never would have appeared at all, and that the operation itself is one by no means dangerous. Very numerous applications of the trephine have been made in the same individual--two, four, six, and even in one case twenty-seven disks having been removed from the same skull, and yet the patients have survived. TUMOURS OF THE SCALP, _Removal of_.--By far the most frequent are the encysted tumours, or wens. These consist of a thick firm cyst-wall, which contains soft, curdy, or pultaceous matter, sometimes almost fluid, at others dry and gritty. They are loosely attached in the subcutaneous cellular tissue, and unless they have become very large, or have been much pressed on, are non-adherent to the skin. The treatment is thus very simple. They should merely be transfixed by a sharp knife, the contents evacuated, and the cyst seized by strong dissecting forceps and twisted out. If they have once become adherent, they must be dissected out in the usual manner, after the adherent portion of skin has been defined by elliptical incisions.