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Show I'ERICBANI UM. renlcnsnmsr. PERICRANIUM. From what has been delivered, we ought to be careful of the pericranium, and not lay the bone naked to a greate r extent than may be necessary for the application of the trephine ; for we should then deprive the bone of its nouris hing vessels. mm. -. manna" .-. if we are operating on a dry or dead bone, the sawings will be dry, and we have to observe if they get moist. As the instrument may be supposed to approach the inner surface, it should be frequently withdrawn, and the point of a tooth-pick should be drawn round the circle in the bone, which sinking through any part that is entire ly cut, gives us warning to incline the handle of the instrument so that When we are operating for large cxtravasations of blood the saw shall press only on the uncut part of the bone. below the skull, it has been observed that the surface of the Through the whole of the operation, the pressure should be light, to cut the bone easily; but now it should be very bone does not bleeda‘s. "VP/WI" 319 It is therefore particularly of impor~ tancc in cases of extravasation, where the dura mater, i. e. slight indeed, for the bone may suddenly give way. the internal periosteum, has been already separated, that we should as little as possible take away the pericraniu m, the external periosteum. The deficiency of blood, when we often happen, however, that we cannot cut the whole circle of the bone through, without injuring the dura mater in scrape otf the membrane, ought to point out to us how we break up the piece before we have perforated the bone are endangering the loss of the entire supply of blood to through the whole circle, when the circul ar piece of bone is the bone. loose. 0n the centre of that spot where we are to apply the he phine, the perforator is placed, and a hole made in which the centre pin of the trephine is to turn, that the circular small levers which I have mentioned may be used. saw may be kept steady. The few first motions of the HO phinc are to be made with a slight hand, and as soon as the circle cut in the bone is deep enough to preser ve the trephine in its place, the centre pin must be taken out. When we have passed the outer table, the saw grates more softly, with less jarring; the sawings are bloody; and the instrument re- quires frequent brushing. Il'e are entering upon the meditullium or diploe, viz. the more vascular centre of the bone. We must again have recourse to our anatomy here : recollect. the thinness of a young skull, where the diploe is not yet form» ed ; how frequently in old people again no distinction of ta‘ bles is to be perceived; and the inequalities of the internal surface of the bone ; in short, we must proce ed as if we had no softer centre to expect. We have especially to recollect, that the inner table is much thinner than the outer one. -‘ See Mr. Abernethy's Essays. some point. It will It will therefore be necessary sometimes to The form of the forceps teaches their use; or the two Hav- ing taken out the circular piece of bone, we introduce the elevator under the depressed bone; but probably we find it jammed and fixed, or, though loose, yet as we raise it on this side, the other side presses down upon the dura mater. If this cannot be prevented by the use of another leverse , the trepan must be again applied. It is impossible to lay down in general rules where we are to plant the trephine, in order successfully to elevate the bone. But I will venture so far against the common rule, as to say, that if we can we ought to place it upon the bone, which we are to take away, if it will bear the necessary pressure without injuring thc dura mater. For if (as I conceive it is) the impulse of the dura mater against the edge of the bone is in proportion to the extent of the bone which is taken away, we ought to be careful that the trephine (as 5" I have used a lever with the point of a hook-like form. It passes under the edge of the firm bone, and catches hold. of the edge of the ‘Dose piece, so as to keep it steady. Vlthl Ml |
