The operation is very simple; it consists in carefully perforating the cranium, and then, by means of an instrument adapted for that purpose, restoring the piece of bone, which has been depressed, to its natural level. In many instances, the proceeding was very successful; but in many others, the cases terminated unfavourably. From what has been already hinted, it is clear that, in many injuries of the head, this trephining must have been unnecessary; in others, inapplicable; and in both (as adding to the injury), mischievous. Still, surgeons went on as before; so that, in a large class of injuries of the head, there was (if the bone was depressed) an almost uniform recourse to the trephine. Again, in cases where it did not immediately appear that the bone was depressed, too often very unnecessary explorative operations were undertaken to determine that circumstance. In short, there was too much of analogy between the matter-of-course adoption of the trephine in severe injuries of the cranium, and that which we have noticed in regard to bleeding in more ordinary accidents. For correcting the abuse of this very serious operation, we are under great obligations to Abernethy and Desault; and we couple these illustrious names together on this occasion, because, although the amount of our obligation to Abernethy is much the greater, we would not willingly omit the justice due to Desault. Desault may have been said to have given the first blow, which so often determines the ultimate fate of a mischievous conventionalism—that blow which _compels the consideration_ of its claims on our common sense. Desault had become extremely disgusted with the results of the operation of the trephine in his hands at the Hôtel Dieu; and, on consideration, although, as it would seem from Bichat's edition of his works, he did not in theory absolutely ignore the occasional propriety of the operation, he practically for ever abandoned it; thus at once cutting the knot he felt it difficult or impracticable to unravel. As this was many years before his death, the principal argument on which he supported the relinquishment of the operation was simply that his success in the treatment of injuries of the head had been much greater since he had altogether laid it aside. This is eminently characteristic of what people call "a practical man;" but, after all, it is not very sound reasoning. Now, here it was that the discriminative excellence of Abernethy began to tell.