A quick and clear perception, for the most part untrammelled by preconceived opinions, led Desault to a vivid appreciation of the immediate results of surgical proceedings; and as these were definite, successful, doubtful, or abortive, he either persevered with a characteristic tenacity of purpose, or at once and for ever abandoned them. He was remarkably happy in his selection and appreciation of the mechanical parts of surgery; and his quick perception disclosed to him several useful points in practice which depend on the more important truths of medical surgery. Now _almost_ all this, as applied to the active portion of Abernethy's life, is equally true of both. But Desault was by no means so deep or so original a thinker as Abernethy. Like Abernethy, he was clear and penetrative; but he did not see nearly so far, nor were his views nearly as comprehensive. Desault was quick at detecting an error in practice, and in sensibly rejecting it. Abernethy would unfold it, examine it, and, by his talents, convert the very defect into usefulness. Desault had by no means, in the same degree, that power of reflection, that suggestive faculty, which, in endeavouring to interpret the meaning of phenomena, can point out the true question which it is desired to ask of nature, as well as the mode of inquiry. All this, and much more, was strikingly developed in Abernethy. The paper before us involves a subject which had engaged the attention both of Abernethy and Desault. They had met with the same difficulty; and the practical solution of it which each obtained, though somewhat different, was extremely characteristic. We will try to make this intelligible. In severe injuries in which the cranium is broken, it frequently happens that a portion of bone is so displaced that it presses on the brain. The consequence of this, in _many cases_, is a train of symptoms sufficiently alarming in themselves, but the actual cause of which many circumstances sometimes concur to complicate or obscure. The same forces which produce the accident not unfrequently involve a violent shock to the whole body. Sometimes fracture or other injury of other parts. Sometimes the patient is deeply intoxicated. Then, again, patients are presented to the surgeon, in different cases, at extremely different intervals after the reception of the injury; so that a case may wear a very different aspect according to period or the phase at which it is first brought under his observation. These and many other circumstances give rise to various modifications of the symptoms, and, _under some complications_, constitute a class of cases which yield to none in importance or difficulty. There is something in the idea of a piece of bone pressing on the brain, which instinctively suggests the expediency of raising it to the natural level. This is, in fact, the object of what is called "trepanning;" or, as we generally term it, "trephining."