The present proposition, of precise or real proportional measurements between fixed points, was communicated by me, about five or six years ago, to James Macartney Esq. lecturer on comparative anatomy, and the laws of organic existence, at St. Bartholomew’s Hospital, and, in my opinion, one of the best comparative anatomists in Europe. I had no sooner announced it, than he assured me that a similar method had struck himself; and I deem this, so far as authority goes, no mean testimony in behalf of its value. Mr Macartney, some time afterward, performed upon the dead subject, in the dissecting room at St. Bartholomew’s, the difficult operation I have just described, according to the direction I have given, and both politely acknowledged me as the proposer, and bestowed on it his approbation. It is my duty to remark, that there is no person whose approbation could give me higher satisfaction. * * * * * I cannot better illustrate the danger arising from ignorance of this principle, than by quoting an instance of repeated failure in the very operation above described, in consequence of such ignorance. A professor at an hospital in London, generally reckoned one of the ablest surgeons of the metropolis, when illustrating this operation in his lectures, first made the usual guessed cut, passing across one half of the Sterno-cleido-mastoid muscle; and, after some tearing and cutting, and innumerable complaints against the knives and the unfortunate assistant, told his pupils, “that they might think he had not yet got the artery, but he knew he had, and he should know it in the living subject, even if it did not pulsate.” The subject, however, was carried from the lecture-room, and the ligature was found to be firmly fixed upon the nerve. This mistake was very properly communicated to the ingenious surgeon, who never forgave the discoverer of it, but very wisely planned how he might avoid the same mistake, or, what he seemed to dread most, its consequences to his own reputation, in the subsequent course. He had thus found, that ligatures left in the dead subject were more troublesome than in the living; and having tied something, which, to judge from the vague way in which he described it, might be a muscle as readily as an artery, he very properly, as his mind was not quite made up upon the subject, secured, to all appearance, his well earned fame, by cutting off and withdrawing the ligature that nobody might see what he had done. Unfortunately, however, curiosity and the wonder that it was possible for so great a surgeon to make so deplorable and dangerous an error, prompted a second examination of the subject. No ligature now remained to direct the enquirer’s search; but alas! artifice could not obtain impunity even to reputation—the artery remained bedded in fat and untouched, the nerve alone was separated from the mass, and bore the mark of the ligature.