The real nature of this case could not possibly have been ascertained, but by an anatomist. The same surgeon has recorded the case of a man who, having fallen twice from his horse, and experienced for several years considerable uneasiness in his back, was at length afflicted with acute pain in the abdomen. At the same time an oval, irregularly circumscribed tumour made its appearance in the right flank. It presented a distinct fluctuation, and had all the appearance of a collection of matter depending on caries of the vertebræ. The pain was seated chiefly at the lower portion of that part of the spine which forms the back, which was, moreover, distorted; and this might have confirmed the opinion that the case was a lumbar abscess with caries. Pelletan, however, who well knew that an aneurism, as it enlarges, may destroy any bone in its neighbourhood, saw that the disease was an aneurism, and predicted that the patient must perish. On opening the body (for the man lived only ten days after Pelletan first saw him) an aneurismal tumour was discovered, which nearly filled the cavity of the abdomen. If this case had been mistaken for lumbar abscess, and the tumour had been opened with a view of affording an exit to the matter, the man would have died in a few seconds. There is no surgeon of discernment and experience whose attention has not been awakened, and whose sagacity has not been put to the test, by the occurrence of similar cases in his own practice. The consequence of error is almost always instantaneously fatal. The catalogue of such disastrous events is long and melancholy. Richerand has recorded, that Ferrand, head surgeon of the Hotel-Dieu, mistook an aneurism in the armpit for an abscess; plunged his knife into the swelling, and killed the patient. De Haen speaks of a person who died in consequence of an opening which was made, contrary to the advice of Boerhaave in a similar tumour at the knee. Vesalius was consulted about a tumour in the back, which he pronounced to be an aneurism; but an ignorant practitioner having made an opening into it, the patient instantly bled to death. Nothing can be more easy than to confound an aneurism of the artery of the neck with a swelling of the glands in its neighbourhood: with a swelling of the cellular substance which surrounds the artery; with abscesses of various kinds; but if a surgeon were to fall into this error, and to open a carotid aneurism, his patient would certainly be dead in the space of a few moments. It must be evident, then, that a thorough knowledge of anatomy is not only indispensable to the proper treatment of cases of this description, but also to the prevention of the most fatal mistakes.