Though the morbid deposit is at first confined, as above related, its limits are afterwards extended; calcareous matter is insinuated, either in minute particles or in broad laminæ, amongst the fibres of the middle coat, is also found external to it, and occasionally situated in the cellular coat. In fine, the various component parts of the parietes of the vessel degenerate, according to the degree of advancement which the disease has attained; and such a condition is the predisposing cause to ulceration of the internal coat, and subsequent effusion of blood. The steatomatous, ulcerated, earthy degeneration of the proper coats of an artery, as Scarpa, the celebrated professor of Pavia, has it, are well exhibited in the accompanying sketch. During violent and sudden exertions the more brittle parts may burst, either at a certain point, or throughout the whole circumference of the artery; and on this such results will supervene as on ulceration of the internal tunic. Ecchymosis then takes place under the cellular coat, which becomes thickened, and incorporated with, and strengthened by, the surrounding tissues; this is the incipient state of an aneurismal tumour. The effusion of blood, gradually increasing, distends the cellular coat, forming the cavity into which it is poured, and produces a tumour of a size proportional to the distensibility of the tunic and the force of the effusion. Sometimes the external coat is separated from the others to a considerable extent by the insinuation of blood. An aneurism, however, may exist from simple dilatation of a portion of the vessel, gradually increasing, and forming a cavity in which the blood accumulates. At one time it was supposed that all spontaneous aneurisms were caused by simple dilatation of the canal; but such an opinion has been long shown to be incorrect, and the term of true aneurism is now confined by many to that tumour and accumulation of blood consequent on the giving way of the internal coat, and situated externally to the canal of the artery. It is true that dilatation may occur previously to the giving way of the coats, and thus the two causes are combined. The dilatation occurs from the calibre of the artery being considerably diminished, in the first instance, at the point where its coats have undergone the calcareous degeneration, and only acts as a predisposing cause to the failure of the coats when thus diseased. When there is mere dilatation, the tumour is generally of an oval form; but when the internal coat gives way, a lateral prominence is formed, and gradually increases in size. The shape of the true aneurism is various: sometimes the tumour is globular, with a narrow neck; and, from this being of considerable length, it becomes difficult, in some situations, as above the clavicle, to ascertain the particular artery which is the seat of disease, the globular extremity of the tumour presenting itself at some distance from the vessel with which its pedicle is connected.