As we purpose limiting ourselves, in the following pages, to the consideration of the conditions of actual dilatation of the artery, we will postpone to another chapter the consideration of false, varicose, and dissecting aneurisms, as subjects which do not appropriately belong to the present place.

After simply referring to the development of the arteries in reference to their calibre and the thickness of their walls, as observed in organs having become hypertrophied (in the gravid uterus and in morbid augmentations of size in the most different organs), we will pass on to dilatation of the arteries based on disease of the arterial coats.

It seems, however, especially necessary to notice, in the first place, the conditions under which dilatation of an artery is established. These are as follow:

1. Some dilatations arise without any visible alteration of texture in consequence of a loss of elasticity and contractility of the coats of the artery, that is to say, of the circular fibrous coat and of the elastic sheath, or in consequence of a mechanically induced continuous and excessive filling of an artery or even of one entire section of the arterial system. Such dilatations are very commonly observed, particularly in advanced life, in the trunk of the aorta, and especially in the ascending arch, in consequence of the first above-named causes. If, as we think, not improbable, Cloquet's two cases of circoid aneurism (or arterial varix), belong to the class in which the aneurism is not the result of extensive chronic inflammation or of deposition, such a dilatation might be extended over the larger portion, or even the whole of the arterial system. In such dilatations the walls of the arteries are thinner, softer, and more yielding, and the circular fibrous coat is paler than usual. The dilatation, especially at the trunk of the aorta, excepting where it exhibits a more or less strongly marked protrusion of the convex portion at the ascending aorta, presents a regularly cylindrical form: in the so-called circoid aneurism, however, it appears so far irregular that it preponderates in one or other portion of the surface, and thus gives rise to the interlaced twisted course of the artery (which seems as if it were coiled round an imaginary axis), and to the bulgings occurring at certain points, as well as to the lengthening of the diseased vessel.

Dilatations arising from mechanical obstructions are observed in the aorta; but more especially in the pulmonary artery and its branches, in consequence of the various diseases of the parenchyma of the lungs which impede the normal injection of the capillaries, in cases of stenosis of the left side of the heart, etc.