Chronic Inflammation Of The Cellular Sheath of the arteries gives rise, as has been already remarked, to dilatation of the vessel, in consequence of its paralyzing the elastic coat. This is especially observable in the trunk of the aorta, and it may, moreover, have laid the foundation of some or other of the recorded cases of cirsoid aneurism. Dilatations of the point of origin of the trunks of the pulmonary artery and the aorta, depending upon inflammation of the cellular sheath, combined with pericarditis, deserve notice on account of their locality (see p. 122). These dilatations are generally of a cylindrical form.

The forms of dilatation based upon the two causes above described, constitute (more particularly in reference to the arterial wall enclosing the dilatation) true Aneurism (Aneurisma verum), which has been distinguished by the designation Arteriectasis from those aneurisms which are regarded as depending for their origin on a more essentially anatomical disturbance.