Aortic aneurysm, illustration. A large saccular-type dilation of the ascending and part of the arch of the aorta. The intimal lining of the sac is irregularly tubercular with some areas being greatly thinned and covered with firm, whitish masses of fibrin. Adjoining non-dilated parts of the aorta show sclerosis i.e. irregular ridges of thickened tissue. Often caused by severe atherosclerosis (but congenital aetiology can occur) the arterial wall/media is weakened by destruction of the normal tissues and replacement by fibrous tissue. From Bollinger, O. 1901 Atlas und Gundriss der Pathologischen Anatomie, vol 1. Lehmann, Munich.