THE LARGEST ARTERY IN THE BODY IS THE aorta, which takes blood from the heart and runs from the chest into the abdomen, lying against the spine until it reaches the pelvis. It divides at that point into iliac arteries that supply blood to the pelvis, buttocks, and legs (see Fig. 1). Because the aorta takes the full force of blood ejected from the heart, it is the most likely artery in the body to weaken.

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• I. ABDOMINAL AORTIC ANEURYSM
  A. Pathogenesis
• II. THORACIC AORTIC ANEURYSM
  Aneurysms of the aorta in the chest are much less com¬mon than abdominal aneurysms. They are classified as the ascending, arch, or descending aortic aneurysms. Because of the etiology and natural history, treatment differs for each of these aortic segments. Aneurysms of the descend¬ing thoracic aorta are the most common and have similar causes as those of abdominal aneurysms. Aneurysms of the descending aorta usually result from a cystic medial necrosis; the etiology of which remains unknown. Also, cystic medial necrosis is observed in nearly all cases of Marfan syndrome. Another rare connective tissue disorder, Ehlers-Danlos syndrome, may involve the aorta. Syphilis was a common cause of ascending thoracic aneurysms, but is now rare because aggressive antibiotic therapy cures the disease in its early stages. In these cases chest x-ray showed typical linear calcification of the dilated ascending thoracic aorta.
• III. AORTIC DISSECTION
  Ascending aortic aneurysms may undergo internal tearing or dissection resulting in an extremely high mortality of up to 1% per minute and 60% in 60 minutes. Thus, time-consuming investigations that are not sufficiently sensitive or specific, such as CTscans, are usually not recom¬mended. Emergency surgery carries the only hope of sur¬vival for patients with dissecting aneurysms, and immediate accurate diagnosis is mandatory to guide therapy.
• IV. BERRY ANEURYSM
  A different type of aneurysm can occur at the base of the brain. The arteries at this site may have a develop¬mental defect and form small berry-like aneurysms that may remain asymptomatic until a rupture occurs when the individual is between 20 and 50 years old. A subarach-noid hemorrhage at the base of the brain may damage the brain substance and cause coma, death, or severe disability.