An aneurysm (or aneurism) (from Gr. a dilatation) is a localized dilation or
ballooning of a blood vessel. Aneurysms most commonly occur in the arteries at the
base of the brain (the circle of Willis) and in the aorta (the main artery coming out
of the heart) - this is an aortic aneurysm.
The layer of the artery that is in direct contact with the flow of blood is the
tunica intima, commonly called the intima. This layer is made up of mainly
endothelial cells. Just deep to this layer is the tunica media, known as the media.
This "middle layer" is made up of smooth muscle cells and elastic tissue. The
outermost layer (furthest from the flow of blood) is known as the tunica adventitia
or the adventitia. This layer is composed of connective tissue.
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