Contains circularly arranged smooth muscle cells, but this layer is much thinner than arteries with a comparable lumen diameter. The smooth muscle cells are not as tightly packed as seen in arteries. Longitudinal bundles of elastin are present. Tunica adventitia . This layer is thicker than the media and contains collagen fibers and elastin fibers.
Larger examples have longitudinally oriented bundles of smooth muscle.
Tunica intima . The endothelium is subtended by a thin connective tissue layer containing smooth muscle cells. The intima often is not distinct from the tunica media. Tunica media . The media of these vessels contains only 2-15 layers of smooth muscle despite a vessel diameter of 1 cm or more.