Tunica media

Artery

• In the smaller arteries it consists principally of smooth muscle fibers in fine bundles, arranged in :wikt:lamella|lamellæ and disposed circularly around the vessel. These lamellæ vary in number according to the size of the vessel; the smallest arteries having only a single layer, and those slightly larger three or four layers - up to a maximum of six layers. It is to this coat that the thickness of the wall of the artery is mainly due.
• In the larger arteries, as the iliac, femoral, and carotid, elastic fibers and collagen unite to form lamellæ which alternate with the layers of smooth muscular fibers; these lamellæ are united to one another by elastic fibers which pass between the smooth muscular bundles, and are connected with the fenestrated membrane of the inner coat.
• In the largest arteries, as the aorta and brachiocephalic, the amount of elastic tissue is considerable; in these vessels a few bundles of white connective tissue also have been found in the middle coat. The muscle fiber cells are arranged in 5 to 7 layers of circular and longitudinal smooth muscle with about 50μ in length and contain well-marked, rod-shaped nuclei, which are often slightly curved.

  Vein

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