Muscular artery. Light micrograph of a transverse section through the wall of a muscular artery, showing its structure. This vessel carries blood under high pressure. Its thick wall is composed of three layers. The innermost tunica intima surrounds the lumen (tube, white). It possesses a fine elastic sheet (convoluted, bottom left) known as the internal elastic lamina. The middle layer, the thick tunica media, is mainly composed of smooth muscle fibres (purple), interspersed with a few connective collagen fibres (blue). The external tunica adventitia (blue) is composed mainly of connective tissue. Trichrome blue stain. Magnification unknown.