 Coronary arteries, considered to be medium-sized muscular arteries, originate from the proximal part of the ascending aorta and lie on the surface of the heart in the epicardium surrounded by adipose tissue. The walls of coronary arteries are usually thicker than those of comparable arteries on the upper or lower limb because of the large amounts of circular smooth muscle layers in the tunica media. In routine H&E preparation, the subendothelial layer of the tunica intima of younger people is inconspicuous, but it progressively thickens by increasing amounts of smooth muscle cell and fibroelastic tissue with aging (Fig. 13.27). The internal elastic membrane is well developed, although it may be fragmented, duplicated, or focally lost in older individuals. The relatively “loose” consistency of the tunica adventitia is reinforced by the longitudinal bundles of collagen fibers that allow for continuous changes of the vascular diameter.
Atherosclerotic changes in coronary arteries that restrict blood ﬂow and oxygen supply to cardiac muscle lead to ischemic heart disease (see Folder 13.3).
 Dural venous sinuses represent venous channels in the cranial cavity. They are essentially broad spaces within the dura mater that are lined with endothelial cells and devoid of smooth muscles.
 The great saphenous vein represents a long subcutaneous vein of the lower limb that originates in the foot and
FIGURE 13.25 • Schematic diagram and photomicrograph of a large vein. a. The cellular and extracellular components are labeled. Note a thin layer of circumferentially arranged smooth muscles of tunica media and the tunica adventitia with a large amount of longitudinally arranged smooth muscle bundles. b. This photomicrograph shows a section through the wall of a human portal vein in a routine H&E preparation. The tunica intima is indiscernible at this magnification. The tunica media contains a layer of circumferentially arranged smooth muscle cells with collagen and elastic fibers. Note the thickest layer of this wall is the tunica adventitia. In addition to an extensive collagen and elastic fiber network, the tunica adventitia contains a broad layer of smooth muscle cells arranged in longitudinal bundles. These bundles are variable in size and separated from each other by connective tissue fibers. 125. (Courtesy of Dr. Donald J. Lowrie Jr., University of Cincinnati College of Medicine.) drains into the femoral vein just below the inguinal ligament. This vein is often described as a muscular vein because of the presence of an unusual amount of smooth muscle (Fig. 13.28). In addition to the thick circular arrangement of smooth muscle in its tunica media, the great saphenous vein possesses numerous longitudinal smooth muscle bundles in the intima and in the well-developed adventitia. A thin, poorly developed internal elastic membrane separates the tunica intima from the media. The great saphenous vein is frequently harvested from the lower limb and used for autotransplantation in coronary artery bypass graft (CABG) surgery when arterial grafts (usually taken from internal thoracic artery) are not available or many grafts are required for multiple-bypass anastomoses. CABG is one of the most commonly performed major surgical operations in the United States.