(tunica adventitia) n. 1. the outer coat of the wall of a *vein or *artery. It consists of loose connective tissue and networks of small blood vessels, which nourish the walls. 2. the outer covering of various other organs or parts.
The arterial system begins at the left ventricle of the heart with the AORTA, which gives o? branches that subdivide into smaller and smaller vessels. The ?nal divisions, called arterioles, are microscopic and end in a network of capillaries which perforate the tissues like the pores of a sponge and bathe them in blood that is collected and brought back to the heart by veins. (See CIRCULATORY SYSTEM OF THE BLOOD.)
The chief arteries after the aorta and its branches are:
(1) the common carotid, running up each side of the neck and dividing into the internal carotid to the brain, and external carotid to the neck and face;
(2) the subclavian to each arm, continued by the axillary in the armpit, and the brachial along the inner side of the arm, dividing at the elbow into the radial and the ulnar,
which unite across the palm of the hand in arches that give branches to the ?ngers;
(3) the two common iliacs, in which the aorta ends, each of which divides into the internal iliac to the organs in the pelvis, and the external iliac to the lower limb, continued by the femoral in the thigh, and the popliteal behind the knee, dividing into the anterior and posterior tibial arteries to the front and back of the leg. The latter passes behind the inner ankle to the sole of the foot, where it forms arches similar to those in the hand, and supplies the foot and toes by plantar branches.
Structure The arteries are highly elastic, dilating at each heartbeat as blood is driven into them, and forcing it on by their resiliency (see PULSE). Every artery has three coats: (a) the outer or adventitia, consisting of ordinary strong ?brous tissue; (b) the middle or media, consisting of muscular ?bres supported by elastic ?bres, which in some of the larger arteries form distinct membranes; and (c) the inner or intima, consisting of a layer of yellow elastic tissue on whose inner surface rests a layer of smooth plate-like endothelial cells, over which ?ows the blood. In the larger arteries the muscle of the middle coat is largely replaced by elastic ?bres, which render the artery still more expansile and elastic. When an artery is cut across, the muscular coat instantly shrinks, drawing the cut end within the ?brous sheath that surrounds the artery, and bunching it up, so that a very small hole is left to be closed by blood-clot. (See HAEMORRHAGE.)... arteries
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