Arteries, the tubes through which the blood is carried from the heart to the various tissues. (Blood-vessels.) The branches of an artery are always smaller than the trunk from which they originate, the smallest arteries or arterioles finally breaking up into minute tubes of microscopic size called capillaries; the blood pumped by the heart through these fine channels is collected again into venules, and these venules combine with other venules to form veins. An artery is composed of three coats, an inner, middle, and outer. The inner coat is lined internally by a smooth layer of endothelium (q.v.), the middle coat consists largely of unstriped muscular tissue (Muscle), while in the outer coat elastic tissue predominates. The calibre of the arteries is controlled by the nervous system by means of nerves, called vasomotor nerves, which terminate in the muscle cells. Thus, in blushing a nervous impulse travelling down the vasomotor nerves of the arteries of the face causes relaxation of muscle cells with resulting increased calibre of arteries, and as a consequence more blood flows into the skin of the cheeks, which become flushed and hot, It is the contraction of the muscular coat of arteries after death which drives blood out of them and causes them to appear empty; hence arose their name (artery signifying air - carrier), the ancients being unaware that the vessel during life was full of blood. The pressure of blood within the arteries is measured by means of the mercurial manometer; it is found that in the carotid of a rabbit this pressure is capable of supporting a column of mercury two or three inches high. The velocity of blood is greatest in the large arteries, and diminishes as the vessel divides and subdivides. The elastic element in the arterial walls serves to convert the intermittent action of the heart into a continuous flow in the capillaries and veins. Thus, if an artery be cut blood spurts out in jets, while in the case of a wounded vein the bleeding occurs in a uniform stream. Arteries are ligatured to check bleeding, as, for example, when a limb is amputated. Of the diseases to which they are subject the most important is atheroma (q.v.); they may also be occluded or plugged (Embolus). In all cases of bleeding from a wounded artery it is important to know that the haemorrhage can almost always be controlled until skilled help is forthcoming by the mere exercise of firm pressure upon the bleeding point. The operation of opening an artery is known as arteriotomy.