Artery (Gr.Artery 100492 fromArtery 100493 air, andArtery 100494 to keep), a blood vessel conveying the blood outward from the heart to the organs; so called because the ancients supposed these vessels to contain " spirits " or air. An artery is distinguished from other blood vessels mainly by the thickness and elasticity of its walls. When cut open, therefore, in the dead body, after most of the blood has collected in the great veins and internal organs, the artery does not collapse as a vein would, but stands open, allow- ing the air to pass into its cavity. It was this circumstance which led the old anatomists to believe that the arteries also contained air during life. They supposed that the air, penetrating the lungs at the moment of inspiration, was partly received by the left ventricle of the heart, and thence distributed by the arteries throughout the body, while the blood was sent out from the right ventricle by the veins. It was not until Galen, in the 2d century, opened the arteries, with some experimental precautions, in the living animal, that it became known that these vessels during life served as conduits for blood and not for air. An artery is composed of three coats, the internal or serous, the middle or fibrous, and the external or cellular.

The external coat is the most resisting of the three, and prevents the vessel, under ordinary circumstances, from being distended beyond a certain point. The middle coat is distensible; but, owing to the peculiar nature of the fibres which constitute its substance, it also has the power of elastic reaction, and in the smaller arteries that of muscular contractility. In the larger and medium-sized arteries, the elasticity of their walls reacts upon the blood during the intervals of the heart's pulsation and urges it onward toward the periphery: so that the current of blood in this part of the circulation, though pulsating in character, is yet continuous, or nearly so, and merely increases in velocity with every pulsation of the heart, and diminishes, without ceasing altogether, in the intervals. In the smaller arteries, the muscular fibres of the middle coat, under the varying influence of the nervous system, contract or relax at certain periods; thus increasing or diminishing the resistance of the vessels to the flow of blood, and causing local variations in the circulation of particular parts.

When an artery is wounded the blood escapes in jets, coming with greater force at the instant of each pulsation of the heart; and it can be distinguished by this feature from haemorrhage from the veins, in which the blood escapes in a comparatively feeble but continuous stream. If the wounded artery be of considerable size, it requires to be secured by a ligature in order to stop the flow of blood.