Page:The American Cyclopædia (1879) Volume I.djvu/810

ARTEMISIUM burning it upon parts of the 1 fody affected with gout or rheumatism. A. tridentata is the com- mon sage bush of the American plains, and is a low irregular shrub, with thick crooked stems, growing in dry alkaline soils, which un- less irrigated will produce little else. Its strong odor maybe noticed at some distance away. With greasewood it serves as the principal if not the only fuel on the plains. All artemisias are easily propagated by seeds or divisions of the roots. ARTEMISIOI, properly a temple of Artemis (Diana), the name of several places in ancient geography. The most important of them was the promontory on the N. coast of Eubcea, off which the Greek ships fought with the fleet of Xerxes, almost simultaneously with the battle of Thermopylffi, in 480 B. C. The success achieved here by the Greeks was soon followed by the great victory at Salamis. ARTERY (Gr. aprripia, from a#/>, air, and rripelv, 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 distin- guished 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 col- lapse 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, pene- trating 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 precau- tions, 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 ex- ternal 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 re- action, and in the smaller arteries that of mus- cular contractility. In the larger and medium- sized arteries, the elasticity of their walls re- acts 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 pulsa- tion of the heart, and diminishes, without ceas- ing altogether, in the intervals. In the smaller arteries, the muscular fibres of the middle coat, ARTESIAN WELLS under the varying influence of the nervous sys- tem, contract or relax at certain periods ; thug 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. ARTESIAN WELLS, small holes sunk in the earth, through which currents of water, struck at great depths, rise toward the surface, and sometimes flow over ; so named from the prov- ince of Artois in France (Lat. Artesium), in which they have for a long time been in use. Water thus pressed up must have its source in some more elevated lands, and be confined in the strata of rock through which it has percolated ; precisely as water is conveyed in pipes below the surface, and pressed up An Artesian Well. into buildings to a height nearly equal to that at which the pipes commence. Water finds its way down into the earth by flowing into the crevices and chasms of the rocks, and by per- colating through the porous strata. In a region of limestone rocks it hollows out for itself its own bed, by dissolving the limestone, and even in this way produces great caves. When forced by the pressure behind, the water is pushed up through any apertures it meets and flows out as a spring or artesian well. There are three conditions essential to the successful boring of an artesian well: 1. A fountain head more elevated than the locality where the boring is to be undertaken. 2. A moderate downward dip of the strata toward the site of the well ; a steep or high angle of inclination of dip is unfavorable, as the water is apt to flow away beyond the reach of the boring, which must needs pass at an acute angle through few layers of rock. 3. Alternations of porous and impervious strata beneath the surface soil. It is sometimes the case that the head of water is at so high an elevation, that the column bursts forth from the ground as a fountain,