297. The blood, in flowing through the arterial trunks and branches to the capillaries, through the arterial to the venous capillaries, and through the venous branches and trunks back to the heart, is exposed to numerous and powerful causes of retardation: such, for example, as the friction between the blood and the sides of the vessels, the numerous curves and angles formed by the branches in springing from the trunks, the tortuous course of the vessels in many parts of the body, and the increasing area of the arterial branches as they multiply and subdivide. Yet the extraordinary fact has been recently discovered, that the blood moves with the same momentum or force in every part of the arterial system, in the aorta, in the artery in the neck which carries the blood to the head (the carotid artery), in the artery of the arm (the humeral artery), in the artery of the lower extremity (the femoral artery); in a word, in the minute and remote capillary, and in the large trunk near the heart. Having contrived an instrument by which the force of the blood as it flows in its vessel could be accurately indicated by the rise of mercury in a tube, M. Poiseuille found that the elevation of the mercury is uniformly the same in the different arteries of the same animal, whatever the size of the artery and its distance from the heart. This tube was inserted, for example, into the common carotid artery of a horse: the diameter of the vessel was 34/100ths of an inch; its distance from the heart was thirty-nine inches; the height to which the mercury rose in the graduated tube was accurately marked. The tube was then inserted into a muscular branch of the artery in the thigh: the diameter of this vessel was 7/100ths of an inch, and its distance from the heart 67½ inches. According to the mean of nine observations, the mercury rose in both tubes to precisely the same elevation. Here is another instance of the beautiful adjustments everywhere established in the living economy. The blood is sent by a living engine, moving under laws peculiar to the state of life, into living vessels, which in their turn acting under laws peculiar to the state of life, so accommodate themselves to the current as absolutely to offer no resistance to its progress; so accommodate themselves to the moving power, as completely and everywhere to obviate the physical impediments to motion inseparable from inorganic matter. 298. That the arterial tubes do possess and exert a truly vital power, modifying the current of the blood they contain, is indubitably established. 1. If in a living animal the trunk of an artery be laid bare, the mere exposure of it to the atmospheric air causes it to contract to such a degree, that its size becomes obviously and strikingly diminished. This can result only from the exertion of a vital property, for no dead tube is capable in such a manner of diminishing its diameter.