When two ligatures are applied at the same time and at some centimetres distant upon two points of an artery which furnishes no branches, we have a portion of artery in which the blood is subjected only to the influence of the parietes. If we make in this portion of the vessel a small opening, almost all the blood that it contains is immediately thrown out, and the artery is much contracted. This experiment has been known for a long time, and uniformly succeeds. The following is one of my own, and places, it seems to me, the phenomenon in a very clear light. I laid bare the crural artery and vein of a dog to a certain extent; I passed under these vessels, near the trunk, a string, which I afterwards drew tightly at the posterior part of the thigh, so that all the arterial blood should come to the limb by the crural artery, and all the venous blood return to the trunk by the crural vein; I then applied a ligature upon the artery, and this vessel was very soon completely empty in the part below the ligature. It is then satisfactorily proved that the force with which the arteries contract upon themselves is sufficient to expel the blood they contain. But what is the nature of this contraction? We have proved that it cannot be attributed to irritability. Every thing leads to the belief that it should be referred to the very great elasticity which the arterial parietes enjoy, an elasticity that is brought into action, when the heart forces a certain quantity of blood into the cavity of these vessels. This property of the arteries being known, it is easy to conceive how the principal agent of the arterial motion, being alternate, the course of fluid is yet continuous. The elasticity of the arterial parietes is similar to that of the reservoir of air in certain pumps with an alternate action, and which notwithstanding throw out the fluid in a continuous manner.