It is not enough to know the kind of influence which the contraction of the arteries has on the motion of the arterial blood; it is necessary to know if this contraction does not influence in a sensible manner the course of the blood in the veins. This is elucidated by the following experiment. Lay bare, as in the preceding experiment, the crural artery and vein of a dog; tie the limb strongly, taking care not to include these vessels; afterwards tie the crural vein, and make a small opening in it below the ligature, of one or two lines in length; the blood flows out in a continuous jet. If the artery be compressed, so as to intercept the course of blood in it, the jet still continues a short time; but it is seen sensibly to diminish, as the artery is becoming empty. It at length ceases entirely when the artery is completely emptied; and though the vein remains distended with blood along its whole extent, it does not flow out at the small wound. If the compression be taken off of the artery, the blood enters it with force, and almost at the same instant it begins again to flow from the opening in the vein, and the jet is reestablished as before. If we check the course of the blood in the artery, there is but a feeble jet from the vein; it is the same if the passage of this fluid is alternately intercepted and permitted. I make the same phenomenon evident in another way; I introduce into the crural artery the extremity of a syringe filled with water at the temperature of 30 degrees of the centigrade thermometer; I push the piston slowly, and soon the blood goes out by the opening in the vein, at first alone and afterwards mixed with water, and it forms a jet the more considerable in proportion to the force with which the piston is pushed. To prove, as we have done, that the heart maintains an evident influence on the course of the blood in the capillary vessels, is not to advance that these vessels have no action on the motion of this fluid. Many physiological phenomena, on the contrary, prove that the capillaries can aid with more or less facility the passage of the blood, and consequently sensibly influence its course.