300. Driven by the heart through the arterial branches into the capillaries, the blood courses along these minute vessels urged by the same power. The most careful observers, from Haller and Spalanzani down to the present time, concur in stating that the pulsatory movement communicated by the heart to the blood in the great arteries is distinctly visible under the microscope in the capillaries. "I have often observed in frogs and tadpoles, and once in the bat," says Wedemeyer, "that when the circulation was becoming feeble, the blood in the finest capillaries advanced by jerks, corresponding with the contractions of the heart. I remarked the same appearance in the fine veins several times in the toad and tadpole, and once in the frog." If an experimenter so dispose the circulation of the limb of an animal that the flow of blood be confined to the branches of a single artery, and a corresponding vein, it is found that the blood stagnates in the vein whenever the current in the artery is stopped by a ligature, but no sooner is the ligature removed from the artery, than the blood begins again to flow freely along the vein, the capillaries of the artery which have to send on the current to those of the vein being now again within the influence of the heart. And if the impulse of the heart be removed from the capillary system, by placing a ligature around the aorta, the capillary circulation is uniformly and completely stopped. 301. It was found by Dr. Hales, that, under ordinary circumstances, the blood rises in a tube connected with a vein to the height only of six inches, while it has been shown (290) that in the artery it ascends as high as ten feet. This prodigious difference between the venous and the arterial tension led to the conclusion that the impulsive force of the heart was all but exhausted before the blood reached the veins, and set physiologists on the search for other powers to carry on the venous circulation. It was overlooked that the blood has an open and ready escape from the great trunks of the veins through the right chambers of the heart, and that in consequence of this free escape of their fluid, these vessels indicate no greater tension than is just sufficient to lift the blood to the heart, and to overcome friction[7]. M. Magendie having laid bare the chief artery and vein of a living limb, and having raised the vessels in such a manner that he could place a ligature around the former, without including the latter, found that the flow of blood from a puncture made below a ligature on the vein, was rapid or slow, according as the heart was allowed to produce a greater or less degree of tension in the artery, which tension was regulated by compressing the artery between the fingers. After a similar preparation of a limb, a ligature was placed around the vein; a tube was then inserted into it; it was found that the blood ascended in the tube from the obstructed vein just as high as from the artery.