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 to pass into the left ventricle, but do not permit it to return into the left auricle. Finally, the valves of the left ventricle allow the blood to pass into the aorta, but prevent it from regurgitating into the same ventricle. The valves with which the veins are equipped permit the blood to travel onward toward the heart, but do not permit it to back up into the arteries.

Galen taught that the arteries pulsated by reason of a "pulsific power" which they derive in direct continuity from the tunics of the heart. He tried to prove the correctness of his doctrine by experimental methods, but in this he failed. Harvey was convinced that the arteries do not pulsate by reason of their own inherent power, but by a force of impulsion communicated to the blood at the heart. He refers to this question in the following terms: "When an artery is opened the blood escapes in jets of unequal force; the alternate jets being stronger than the intermediate, and the stronger jets corresponding in time of occurrence, not with the systoles but with the diastoles of the artery. The artery, therefore, must be distended by impulsion, by the shock of the blood. If the artery dilates by reason of its own inherent power, the blood would not be expelled with the maximum force at the very moment when this dilatation occurs." As evidence of the non-existence of Galen's assumed "pulsific power," Harvey mentions the fact that, in the case of a patch-shaped calcification of the crural artery which came under his observation, the pulsation took place as usual, but at a point below (distal to) the edge of the patch. The intervening patch of rigid calcareous matter was not able to prevent the traveling onward of the propelling power.

Harvey next takes up the consideration of the veins, and, after showing that they permit a flow of the contained blood in only one direction,—viz., that from the extremities toward the heart,—he calls attention to certain experiences which he has had: (1) When a cord is tied lightly around a limb the flow of blood is arrested only in the veins, because these vessels are located near the surface of the skin; but, if the cord is tied more tightly, the flow of blood is also