The discoveries of the nerves and the valves of the heart had made great difficulties for the Aristotelian psychology and physiology shortly after Aristotle's time. We have seen that the semilunar valves were described, and their use noted, in a treatise included in the Hippocratic collection;[141] and all the valves, both arterial and auriculo-ventricular, were well recognized by Erasistratus, whose acquaintance we have made already, and who flourished about 300 B.C., Aristotle having died in 322 B.C. Erasistratus, we remember, was more than four centuries earlier than Galen and more than nineteen centuries earlier than Harvey. That the heart throughout life is not only the source of the perfected blood, but gives out blood to the vena cava for distribution, had been rendered a hard saying, especially by the recognition of the tricuspid valve.[142] Galen, however, like the somewhat earlier Greek physician Aretæus, the Cappadocian,[143] was not confronted by this difficulty, for they both adhered to an ancient doctrine to be found in the Hippocratic treatise "On Nourishment," and there sketched with mingled clearness and vagueness in the following pithy saying:-- "Root of the veins, the liver; root of the arteries, the heart. Out of these wander into all parts blood and spirits, and through these heat comes in."[144] Obviously the doctrine here foreshadowed was quite irreconcilable with the views of Aristotle. In studying the works of Harvey and of his contemporaries and predecessors it must be borne in mind that, from ancient times past the time of Harvey to more modern days, the word "heart" was very commonly used by physicians and men of science to mean simply the ventricular mass, without the auricles, which were reckoned in with the great vessels. In slaughterhouses the word is still used in this ancient sense. Harvey's practice was fluctuating; for the word is used by him sometimes to mean the ventricular mass only, sometimes, as in the science of to-day, to mean the ventricular mass and the auricles taken together.