Such was the spirit of the times in which Harvey lived, and such the influences which surrounded his teachers before him and himself in turn. It makes a long preface to a consideration of what Harvey himself accomplished, but it is not without its interest because men and their deeds must be judged largely by their environment. Now, to speak more particularly of Harvey himself, and what was known of the circulation when he undertook his investigations. The liver had been considered, from time immemorial, as the principal factor in the production and movement of the blood. The ancients supposed that here the veins took their origin and that through them the blood flowed to all parts of the body, returning to its source by an undulating movement or series of alternate waves. The arteries had been supposed to contain only vital spirits, whose great reservoir was the heart, although Erasistratus had admitted that in certain cases blood might escape into the arterial channels. Later Galen showed that the arteries always contained blood, and he knew that blood was poured into the right side of the heart by the great veins, but believed that only a little of it passed from the right ventricle into the lungs, the greater part of it passing through hypothetical pores in the septum and thus into the left ventricle. This opinion, like Galen's in other respects, remained unchanged until the middle of the 16th century. It was also known that valves existed within the veins, and that if an artery were tied on a living animal blood would cease to flow and pulsation be checked below the ligature, while if a vein were tied it shrunk above the ligature and became distended below.