Erasistratus thus, in a manner, laid the foundations of the theory of the motion of the blood. But it was not given to him to get any further. What the contents of the heart were, and whether they moved or not, was a point which could be determined only by experiment. And, for want of sufficiently careful experimentation, Erasistratus strayed into a hopelessly misleading path. Observing that the arteries are usually empty of blood after death, he adopted the unlucky hypothesis that this is their normal condition, and that during life they are filled with air. And it will be observed that it is not improbable that Erasistratus' discovery of the valves of the heart and of their mechanical action strengthened him in this view. For, as the arteria venosa branches out in the lungs, what more likely than that its ultimate ramifications absorb the air which is inspired; and that this air, passing into the left ventricle, is then pumped all over the body through the aorta, in order to supply the vivifying principle which evidently resides in the air; or, it may be, of cooling the too great heat of the blood? How easy to explain the elastic bounding feel of a pulsating artery by the hypothesis that it is full of air! Had Erasistratus only been acquainted with the structure of insects, the analogy of their tracheal system would have been a tower of strength to him. There was no _prima-facie_ absurdity in his hypothesis--and experiment was the sole means of demonstrating its truth or falsity. More than four hundred years elapsed before the theory of the motion of the blood returned once more to the strait road which leads truthward; and it was brought back by the only possible method, that of experiment. A man of extraordinary genius, Claudius Galenus, of Pergamus, was trained to anatomical and physiological investigation in the great schools of Alexandria, and spent a long life in incessant research, teaching, and medical practice. More than one hundred fifty treatises from his pen, on philosophical, literary, scientific, and practical topics, are extant; and there is reason to believe that they constitute not more than a third of his works. No former anatomist had reached his excellence, while he may be regarded as the founder of experimental physiology. And it is precisely because he was a master of the experimental method that he was able to learn more about the motions of the heart and of the blood than any of his predecessors, and to leave to posterity a legacy of knowledge which was not substantially increased for more than thirteen hundred years.