His work as a physiologist was even more important, for, so far as we know, he was the first to carry out experiments on a large scale. In the first place, he was within an ace of discovering the circulation of the blood. You may remember that through the errors of Praxagoras and Erasistratus, the arteries were believed to contain air and got their name on that account: Galen showed by experiment that the arteries contain blood and not air. He studied particularly the movements of the heart, the action of the valves, and the pulsatile forces in the arteries. Of the two kinds of blood, the one, contained in the venous system, was dark and thick and rich in grosser elements, and served for the general nutrition of the body. This system took its origin, as is clearly shown in the figure, in the liver, the central organ of nutrition and of sanguification. From the portal system were absorbed, through the stomach and intestines, the products of digestion. From the liver extend the venae cavae, one to supply the head and arms, the other the lower extremities: extending from the right heart was a branch, corresponding to the pulmonary artery, the arterial vein which distributed blood to the lungs. This was the closed venous system. The arterial system, shown, as you see, quite separate in Figure 31, was full of a thinner, brighter, warmer blood, characterized by the presence of an abundance of the vital spirits. Warmed in the ventricle, it distributed vital heat to all parts of the body. The two systems were closed and communicated with each other only through certain pores or perforations in the septum separating the ventricles. At the periphery, however, Galen recognized (as had been done already by the Alexandrians) that the arteries anastomose with the veins, ". . . and they mutually receive from each other blood and spirits through certain invisible and extremely small vessels."