Galen had an eclectic mind and could not identify himself with any of the prevailing schools, but regarded himself as a disciple of Hippocrates.  For our purpose, both his philosophy and his practice are of minor interest in comparison with his great labors in anatomy and physiology.

In anatomy, he was a pupil of the Alexandrians to whom he constantly refers.  Times must have changed since the days of Herophilus, as Galen does not seem ever to have had an opportunity of dissecting the human body, and he laments the prejudice which prevents it.  In the study of osteology, he urges the student to be on the lookout for an occasional human bone exposed in a graveyard, and on one occasion he tells of finding the carcass of a robber with the bones picked bare by birds and beasts.  Failing this source, he advises the student to go to Alexandria, where there were still two skeletons.  He himself dissected chiefly apes and pigs.  His osteology was admirable, and his little tractate “De Ossibus” could, with very few changes, be used today by a hygiene class as a manual.  His description of the muscles and of the organs is very full, covering, of course, many sins of omission and of commission, but it was the culmination of the study of the subject by Greek physicians.

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.”