Galen insisted upon the study of anatomy as essential, and in this respect was in conflict with the view held by the Methodists and the Empirics who believed that a physician could understand diseases without any knowledge of the exact structure of the body. His books on anatomy were originally fifteen in number. The last six of these are now extant only in an Arabic translation, two copies of which are preserved in the Bodleian Library at Oxford.
The directions he gives for dissection show that he was a master of the art. In dissecting out the portal vein and its ramifications, for instance, he advises that a probe should be inserted into the vein, and the point of the probe gradually advanced as the surrounding tissue is cut away, so that finally the minute branches are exposed; and he describes the use of the blowpipe, and other instruments used in dissection. He carried out the experiment of tying the iliac and axillary arteries in animals, and found that this procedure stopped the pulse in the leg and arm, but caused no serious symptoms, and he found that even the carotid arteries could be tied without causing death. He also pointed out that tying the carotid artery did not cause loss of voice, but that tying the artery carelessly so as to include the nerve had this effect. He was the first to describe the ductus arteriosus, and the three coats of the arteries.
It is highly improbable that Galen dissected human bodies in Rome, though he dissected a great variety of the lower animals. He writes that the doctors who attended Marcus Aurelius in the German wars dissected the dead bodies of the barbarians. The chief mistakes made by Galen as an anatomist were due to his assumption that what is true of the anatomy of a lower animal is true also when applied to man.
Galen greatly assisted the advance of physiology by recognizing that every part of the body exists for the purpose of performing a definite function. Aristotle, like Plato, had taught that "Nature makes nothing in vain," and Galen's philosophy was greatly influenced by the teaching of Aristotle. Galen regarded his work as "a religious hymn in honour of the Creator, who has given proof of His Omnipotence in creating everything perfectly conformable to its destination."
He regarded the structure of various parts, such as the hand and the membranes of the brain, as absolute perfection, although his idea of the human hand was derived from a study of the ape's, and he had no knowledge of the arachnoid membrane of the brain, but it would be unfair to criticize his conclusions because of his failure to recognize a few comparatively unimportant details. He discovered the function of the motor nerves by cutting them experimentally, and so producing paralysis of the muscles; the platysma, interossei, and popliteus muscles were first described by him. He was the greatest authority on the pulse, and he recognized that it consisted of a diastole (expansion) and a systole (contraction) with an interval after the diastole, and another after the systole. Aristotle thought that arteries contained air, but Galen taught that they contained blood, for, when an artery was wounded, blood gushed out. He was not far from the discovery of the circulation. He described the heart as having the appearance of a muscle, and considered it the source of natural heat, and the seat of violent passions. He knew well the anatomy of the human skeleton, and advised students to go to Alexandria where they might see and handle and properly study the bones. He recognized that inspiration is associated with enlargement of the chest, and imagined that air passed inside the skull through the cribriform plate of the ethmoid bone, and passed out by the same channel, carrying off humours from the brain into the nose. But some of this air remained and combined with the vital spirits in the anterior ventricles of the brain, and finally exuded from the fourth ventricle, the residence of the soul. Aristotle had taught that the heart was the seat of the soul, and the brain relatively unimportant.