During this epoch many modifications were introduced and improvements made in the teaching of medicine. Permanent amphitheaters were established for dissection, and chairs of anatomy created, their incumbents being paid out of the public treasury. The popes, appear to have taken the initiative in this respect,{ 112} which accounts for the great number of subjects with which Eustachius was supplied, as compared with Vesalius, who obtained only two or three in a year. Up to this time _the razor had been the sole instrument of dissection_, but was now replaced by the scalpel, which remains in use to-day. By the labors of the few men mentioned anatomy acquired a degree of perfection which it had never attained under the Greeks. Skillful artists put their labors upon paper, and plates and descriptions made from anatomical preparations represented the various parts of the human body with more fidelity than had been supposed possible. Nerves, tendons, and ligaments were no longer confused, but traced so far as possible from origin to ramifications. Ancient errors generally were corrected. It was proven that there was no bony structure in the tissue of the heart, that the partition between its cavities was not porous; and attentive examination of its valves led to the discovery of the lesser circulation by Columbus. Michael Servetus, whom John Calvin burned at the stake, was perhaps the first to note this phenomenon. He saw that the blood could not penetrate directly from the right into the left cavity of the heart, but that it was necessary for the whole fluid to pass through the lungs, where it became impregnated with the vital spirit of the atmosphere, and reached afterward the left auricle; the position of the valves in the pulmonary arteries and veins clearly confirmed his conjecture. Moreover the size of the pulmonary arteries was enormous, and disproportionate to the quantity of blood necessary for the nutrition of the lungs, which seemed to prove that this was not, as had been believed, the sole purpose of those vessels. It was about this time that Fabricius ab Aquapendente pointed out valves in veins in various parts of the body, and that Columbus and Andreas Cesalpinus explained more fully the mechanism of the lesser circulation; in fact, the former so closely approached an appreciation of the{ 113} purpose of the vascular system that some have thought he really knew it, but the passages in his writings thought to sustain this opinion are not at all conclusive. He seems to have confused the action of the heart during sleep with that during the waking hours; and although he realized that the blood could not flow backward through the arteries, that the vena cava was the only vessel which permitted the entrance of blood into the heart, and though he spoke of anastomosis between arteries and veins and remarked that if a band be applied around a limb the veins swell below the ligature, he contented himself with comparing the motion of the blood with the flux and reflux of Euripus, as Aristotle had done. It is even thus that he tortured his mind in trying to reconcile two irreconcilable theories,--i.e., the opinion of the ancients on the motion of the blood and recent discoveries in the anatomy of the vascular system.