Next it was shown that pulmonary veins carried nothing to the heart except blood. And now, during this Reform{160} Period, the purpose of the movements of the chest was better studied, for Borelli, Helvetius, and Haller made many experiments, as the result of which it was determined that during inspiration the thorax is enlarged in all directions, and during expiration partly collapsed by relaxation of muscles, and that there never is any empty space between the lungs and the sides of the chest; further, that air is drawn into the chest by the tendency of all gases or fluids to maintain an equilibrium, or, in other words, because Nature abhors a vacuum. This being settled, various pneumatic theories were adopted and abandoned, all of which had subsequently to give way before a knowledge of what really occurs. The truth was conceived of by Mayow in 1668. It had been noticed that blood which appeared black in issuing from the veins, became red in contact with the air, and direct observation proved a similar change of color to take place during its passage from the pulmonary veins during life. Goodwin, opening the thorax of a frog, was the first to see this, and Hessenfratz filled a silk bladder with venous blood, and, plunging it into an atmosphere of oxygen, saw the blood change from black to red. In this way and by the later labors of Bichat and Lavoisier were clearly established the mechanism and the purpose of the function of respiration. The discovery of the lymphatic vessels and their purpose was scarcely less remarkable than that of the circulation, though marked by less eclat because it was not the work of one man, but a matter of slow development. Herophilus and Erasistratus had seen white vessels connected with the lymph-nodes in the mesentery of animals, and supposed them to be arteries full of air. Galen disputed this, for he believed that the intestinal chyle was carried by the veins of the mesentery into the liver. In 1563 Eustachius described the thoracic duct in the horse. In 1622 Aselli, Professor of Anatomy at Milan, discovered the lacteal vessels in a dog which had been killed immediately after partaking{161} of food; having pricked one of these by mistake, he saw a white fluid issue from it. Repeating the experiment, he became certain that the white threads were vessels which drew the chyle from the intestines. He observed the valves with which they are supplied, and supposed these vessels all met in the pancreas and continued on into the liver. In 1647 Pecquet, while still a student at Montpellier, discovered the lymph-reservoir, or _receptaculum chyli_, and the canal which leads from it (the thoracic duct), which he followed to its termination in the left subclavian vein. Having ligated the duct, he saw it swell below and become empty above the ligature. He studied the courses of the lacteals, and convinced himself that they all entered into the common reservoir.