This discovery gave the last blow to the ancient theory which attributed to the liver the function of blood-making, and confirmed the doctrine of Harvey. Strangely enough, the latter united with Riolan in opposing the discovery of Pecquet and denying its significance. From this time the lymphatic vessels and glands became objects of common interest and were investigated by many anatomists,--by Bartholin, Ruysch, the Hunters, Hewson, and, above all, by Mascagni, who was the first to give a graphic description of the whole lymphatic apparatus. The ancients confounded, under the name "neuron," nerves, tendons, ligaments, and membranes; even Aristotle regarded the brain as an inert mass devoid of sensation, and supposed the nerves to originate in the heart. Rufus, of Ephesus, remarked that Herophilus distinguished three sorts of nerves,--the first serving for sensation and motion and proceeding from the brain and spinal marrow, the second and third serving to unite bones and muscles. Galen also shared in this error, but, nevertheless, described the brain-membranes and the difference between white and gray matter; he supposed the cerebrum to be the seat of the soul and origin of sensory nerves, and that the cerebellum{162} gave rise to nerves of motion; the pulsation of the cerebrum exposed was held to be a sort of brain respiration. Galen came very near recognizing the distinction between nerves and tendons, but nevertheless confused them. The anatomists of the sixteenth century described certain portions of the nervous system with, more exactness than did Galen, but not with such positiveness as to prevent Cesalpinus from renewing the Aristotelian theory that the heart was the origin of sensation and the seat of the soul. Nearly two centuries later Baglivi advanced a theory which referred vital movement to the heart and the dura mater. The progress which accrued to comparative anatomy and physiology, and the experiments which were made on animals, during this period, shed a great deal of light upon the nervous system. The researches of Vieussens, Haller, Meckel, Vicq d'Azyr, Scarpa, Soemmering, and others had already rendered it manifest that the brain was the organ of sensation and voluntary motion, and Bichat had proposed to divide the nervous system into cerebrospinal and sympathetic branches. Now, too, Kepler discovered that the crystalline lens was not the seat of vision, as had been supposed, but that its function, like that of other lenses, is the refraction of light. He observed that the image of objects is depicted upon the retina, and (with Schemer) demonstrated that the expansion of the optic nerve in the retina is the essential part in the organ of sight. Obviously, also, interest in the anatomy of the eye, which these observations everywhere stimulated, was, in a great measure, aided by the researches of Newton on light and color.