Two of the most celebrated physicians of that period (305–280 B. C.) were Erasistratus and Herophilus, both of whom were distinguished as the founders of schools or sects of medicine at Alexandria. They had received their early training as physicians from Chrysippus, a widely known Stoic philosopher, who, according to Albert von Haller, had taught at the school of Cnidus and had also written on medical topics; and, among the other teachers, it is stated that Anaxagoras of Cos had instructed Herophilus, and that Metrodorus, the son-in-law of Aristotle, had performed the same service for Erasistratus. So far as fundamental principles are concerned, the schools founded by these two physicians at Alexandria differed very little from each other, and the men themselves also gained their distinction in very much the same branches of medical knowledge, both of them having made a number of original discoveries in anatomy and both of them having become eminent practitioners. Herophilus was born at Chalcedon, a Greek city on the Propontus, nearly opposite to Byzantium. We possess no knowledge whatever regarding the earlier years of his career, notwithstanding the fact that no fewer than four different men devoted their energies to the writing of his biography. The books themselves have been either lost or destroyed. Herophilus showed a decided leaning toward the study of anatomy, and his contributions to this branch of medicine are among the earliest which we possess. Herophilus strove to supply one of the most conspicuous deficiencies in the Hippocratic system of medicine, viz., inadequate knowledge of the nervous system; and to this end he conducted a series of the most careful investigations, as a result of which he was successful in establishing several facts previously unknown. He described the membranes of the brain, the choroid plexus, the venous sinuses, the structure which bears his name,--the torcular Herophili,--the cerebral ventricles, and the calamus scriptorius; he traced the course of the nerve trunks for some distance from their origin in the brain and spinal cord; and it was he who established the fact that two different sets of nerves exist--one for conveying sensations to the brain and the other for producing motion. In addition, he investigated the corpus vitreum, the retina, the optic nerve, etc. He also called attention to the peculiar mode of construction of the duodenum, and to the fact that the walls of the arteries are thicker than those of the veins. Some idea of the accurate manner in which he carried on his anatomical researches may be gained from the fact that he noted the circumstance that the left vena spermatica occasionally originates in the vena renalis.