that cardiac systole is synchronous with the arterial pulse. He not only verified Estienne's findings on the valves of the hepatic veins, but also described the azygos vein, and discovered the canal which passes into the fetus between the umbilical vein and vena cava. Nervous system Vesalius defined a nerve as the mode of transmitting sensation and motion and thus refuted his contemporaries' claims that ligaments, tendons and aponeuroses were three types of nerve units. He believed that the brain and the nervous system are the center of the mind and emotion in contrast to the common Aristotelian belief that the heart was the center of the body. He correspondingly believed that nerves themselves do not originate from the heart, but from the brain—facts already experimentally proved by Herophilus and Erasistratus in the classical era, but suppressed after the adoption of Aristotelianism by the Catholic Church in the Middle Ages. Upon studying the optic nerve, Vesalius came to the conclusion that nerves were not hollow. Abdominal organs In De fabrica, he corrected an earlier claim he made in Tabulae about the right kidney being set higher than the left. Vesalius claimed that the kidneys were not a filter device for urine to pass through, but rather that the kidneys serve to filter blood as well, and that excretions from the kidneys travelled through the ureters to the bladder. He described the omentum, and its connections with the stomach, the spleen and the colon gave the first correct views of the structure of the pylorus. He also observed the small size of the caecal appendix in man and gave the first good account of the mediastinum and pleura. Vesalius admitted that due to a lack of pregnant cadavers he was unable to come to a significant understanding of the reproductive organs. However, he did find that the uterus had been falsely identified as having two distinct sections. Heart Through his work with muscles, Vesalius believed that a criterion for muscles was their voluntary motion. On this claim, he deduced that the heart was not a true muscle due to the obvious involuntary nature of its motion. He identified two chambers and two atria. The right atrium was considered a continuation of the inferior and superior venae cavae, and the left atrium was considered a continuation of the pulmonary vein. He also addressed the controversial issue of the heart being the centre