The third part of the _Fabrica_, comprising sixty pages, is devoted to the veins and arteries. Vesalius begins with the definition of a vein, and describes the structure of these vessels in general. The term “artery” is treated in like manner. He introduces several small illustrations which serve to elucidate this part of the text. His first large plate in this section is devoted to the venae portae. This is followed by a full-page picture of the entire venous system. The arterial system is fully described and elaborately illustrated. To these is added another plate, in which both arteries and veins are represented in their natural order. In other plates he shows the special circulations—cerebral, portal, and pulmonary. [Illustration: DISSECTION OF THE ABDOMEN BY VESALIUS (From the “Fabrica”, 1543)] Vesalius described the valve which guards the foramen ovale in the foetus, and also noticed the valve-like fold which guards the entrance of each hepatic vein into the inferior vena cava. He also gave an admirable description of the vena azygos. Blinded by the ancient theory of the movement of the blood—a sort of flux and reflux in the veins, he overlooked the function of the venous valves. He described them as eminences, or projections, or accidental rugosities, which in no way interfere with the flux and reflux of the blood. [Illustration: DISSECTION OF THE HEART BY VESALIUS (From the “Fabrica”, 1543)] The Fourth Book Vesalius devotes forty pages to the cerebral and spinal nerves. The anatomy of the brain is treated in the seventh book. His representations of the nerves are very creditable. He mentions eleven pairs of cranial nerves: the olfactory, the optic, the motores oculorum, the trifacial, the abducens, the portio dura, the portio mollis, the glosso-pharyngeal, the pneumogastric, and the spinal accessory. His account of the brain—contained in the seventh book—is elaborately minute considering the time when it was written. His illustrations and description of this organ surpass those of scores of later authors. Vesalius fully describes the position of the brain; the membranes which cover it; the cavities, or ventricles, within it; the divisions of cerebrum, cerebellum, and medulla; the anatomy of the base, and the origins of the cerebral nerves. These structures are illustrated from different points of view. The Fifth Book The fifth book, comprising more than one hundred pages, is devoted to the organs of nutrition. Here we find an admirable account of the peritoneum, the mesentery, the omentum, the stomach and intestines, the liver, the spleen, and the genito-urinary tract—all of which structures are described and fully illustrated. In this book Vesalius also describes the foetus in utero. The Sixth Book