B B.* First pair of ribs. C C.* Second pair of ribs. D. Aorta, with left vagus and phrenic nerves crossing its transverse arch. E. Root of pulmonary artery. F. Right ventricle. G. Right auricle. H. Vena cava superior, with right phrenic nerve on its outer border. I I*. Right and left lungs collapsed, and turned outwards, to show the heart's outline. K K*. Seventh pair of ribs. L L*. The diaphragm in section. M. The liver in section. N. The gall bladder with its duct joining the hepatic duct to form the common bile duct. The hepatic artery is seen superficial to the common duct; the vena portae is seen beneath it. The patent orifices of the hepatic veins are seen on the cut surface of the liver. O. The stomach. P. The coeliac axis dividing into the coronary, splenic and hepatic arteries. Q. Inferior vena cava. R. The spleen. S S* S**. The transverse colon, between which and the lower border of the stomach is seen the gastro-epiploic artery, formed by the splenic and hepatic arteries. S***. Ascending colon in the right iliac region. T. Convolutions of the small intestines distended with air. [Illustration: Chest and abdomen, showing bones, blood vessels, muscles and other internal organs.] Plate 23 COMMENTARY ON PLATE 24. THE RELATIONS OF THE PRINCIPAL BLOODVESSELS TO THE VISCERA OF THE THORACICO-ABDOMINAL CAVITY. The median line of the body is occupied by the centres of the four great systems of organs which serve in the processes of circulation, respiration, innervation, and nutrition. These organs being fashioned in accordance with the law of symmetry, we find them arranged in close connexion with the vertebrate centre of the osseous fabric, which is itself symmetrical. In this symmetrical arrangement of the main organs of the trunk of the body, a mechanical principle is prominently apparent; for as the centre is the least moveable and most protected region of the form, so have these vitally important structures the full benefit of this situation. The aortal trunk, G, of the arterial system is disposed along the median line, as well for its own safety as for the fitting distribution of those branches which spring symmetrically from either side of it to supply the lateral regions of the body.