The azygos system consists of the major azygos vein, which starts at the right external iliac vein and empties into the descending vena cava; the minor azygos vein which starts at the left external iliac vein and empties into the major azygos vein back of the heart; and the tertiary azygos vein, which starts at the left subclavian vein and empties into the minor azygos vein. The azygos veins collect all the blood from the side walls of the body and form a perfect collateral circulation between the superior and inferior caval systems, and thoroughly equalizes the blood pressure all over the body. The major azygos vein receives the following: the right intercostal veins, excepting the first; the azygos minor; the right bronchial vein; the esophageal vein; the pericardiac; and the posterior mediastinal veins. The minor azygos vein receives the following: the tertiary azygos vein; the lower five left intercostal veins; the small left mediastinal veins; the lower left esophageal veins. The tertiary azygos receives the following: the fifth, sixth and sometimes the seventh intercostal veins; the lower end of the lower left superior intercostal vein; and the left bronchial vein. The inferior vena cava receives the following veins: the lumbar veins; the hepatic veins; the phrenic veins; the renal veins; the right suprarenal vein; the right spermatic or ovarian vein. The left spermatic or ovarian vein and the left suprarenal vein empty into the left renal vein. =The Pulmonary Circulation.=—This is the circulation existing between the right ventricle of the heart through the lungs back to the left auricle of the heart. The pulmonary artery takes its origin from the summit of the right ventricle. It is about two inches in length, and is directed upward, backward and slightly towards the left, and beneath the arch of the aorta it divides into the right and left pulmonary arteries. These end in a system of capillaries in between the air cells of the lungs, where carbon dioxide is thrown off and oxygen taken on. The pulmonary veins are four in number, two passing from the root of each lung to the posterior surface of the left auricle of the heart. Each vein is formed at the root of the lung by the union of a number of smaller vessels which take origin ultimately from the capillary net work formed from the branches of the pulmonary artery, and to a certain extent from that formed by the bronchial arteries. Each pulmonary vein is about six inches in length.