The vena azygos receives several .veins; in the abdomen and in its passage through the diaphragm it is joined by one or two of the superior lnmbars, and by small branches from the diaphragm; in the thorax it receives the intercostals; the seven or eight inferior inter costals of the right side enter it distinctly ; the corresponding number of the left side some times join it in a similar manner, but most com monly they first unite into a trunk, called the left or" minorazygos, of which we shall speak presently. The three or four superior inter costal veins of the right side unite into one or two branches which end in the convexity at the upper extremity of the azygos major, which also receives the right bronchial veins in the same situation, and at a lower point the oeso phageal ; the latter, like the arteries of the same name, are irregular in number and in situation.
The left vena azygos, azygos minor, semi azygos, is smaller, but in other respects similar to the right ; it commences by small branches from the superior left lumbar, capsular, and renal veins, which unite into a delicate vessel that sometimes communicates with the right azygos, and sometimes with the inferior Cava; it then passes through the aortic opening in the diaphragm, or through or external to its left crus in company with the left splanchnic nerve, and ascends along the anterior and left side of the dorsal vertebra as high as the seventh or eighth ; it then crosses the spine behind the aorta, oesophagus, and thoracic duct, to join the right or great vena azygos. The azygos minor receives the six or seven inferior left intercostal veins, and as it is passing across the spine it is generally joined by a large descending branch which is formed by the confluence of some of the superior of these vessels ; the azygos minor also receives the left bronchial veins as well as some branches from the diaphragm, oesophagus, and medias tinum. In some subjects this vein is wanting; in such cases the left intercostals join the proper azygos either individually, or by two or three uniting into a large branch.
The intercostal veins are eleven or twelve in number on each side ; in their course and dis tribution they correspond to the intercostal arteries ; they commence each by the union of small branches near the sternum, which anas tomose with the internal mammary veins ; they then accompany the intercostal arteries along the groove in the lower border of each rib ; near the spine they increase in size, being joined by several veins from the exterior mus cles of the spine, which pass through the internal part of each intercostal space along with the posterior branches of the intercostal arteries ; in this situation also they receive veins from the vertebral canal, communicating with the vertebral sinuses on the posterior surface of the bodies of the vertebrae, and passing through the intervertebral holes along with the spinal nerves. All the intercostal veins com
municate with each other over the heads of the ribs, either by many small or by a few larger branches ; the veins of the opposite sides also communicate by transverse branches, so as to give to the anterior surface of the dorsal ver tebrae, in a successful injection of the venous system, an appearance somewhat analogous to the vertebral sinuses on their posterior sur face. The first intercostal vein of the right side generally ascends over the neck of the first rib, and over the first dorsal ganglion of the sympathetic nerve, and joins the subclavian vein or some of its deep cervical branches ; the second intercostal frequently joins the first, and sometimes the third also terminates in a similar manner, but usually the fourth, third, and often the second open into the arch of the azygos by one or two branches : these superior intercostal veins always communicate with each other and with the azygos below, as well as with the subclavian above. The remaining intercostal veins of the right side enter the azygos separately, or two or three occasionally unite and end by a common opening; the infe rior ascend, the middle take a transverse course, and the superior descend ; near the spine they all anastomose freely with each other, so that the heads of the ribs support a chain or net work of these vascular inosculations, as is well represented in Breschet's plates of the venous system.
The superior intercostal vein of the left side always joins the left subclavian or some of its large branches, the internal mammary in parti• cular ; it is usually a large vein, but it presents great varieties ; in some it appears as a third vena azygos, and might be named the left superior azygos; in such cases it communicates below with the inferior azygos about the sixth dorsal vertebra and above with the left subcla vian ; in the intermediate space it receives the corresponding intercostal veins, also the ce so phagxal, mediastinal, and left bronchial; this vein sometimes also communicates directly with the right azygos. The remaining left intercostal veins enter the lesser azygos, or if this vessel be absent, they cross the spine behind the cesophagus, aorta, and thoracic duct, and enter the great azygos separately, or two or three conjoined. The superior and in ferior azygos veins of the left side are some times continuous, and enter the left subclavian, thus taking a parallel and very similar course to the vein on the right side, particularly when the latter opens so high as into either of the venze innominatm.