The bronchial veins arise in the cellular tissue of the lungs from the extremities of the bron chial arteries ; as the branches unite into larger vessels, these are found to accompany very closely the divisions of the bronchial tube ; they leave the root of each lung two or three in number ; on the right side one joins the arch of the azygos or the superior vena cava, the others open into the azygos lower down, or into some of the mediastinal or intercostal branches. The left bronchial veins arise in a similar manner, escape from the root of the left lung, and open either into some of the superior in tercostal veins, or into the superior or inferior azygos minor. In minute injections of the lungs these veins are found to inosculate with the capillary terminations of the pulmonary arteries. Both the right and left vena azygos receive numerous branches from the posterior mediastinum, from the coats of the aorta, pericardium, cesophagus, bronchial glands, trachea, &c. &c.; these veins pursue no regular course ; they receive names either from the arteries they accompany, or from the organs whence they are derived ; they require no par ticular description.
The vena azygos is the principal vein apper taining to the parietes of the chest ; it not only serves to receive the several branches which have been mentioned, but also maintains numerous communications between different portions of the venous system, which must prove of essential service in case of obstruction to the circulation in any of the principal trunks : thus, its abdominal portion communi cates either directly with the inferior vena cava, or indirectly through the medium of the lumbar, phrenic, renal, or spermatic veins, while its thoracic end joins the superior cava, and at the same time anastomoses on either side with the subclavian vein or some of its branches.
On both sides of the thorax again it inoscu lates by its intercostal communications not only with the internal mammary, but also with the thoracic branches of the axillary veins, and along the vertebrae it communicates with the vertebral sinuses, opposite each foramen of conjunction. This vein, consequently, ap pears not only as one of the roots of the cava, but also as a loop or second channel between the two cavze, which, in case of the obstruction of either, more particularly of the inferior, would convey the blood to the heart, and thus obviate any impediment to the venous circulation of the lower segment of the body. Cases have even occurred in which the inferior cava has been obstructed or nearly obliterated by the pressure of a tumour or of a diseased liver, and in these this anastomosis, and indeed the whole vena azygos have been found greatly increased in size. The vena azygos appears moreover to have been formed as a convenient means for receiving numerous venous branches which could not reach any of the large vessels without some more complicated provision : thus the inferior intercostal veins could not join the inferior cava, where the latter is imbedded in the liver, without perforating the diaphragm ; neither could the middle and superior inter costal, the mediastinal, and the bronchial veins arrive at the superior cava or at the right auricle of the heart without a much more complex disposition of all these parts than we observe.