366 



AZYGOS. 



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 oeso- 

 phagaeal, 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 oesophagus, 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 

 venae innominatee. 



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, oesophagus, 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 cavee, 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 



