532 THE AZYGOS VEINS. 



first space passes forwards in company witli the superior intercostal artery, and opens 

 into the innominate vein or one of its branches, most frequently the vertebral. 



The foonchwl veins are of small size and return only a portion of the blood 

 conveyed to the lungs by the bronchial arteries. They are formed by the union of 

 branches from the larger bronchial tubes, as well as from the other structures of the 

 lung, and issue at the hilum of that organ, where they receive other twigs from the 

 lower part of the trachea and from a fine plexus in the posterior mediastinum. The 

 vein of the right side opens into the large azygos vein near its termination ; that of 

 the opposite side ends in the left upper azygos vein. The minute veins from the 

 smaller bronchial tubes, and some of those from the larger ones, terminate in the 

 pulmonary veins, as do also several branches from the mediastinal plexus (Zucker- 

 kandl). 



As the azygos veins communicate below with the inferior vena cava or some of 

 the branches of that vessel, while they terminate above in the superior cava, they 

 form a supplementary channel by which blood can be conveyed from the lower part 

 of the body to the heart in cases of obstruction of the inferior trunk. There is 

 generally a valve, most frequently consisting of two segments, in the arch or at the 

 upper end of the ascending portion of the large azygos vein, but in the majority of 

 cases it is not sufficient to close the vessel completely. In rare instances only is a 

 valve present in the left lower azygos vein at or near its termination. The inter- 

 costal veins have valves near their openings into the azygos veins. 1 



Varieties. The azygos veins of the left side present many varieties both in the number 

 of intercostal veins which each receives, and in the manner in which they are connected with 

 the large azygos vein. Two or three of the middle intercostal veins of the left side frequently 

 unite into a short intermediate trunk, which passes directly into the large azygos vein ; or the 

 left upper azygos vein may be absent, the intercostal veins which usually form that vessel 

 being continued across the spine and opening independently into the main stem. The left 

 superior intercostal and upper azygos veins are sometimes represented by a single vessel, 

 which may join either the corresponding innominate vein above, or the large azygos vein 

 below. Occasionally all the intercostal veins of the left side are collected into a longitudinal 

 trunk which terminates in the left innominate vein, the arrangement corresponding to that 

 on the right side ; and, on the other hand, instances are sometimes met with in which there 

 is only a single azygos vein ascending on the front of the spine and receiving the intercostal 

 veins of both sides. Transposition of the azygos veins is recorded by G-ruber, the larger vessel 

 being placed on the left side, receiving the smaller right veins, and then arching forwards 

 over the root of the left lung to open into the left end of the coronary sinus of the heart, 

 thus resembling the condition which is normal in the sheep and some other animals. The 

 foregoing varieties are readily explained by reference to the mode of development of these 

 vessels (see Vol. I). 



In several cases the inferior vena cava has been seen continued into the azygos vein, which 

 is then of course extremely large (see varieties of the inferior cava) ; and the spermatic vein 

 or, on the left side, the renal and suprarenal veins have also been observed terminating in the 

 same manner. 



VEINS OF THE SPINE. 



The veins of the spine form plexuses extending along the whole length of the 

 column, and may be divided into the following sets : 1, the dorsal, placed deeply in 

 the vertebral grooves ; 2, the veins of the bodies of the vertebrae ; 3, the anterior 

 longitudinal, lying within the canal at the back of the bodies of the vertebrae ; 4, 

 the posterior longitudinal, also situated within the canal, along the fore part of the 

 arches of the vertebrae ; and 5, the veins of the spinal cord. There are likewise 

 branches of communication, some of which unite the several sets with one another, 

 while others bring them into connection with the general venous system. The veins 

 of the spine have no valves. 



The dorsal spinal veins are derived from the muscles and integument of the 



1 W. Braune, " Ueber die Intercostal venen des menschl. Korpers." Ber. d. kgl. sachs. Gesellsch., 

 1883. 



