THE AZYGOS VEINS. 



531 



side, with the exception of that from the first space, the upper two or three of these 

 vessels being united into a short common trunk which is known as the superior inter- 

 costal vein (p. 512), and which opens into the commencement of the arch of the 



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Fig. 411. SKETCH OF THE PRINCIPAL SYSTEMIC VENOUS y\lT g' \ 3'). 



TRUNKS, SHOWING THE AZYGOS VEINS. (Allen Thomson. ) | u^~ .~J^_ 



For the detailed description of this figure see p. 511. The 

 following indications relate to the accompanying part of the 

 text : 8, left superior intercostal vein, continued below into 

 the left upper azygos vein ; the superior intercostal vein of 

 the right side is seen passing downwards into the large azygos 

 vein ; 9, 9, 9, large azygos vein ; 9', left lower azygos vein ; 

 10, thoracic duct ; 11, inferior vena cava ; 12, union of a 

 branch of the left lower azygos with the left renal vein ; 13, 13', 

 right and left ascending lumbar veins, continued upwards into 

 the corresponding azygos veins ; 15, union of lumbar, ilio- 

 lambar, and sacral veins. 



azjgos vein. It is also joined by the left azygos 

 veins, by the right bronchial vein, and by several 

 small cesophageal, pericardial, and posterior medi- 

 astinal veins. 



The left lower or small azygos vein (v. 

 hemi azygos) commences as the ascending lumbar 

 vein of the left side, and in most cases has also 

 a communication with the corresponding renal 

 vein, 1 seldom with the suprarenal or spermatic. It 

 enters the thorax through the left crus of the 

 diaphragm and ascends upon the spine, in front 

 of the lower intercostal arteries, to the level (most 

 frequently) of the ninth dorsal vertebra, where it 

 crosses to the right behind the aorta and opens 

 into the large azygos vein. It receives the inter- 

 costal veins from the lower three or four spaces of 

 the left side, and some small branches from the 

 mediastinum. 



The left upper azygos vein (v. hemiazygos 

 accessoria) is formed by the union of the veins from 

 four or five intercostal spaces, generally from the 

 fourth to the seventh or eighth inclusive, and it 

 receives also some mediastinal branches and the 

 left bronchial vein. It communicates above with 

 the superior intercostal vein, and below it opens 

 into the large azygos vein, either separately or in 

 common with the left lower azygos vein. 



The intercostal veins are single vessels lying in 

 the intercostal spaces above the arteries, which they 

 follow closely in their ramifications. They are 

 joined by large posterior branches, collecting blood 

 from the muscles of the back, the dorsal spinal 

 plexus and the spinal canal, and by small twigs from the bodies of the vertebrae, 

 before terminating in the superior intercostal or azygos veins. The vein from the 



1 Lejars found the left azygos vein communicating with the renal in 88 percent. (" Les voies de 

 sdret6 de la veine renale," Bull. Soc. Anat., Paris, 1888). 



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