VENOUS SYSTEM. 



1403 



the great azi/gos vein, which joins that trunk 

 at the moment of its entrance into the cavity 

 of the pericardium. 



Azygos veins* The azygos veins ami their 

 tributaries (which are described by Cruveil- 

 hier amongst the superficial rachidian veins) 

 constitute a remarkable system which is of 

 great importance, as it assists in maintaining 

 the circulation through the spinal system of 

 veins, and at the same time connects the 

 brandies of the superior with those of the 

 inferior vena cava. 



The veins of the azygos system usually 

 consist of three principal trunks, the greater 

 and lesser azygos veins, and the left superior 

 interc sfal vein. 



The venn azi/go.^ major commences in the 

 lumbar region, by the union of several branches 

 from the upper 'lumbar veins, through which it 

 communicates with the inferior vena cava. 

 Sometimes the azygos vein receives a branch 

 directly from that vessel, and frequently another 

 from the rcnrtl vein. 



After a very short stage in the abdomen, 

 the great azygos vein enters the thorax through 

 the aortic canal in the diaphragm, and ascends 

 on the vertebral column to the right of the 

 aorta, from which it is separated by the 

 thoracic duct. Having arrived at the third 

 dorsal vertebra, the vein now changes its di- 

 rection, and passing forwards and to the right 

 side, leaves the posterior mediastinum, arches 

 over the right bronchus, and descends to ter- 

 minate in the superior vena cava, where that 

 vessel is about to enter the pericardium. 



The vena azygos major is joined by all the 

 intercostal veins of the right side, anil by the 

 lesser azygos vein, through the medium of 

 which it communicates with the lower inter- 

 costal veins of the left side also. Two or 

 three of the middle intercostal veins of the 

 left side open directly into the great azygos 

 vein, it likewise communicates very freely with 

 the left superior intercostal vein. 



The lesser azygos vein (or left or inferior 

 azygos vein), likewise has its origin in the 

 lumbar region, from some of the upper Inmlvir 

 veins on the left side of the spine ; it anas- 

 tomoses with the neighbouring veins which 

 ramify on the surface of the vertebrae, and is 

 connected, more frequently even than the 

 greater azygos vein, with the corresponding 

 renal vein (Breschet). The lesser azygos 

 vein enters the thorax, either through the 

 aortic canal in the diaphragm, or by per- 

 forating its left cms along with the left 

 splanchnic nerve, ascends on the left side of 

 the spine, as high as the fifth or sixth dorsal 

 vertebra, where, crossing the front of the 

 spinal column, behind the aorta, to terminate 

 in the great azygos vein. 



The veins from the five or six lower inter- 



* As these veins have already l>een described in 

 an article specially devoted to the subject, they are 

 but briefly noticed in this place, and only in so far 

 as was necessary to cany out the design of giving 

 a continuous description of the elements of the 

 venous system. Vide Art. AZYGOS (Dr. Harrison)) 

 Vol. I. p". 304. 



VOI,. IV. 



costal spaces terminate in this vessel. In 

 some instances there is no venous trunk cor- 

 responding to that above described, in which 

 case the left intercostal veins cross the spinal 

 column behind the thoracic aorta, to join the 

 great vein of the right side, which, under 

 these circumstances only, is appropriately 

 termed " azygos." 



77/6' left superior intercostal vein (left supe- 

 rior azi/gos vein) is formed by the union of the 

 three or four superior intercostal veins of the 

 left side. Its course is subject to some variety, 

 but it always serves to e>tabli>h a connexion 

 between the azygos system and the deep veins 

 of the neck. In many instances this vein 

 passes upwards, and opens into the left vena 

 innominata, but more frequently it descends, 

 increasing in size as it approaches its termi- 

 nation in the great azygos vein. Sometimes 

 the left superior intercostal vein terminates in 

 the azygos minor. 



There is sometimes a similar branch on the 

 right side, which corresponds to the right su- 

 perior intercostal artery ; this vein is always 

 inferior in size to that of the left side, and, 

 like it, terminates in the great azygos vein. 



The bronchial veins are the " venae comites " 

 of the bronchial arteries, from the capillaries 

 of which they are derived, and which they 

 accompany throughout the lungs. They leave 

 the root of the lung, having formed into two 

 or three trunks, and terminate, on the right 

 side, in the superior vena cava, or in the great 

 azygos vein, and on the left side in the left 

 superior intercostal vein, or in the azygos 

 minor. 



In the pelvis the middle and lateral sacral 

 veins represent the azygos system; they com- 

 municate freely with the veins in the sacral 

 canal, and with the vesical and hsemorrhoidal 

 plexuses, and end in the common iliac veins. 

 The sacral veins thus establish a communi- 

 cation between the general venous system and 

 the system of the vena portae. 



There are no valves in the azygos veins, 

 although they exist in great numbers in their 

 tributaries, the intercostal veins. 



Veins of the spine.* (RacJiidian veins : 

 Breschet.) These veins, imperfectly known 

 to Chaussier, were first accurately described 

 by Dupuytren and Breschet f ; they have been 

 comprehensively arranged by Cruveilhier, who 

 includes, under the head of spinal veins, a 

 larger portion of the venous system than the 

 author first quoted. By Cruveilhier the veins 

 of the spinal system are divided into A. The 

 superficial or extra-spinal, anil B., the deep or 

 intra-spintil veins. 



A. The superficial veins of the spine are sub- 

 divided into the anterior and the posterior. 



* Such of the veins of the spine as are situated in 

 the interior of the spinal canal, are described in Hie 

 Article NERVOUS SYSTEM (Dr. Todd), at page 629, 

 Vol. 111. of this work; any other than a brief 

 notice of the subject in this place is, therefore, un- 

 necessary. 



t Kssai sur les Yeines du Each is, A to. Ee- 

 cherches Anatomiques sur le Systcmc. Yeineux, ibl. 

 avec Planches. 



