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, arid, like it, terminates in the great azygos vein.
The bronchial veins are the " vense 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 hmmorrhoidal 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 porter.
There are no valves in the azygos veins, although they exist in great numbers in their tributaries, the intercostal veins.
Veins of the spine.* (I?achidian veins : 13reschet.)—These veins, imperfectly known to ('haussier, were first accurately described by Dupuytren and Breschett ; 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, and B., the deep or intra-spinal veins.
A. The superficial veins of the spine are sub divided into the anterior and the posterior.
1. The anterior superficial spinal (or rachi dian) veins, include, according to this au thority, the vena azygos major, the vane azygos minor, the trunk of the right and left superior intercostal veins, the lumbar and ilio-lumbar veins, and the lateral and middle sacral veins.
2. The posterior superficial spinal veins, "form an exceedingly complicated network, the meshes of which surround the spinous processes and laminm, and the transverse and articular processes of all the vertebrat."
B. The deep or veins comprise the following ; — I. The veins of the bodies of the 2. The great anterior longitudinal veins or sinuses (Willis) ; 3. The posterior spinal veins and plexuses; all these are external to the " theca verte bralis :" and 4. The veins of the spinal cord itself ; in ternal to the theca vertebralis.
Superficial spinal veins. — The anterior super ficial spinal veins have been already described as constituting a part of the azygos system.
The posterior superficial spinal veins (the veins of Dupuytren and Breschet) are derived from the muscles which fill the vertebral grooves, and thus cover the laminm and the spinous and transverse processes with a series of anastomosing vessels. In the neck they form a complicated plexus, from which proceed two large veins (posterior jugular, Cruveilhier), which communicate freely with the vertebral veins, and join the venm in nominat.
From the numerous venous circles and plexuses formed by the "dorsi-spinal " veins, communicating branches are given off, which perforate the ligaments subilava, or pass through the intervertebral foramina, and unite freely with the deep spinal veins.
Deep spinal veins. — 1. Veins of the bodies of the vertebrce : (veines basi-vertebrales, Bres chet), [vide Fig. 361. Vol. III. p. 630.], are contained in bony canals in the bodies of all the vertebra:, and are analogous to the diploic veins of the cranium. The basi-vertebral veins originate it) the canallated tissue of the bone, and form larger trunks, which converge towards the posterior surfaces of the bodies of the vertebrm, where the orifices of the bony canals in which they are contained are very apparent ; on emerging from the vertebroa these veins form a plexus (transverse plexus) interposed between the bones and the pos terior common ligament of the spine, and from which veins pass laterally to terminate in the anterior longitudinal sinuses. Some of the smaller venous canals pass forwards to open on the anterior surface of the vertebral column, where they anastomose with the su perficial veins.