612 THE BLOOD-VASCULAR SYSTEM. 



in the right azygos vein. It receives the four or five lower intercostal veins of the 

 left side, and some oesophageal and mediastinal veins. 



The left upper azygos vein varies inversely with the size of the left superior 

 intercostal. It receives veins from the intercostal spaces between the left superior 

 intercostal vein and highest tributary of the left lower azygos. They are usually 

 three or four in number, and join to form a trunk which ends in the right azygos 

 vein or in the left lower azygos. It sometimes receives the left bronchial vein. 

 When this vein is small or altogether wanting, the left superior intercostal vein 

 will extend as low as the fifth or six intercostal space. 



Surgical Anatomy. In obstruction of the inferior vena cava the azygos veins are one of 

 the principal means by which the venous circulation is carried on, connecting as they do the superior 

 and inferior venae cavae, and communicating with the common iliac veins by the ascending lumbar 

 veins, and with many of the tributaries of the inferior vena cava, 



The bronchial veins return the blood from the substance of the lungs ; that of 

 the right side opens into the vena azygos major near its termination ; that of the 

 left side, into the left superior intercostal vein or left upper azygos vein. 



THE SPINAL VEINS. 



The numerous venous plexuses placed upon and within the spine may be 

 arranged into four sets : 



1. Those placed on the exterior of the spinal column (the dorsi-splnal vein*). 



2. Those situated in the interior of the spinal canal, between the vertebrae and 

 the theca vertebralis (meningo-rachidian veins). 



3. The veins of the bodies of the vertebrae (vence basis vertebrarurn). 



4. The veins of the spinal cord (medulli-spinal). 



1. The Dorsi-spinal Veins commence by small branches which receive their 



The dorsi-spmal veins. 



FIG. l>32. Transverse section of a dorsal vertebra, showing the spinal veins. 



blood from the integument of the back of the spine and from the muscles in the 

 vertebral grooves. They form a complicated network, which surrounds the spinous 

 processes, the laminae, and the transverse and articular processes of all the ver- 

 tebrae. At the bases of the transverse processes they communicate, by means of 

 ascending and descending branches, with the veins surrounding the contiguous 

 vertebrae, and they join with the veins in the spinal canal by branches which 

 perforate the ligamenta subflava. Other branches pass obliquely forward, 

 between the transverse processes, and communicate with the intraspinal veins 

 through the intervertebral foramina. They terminate by joining the vertebral 



