AZYGOS SPINAL. 471 



the spine, as high as the sixth or seventh vertebra, passes across the column, 

 behind the aorta and thoracic duct, to terminate in the right azygos vein. It 

 Teceives the four or five lower intercostal veins of the left side, and some oesopha- 

 geal and mediastinal veins. 



The left upper azygos vein varies according to the size of the left superior 

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

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

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

 vein, or in the left lower azygos. When this vein is small, or altogether wanting, 

 the left superior intercostal vein will extend as low as the fifth or sixth intercostal 

 space. 



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, in the left superior intercostal vein. 



THE SPIXAL 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 (dorsi-spinal veins). 



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

 the theca vertebralis (meningo-rachidian veins). 



3. The veins of the bodies of the vertebra (vense basis vertebrarum). 



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



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

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

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

 of ascending and descending branches, with the veins surrounding the contiguous 

 vertebras, and they join with the veins in the spinal canal by branches which per- 

 forate the ligamenta subflava; in the intervals between the arches of the vertebrae, 

 they terminate in the vertebral veins in the neck, in the intercostal veins in the 

 thorax, in the lumbar and sacral veins in the loins and pelvis. 



2. The veins contained in the spinal canal are situated between the theca 

 vertebralis and the vertebras. They consist of two longitudinal plexuses, one 

 of which runs along the posterior surface of the bodies of the vertebrae, 

 throughout the entire length of the spinal canal (anterior longitudinal spinal veins), 

 receiving the veins belonging to the bodies of the vertebras (venas basis verte 

 brarum). The other plexus (posterior longitudinal spinal veins) is placed on the 

 inner or anterior surface of the laminae of the vertebrae, and extends also along 

 the entire length of the spinal canal. 



The Anterior Longitudinal Spinal Veins consist of two large, tortuous venous 

 canals, which extend along the whole length of the vertebral column, from the 

 foramen magnum to the base of the coccyx, being placed one on each side of the 

 posterior surface of the bodies of the vertebrae, external to the posterior common 

 ligament. These veins communicate together opposite each vertebra, by transverse 

 trunks, which pass beneath the ligament, and receive the large venae basis verte- 

 brarurn, from the interior of the body of each vertebra. The anterior longitudinal 

 spinal veins are least developed in the cervical and sacral regions. They are not 

 of uniform size throughout, being alternately enlarged and constricted. At the 

 intervertebral foramina, they communicate with the dorsi-spinal veins, and with 

 the vertebral veins in the neck, with the intercostal veins in the dorsal region, 

 and with the lumbar and sacral veins in the corresponding regions. 



The Posterior Longitudinal Spinal Veins, smaller than the anterior, are situated 

 one on either side, between the inner surface of the laminae and the theca verte- 

 bralis. They communicate, like the anterior, opposite each vertebra, by transverse 



