740 THE VASCULAR SYSTEMS 



It receives the four or five lower intercostal veins of the left side, and some 

 oesophageal and mediastinal veins. 



The vena azygos minor superior (v. hemiazyyos accessorial) varies inversely in 

 size with the left superior intercostal. It receives veins from the intercostal 

 spaces between the left superior intercostal vein and highest tributary of the vena 

 azygos minor inferior. They are usually three or four in number, usually the 

 fourth, fifth, sixth, and seventh left posterior intercostal veins. They join to 

 form a trunk which ends in the major azygos vein or in the vena azygos minor 

 inferior. When this vein is small or altogether wanting, the left superior inter- 

 costal vein will extend as low as the fifth or sixth intercostal space. It sometimes 

 receives the left bronchial vein. 



Applied Anatomy. In obstruction of the inferior vena cava, the azygos veins are one ol 

 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 ascend- 

 ing lumbar veins and with many of the tributaries of the inferior vena cava. 



Thrombosis of the superior vena cava is oftenest due to pressure exerted on the vessel by 

 an aneurism or a tumor; it may also occur by propagation of clotting from a tributary peripheral 

 vein. If occlusion of the vessel take place slowly, a collateral venous circulation may be estab- 

 lished; the patient will have some oedema with dilatation and congestion of the veins about th( 

 head and neck, and may also suffer fron> attacks of dyspnea and recurrent pleural effusion, 

 In most cases, however, the blockage of the superior vena cava takes place rapidly, and is 

 rapidly fatal. 



The bronchial veins (vv. bronchiales) return the blood from the larger bronchi 

 and from the structures at the roots of the lungs; that of the right side opens intc 

 the vena azygos major, near its termination; that of the left side opens into th 

 left superior intercostal vein or the vena azygos minor superior. A considerable 

 quantity of the blood which is carried to the lungs through the bronchial arteries 

 is returned to the left side of the heart by the pulmonary veins. 



The Spinal Veins. 



The spinal veins may be arranged into four groups viz. : 



1. The extraspinal veins. 



2. The intraspinal veins. 



3. The veins of the bodies of the vertebrae. 



4. The veins of the spinal cord. 



1. The extraspinal veins (plexus venosi vertebrates extern i) commence bj 

 small branches which receive their blood from the integument of the back of th 

 vertebral column and from the muscles in the vertebral grooves. They forn 

 two plexuses, an anterior plexus (plexus venosi vertebrates anteriores] upon the 

 vertebral bodies and a posterior plexus (plexus venosi vertebrates posteriorcs) 

 which surrounds the spinous processes, the laminae, and the transverse anc 

 articular processes of all the vertebrae. At the bases of the transverse processe; 

 they communicate, by means of ascending and descending branches, with th 

 veins surrounding the contiguous vertebrae, and they join with the veins in th( 

 vertebral canal by branches which perforate the ligamenta subflava. Othe] 

 branches pass obliquely forward, between the transverse processes, and com 

 municate with the intraspinal veins through the intervertebral foramina (vv 

 intervertebrales). The extraspinal veins terminate by joining the vertebral veini 

 in the neck, the intercostal veins in the thorax, and the lumbar and lateral sacra 

 veins in the loins and pelvis. 



2. The intraspinal veins (plexus venosi vertebrates interni) are situated betweei 

 the spinal dura and the vertebrae. They consist of two longitudinal plexuses, oni 



