664 THE BLOOD-VASCULAR SYSTEM 



means of the anterior sacral plexus, with the middle and lateral sacral veins, and 

 with the common ihac, hypogastric and ilio-lumbar veins. It ascends in front of 

 the lumbar transverse processes communicating with the lumbar veins, the vena 

 cava inferior and, usually, with the renal vein. The right vein enters the 

 thorax between the aorta and the right medial crus of the diaphragm, and is 

 continued upward as the vena azygos. The left vein pierces the left medial crus 

 and becomes the hemiazygos. 



The intercostal veins [vv. intercostales]. — The intercostal veins are twelve in 

 number on each side, the last one being subcostal. They correspond to the inter- 

 costal arteries. There is one vein to each artery, the vein lying above the artery 

 whilst in the intercostal space. Each vein receives a dorsal tributary which 

 accompanies the posterior ramus of an intercostal artery between the transverse 

 process of the vertebras and the neck of the rib. These dorsal branches not only 

 return the blood from the muscles of the back, but receive a spinal branch from 

 the vertebral venous plexuses. The intercostal veins also receive small tributaries 

 from the bodies of the vertebrae. The termination of the intercostal veins is 

 different on the two sides and also varies greatly in different individuals. The 

 intercostal vein from the first space on either side may join the superior inter- 

 costal vein, but commonly opens directly into the innominate or one of its tribu- 

 taries, most frequently the vertebral. 



On the right side. — The second intercostal vein joins with the third or with the third and 

 fourth to form the right superior intercostal vein [v. intercostalis suprema dextra]. This vein 

 opens into the azygos vein as the latter is arching over the root of the right lung. The rest 

 join the azygos directly. The upper of these have well-marked valves where they join the 

 azygos vein; in the lower veins these valves are imperfect. AU the intercostal veins are pro- 

 vided with valves in their course between the muscles. 



On the left side the second intercostal vein joins the third and fourth to form a single 

 trunk, the left superior intercostal vein [v. intercostalis suprema sinistra]. This vein passes 

 upward across the arch of the aorta and opens into the left innominate vein. The left superior 

 intercostal frequently communicates at its lower end with the accessory hemiazygos vein, 

 which is occasionally tributary to it. In most cases a small tributary runs up over the front 

 of the aortic arch to join the superior intercostal vein; it is a vestige of the left common cardinal 

 and from it a small fibrous cord can often be traced through the vestigial fold of the pericardium 

 to the oblique vein of the left atrium (p. 523). 



The left fifth, sixth and seventh intercostal veins commonly open into the accessory 

 hemiazygos, and the eighth or ninth and succeeding veins into the hemiazygos. The method 

 of termination of the intercostal veins of the left side is subject to such variation that a normal 

 arrangement can scarcely be said to exist at all. The eighth may open directly into the azygos, 

 as may the seventh and ninth or even more of the veins; the hemiazygos and accessory hemi- 

 azygos being correspondingly reduced in size. 



The posterior bronchial veins [vv. bronchiales posteriores] correspond to the bronchial 

 arteries, but do not return the whole of the blood carried to the lungs by those vessels — that 

 part which is distributed to the smaller bronchial tubes and the alveola) being brought back by 

 the pulmonary veins. The posterior bronchial veins issue from the lung substance behind the 

 structures forming the root of the lung. The right vein generally joins the vena azygos just 

 before the latter vein enters the superior vena cava. The left vein opens into accessory hemi- 

 azygos vein. The bronchial veins at the root of the lung receive small tributaries from the 

 bronchial glands, from the trachea, and from the posterior mediastinum. 



The oesophageal veins [vv. oesophageal] from the thoracic portion of the oesophagus end 

 in part in the vena azygos, and in part in the vena hemiazygos. They anastomose with the 

 upper oesophageal veins, and with the coronary vein. 



The posterior mediastinal veins, small and numerous, open into the azygos and hemiazygos 

 veins. 



2. THE VEINS OF THE VERTEBRAL COLUMN 



The venous plexuses around and within the vertebral column extending from 

 the cranium to the coccyx may be divided into two categories: — (1) the external 

 and (2) the internal vertebral venous plexuses. The external plexuses consist 

 of two parts, the anterior vertebral venous plexuses situated on the anterior aspect 

 of the vertebral bodies and the posterior vertebral venous plexuses ramifying over 

 the posterior aspect of the vertebral arches, spines, and transverse processes. 

 The internal plexuses con.sist of tivo longitudinal venous sinuses situated between 

 the vertebrae and tlie posterior longitudinal ligament, and of two vertebral venous 

 retia placed immediately external to the dura mater. The sinuses of the internal 

 plexuses communicate freely with one another and with the internal retia and 

 external plexu.ses. They receive the external spinal veins and the basivertebral 

 veins from the bodies of the vertebrae. The venous circulation of the vertebral 



