628 THE VENA AZYGOS. 



The sinuses of the brain, emerging at the base of the 

 cranium, become the internal jugular veins, which, with 

 the external jugulars, descend the neck, and at the root of 

 the latter unite with the subclavian upon either side, and 

 form the right and left vena-innominata, the junction of 

 which, behind the cartilage of the first rib upon the right 

 aide, constitutes the superior cava. 



This great vein is about three inches in length, and de- 

 scends within the pericardium to enter the upper portion 

 of the right auricle. It has in front of it the remains of 

 the thymus gland and some cellular structure ; behind is 

 the right pulmonary artery and the pulmonary vein ; upon 

 the right is the phrenic nerve and right lung ; upon the 

 left or internally is the ascending aorta. Just as it enters 

 the pericardium it receives at its posterior part the vena 

 azygos. 



The vena azygos (Fig. 9) returns the blood chiefly of the 

 parietes of the chest. It commences in the abdomen, op- 

 posite the second lumbar vertebra, by branches from the 

 superior lumbar veins, and sometimes also from the renal 

 and spermatic, with an occasional branch from the inferior 

 cava. Thus formed, it ascends through the aortic opening 

 in the diaphragm to the thorax, and continues upward in 

 the posterior mediastinum upon the right side of the verte- 

 bral column, having the thoracic duct and aorta upon its 

 left, and the splanchnic nerve upon its right. It increases 

 in size as it ascends, and about the fourth dorsal vertebra 

 arches over the root of the right lung to terminate in the 

 superior cava, where a valve~Ms found to prevent regurgita- 

 tion. 



This vein in its course receives the intercostal veins of 

 the right side, the azygos minor or vena Jiemiazygos of the 

 left side, which is formed from the superior left lumbar 

 veins, *nd as it ascends receives the six or seven left infe- 

 rior intercostals. It crosses the spine about the sixth or 

 seventh dorsal vertebra behind the aorta and thoracic duct, 

 and joins the right or great vena azygos. The five or six 

 left superior intercostal veins constitute what has been 



