312 SUPERIOR VENA CAVA AND ITS GREAT BRANCHES. 



Of the Superior or descending Vena Cava, and the veins 

 which communicate with it. 



This great vessel proceeds upwards from the superior and 

 posterior part of the right sinus or pouch of the heart ;* and a 

 portion of it is so involved by the pericardium, that it seems to 

 be included in that sac, as the heart is, in this situation. It is 

 somewhat anterior as well as to the right of the aorta. It 

 continues above the pericardium, adhering to the right lamen 

 of the mediastinum, and rather inclining forward. When it 

 is as high as the lower margin of the u[)per rib, it sends off a 

 very large branch, which conveys the venous blood of the left 

 arm and the left side of the head and neck. This large vein, 

 which is very important, both on account of its size and situa- 

 tion, proceeds in a transverse direction within the sternum, 

 almost in contact with, and but little below, the upper and 

 internal margin of that bone. Immediately behind, or within 

 the origin of the left sterno-mastoid muscle, it divides into the 

 left subclavian, which preserves a transverse course, and the 

 left internal jugular, which proceeds to the cavity of the cran- 

 ium by the foramen lacerum. 



After sending off this transverse branch of the left, the 

 great vein continues upwards and behind the right sterno- 

 mastoid muscles, and there sends off, nearly at right angles, 

 the right subclavian vein. After it has parted with this vein, 

 it takes the name of Internal Jugular, and continues to the 

 right foramen lacerum, in the basis of the cranium. The 

 superior vena cava is, therefore, principally formed by the 

 union of the subclavians and internal jugulars from each side of 

 the body. 



Immediately after the superior cava rises above the pericar- 

 dium, before it divides as above stated, it sends off, from its 

 posterior part, a large vein which is single, and, therefore, called 



Vena Azygos. 



This vessel projects backwards above the right pulmonary 

 artery and right branch of the trachea, and then curves down- 



* See description of the heart, vol. i. page 504. 



k. 



