256 CIRCULATORY SYSTEM. 



mary vein discharges itself on its respective side into the vena 

 innominata near the superior intercostal. 



5. The Vena Azygos is placed in the posterior mediastinum^ 

 on the right anterior margin of the Dorsal Vertebrae, and dis- 

 charges itself in making an arch forwards over the root of the 

 right lung, into the descending cava, just above the introduction 

 of the latter into the pericardium. Its orifice there is supplied 

 with a membranous duplicSture or valve, which prevents the 

 blood once discharged from re-entering it. The valve is some- 

 times double, and also somewhat removed from the orifice. It 

 was the observation of these valves, first of all, which called 

 the attention of Sylvius and of Charles Etienne to their exist- 

 ence in other veins. 



This vein begins in the abdomen, either by an anastomosis 

 with the ascending cava, or with the upper lumbar vein; it then 

 ascends into the thorax through the aortic orifice of the dia- 

 phragm, and continuing to mount upwards beneath the pleura 

 it lies on the right side of the thoracic duct, and crosses in front 

 of the intercostal arteries of the right side. In this course, it 

 receives from the ten inferior intercostal spaces of the right side, 

 their respective intercostal veins corresponding in their origin 

 and course with the intercostal arteries. About the sixth ver- 

 tebra of the back, it receives a trunk (Vena Hemiazygos) which 

 is formed from the four or six lower intercostal veins of the. left 

 side, and commences in the abdomen, also, by anastomosis with 

 the left emulgent vein or the left superior lumbar, and which 

 gets into the thorax either through the aortic orifice of the dia- 

 phragm, or through a special opening to the left of it. The 

 vena azygos also receives some small ramifications from the 

 oesophagus, and near the fourth dorsal vertebra, the right bron- 

 chial vein is discharged into it. Other small ramifications join 

 it from the parietes of the descending cava, of the aorta, and of 

 the right pulmonary artery. Branches also proceed to it, or to 

 the intercostal veins, from the interior of the vertebral canal at 

 each intervertebral foramen. 



This vein is commonly spoken of by anatomists, as forming 

 a great anastomosis between the ascending and the descending 

 cava. The tendency to establish this anastomosis is strikingly 

 confirmed by a preparation made by myself, when I was a stu- 



