VENA AZYGOS. 313 



wards behind them. It proceeds down the spine to the right 

 of the aorta and at a small distance from it, into the abdomen, 

 between the crura of the diaphragm, and sometimes between 

 some of the portions of that nmscle, which are attached to the 

 dorsal vertebrae. In the abdomen it often anastomoses either 

 with the lumbar veins or the vena cav^a. 



The azygos frequently sends off several small veins from its 

 curvature to the contiguous parts, and also the right Bronchial 

 Vein, which passes along the ramifications of the trachea into 

 the substance of the lungs.* In its course downwards it gives 

 off branches to the CESophagus, some of which are of conside- 

 rable size. 



The Inferior Intercostal Veins originate directly or indirectly 

 from the azygos. In some cases there is no superior intercostal 

 on the right side ; and then the two or three uppermost of the 

 right intercostals are also derived from the azygos; and often 

 originate from it by a common trunk, which soon divides. 

 Most commonly the ten inferior intercostals on the right side 

 proceed directly from the azygos, and accompany the inter- 

 costal arteries. Their posterior branches pass into the ver- 

 tebral cavity, and communicate with the veins which are 

 there. 



About the sixth or seventh rib, the vena azygos frequently 

 sends off a branch to the left, which descends on the left side 

 of the vertebrae, and sends off those left intercostal veins which 

 are below its origin. It passes through the diaphragm with 

 the aorta, or to the left of it, and anastomoses either with the 

 azygos itself, or in a way which is analogous to the anastomosis 

 of that vessel. 



The Vena Azygos may be regarded as the great trunk of 

 the veins of the parietes of the thorax, which are thus collected, 

 because they could not with convenience pass singly to the vena 

 cava, as the arteries do to the aorta. 



Soon after sending off the vena azygos, the Superior Cava 

 sends off the great transverse branch above mentioned. From 

 this it continues upwards but a short distance, when it divides, 

 * This bronchial vein sometimes arises from the superior cava. 

 VOL. II. 27 



