I030 A MANUAL OF ANATOMY 



tebra, lying in front of the lower aortic intercostal arteries, and 

 it takes up the lower four left intercostal veins. It then crosses 

 the vertebral column from left to" right, under the name of the 

 lower transverse azygffs vein, passing behind the descending aorta 

 and thoracic duct, and thereafter it opens into the right azygos 

 vein. 



Tributaries. — These are as follows : 



1. The left subcostal vein. 



2. The lower four left intercostal veins. 



3. Some oesophageal veins. 



The upper left, or accessory, azygos vein is formed by the union 

 of the fifth, sixth, and seventh left intercostal veins (sometimes 

 also the fourth). At the level of the seventh thoracic vertebra it 

 crosses the vertebral column from left to right, under the name of 

 the upper transverse azygos vein, passing behind the descending 

 aorta and thoracic duct, and thereafter it opens into the right 

 azygos vein. It communicates above with the left superior inter- 

 costal vein, and below with the lower left azygos vein. 



Tributaries. — ^These are as follows : 



1. The fifth, sixth, and seventh left intercostal veins (sometimes 

 also the fourth). 



2. The left bronchial vein, as a rule. 



3. Some oesophageal veins. 



The lower and upper left azygos veins, which are subject to much 

 variation, sometimes unite to form one transverse azygos vein. 



Summary of the Azygos Veins — Right Azygos Vein. — This vessel receives 

 (i) the right subcostal vein; (2) the lower seven (sometimes the lower eight) 

 right intercostal veins ; (3) the right superior intercostal vein ; (4) the right 

 bronchial vein ; (5) some "oesophageal veins ; (6) some pericardial veins ; and 

 (7) the lower and upper transverse azygos veins. 



Lower Left Azygos Vein. — This vessel receives (i) the left subcostal vein; 

 (2) the lower four left intercostal veins ; and {3) some cesophageal veins. 



Upper Left Azygos Vein. — This vessel receives (i) the fifth, sixth, and 

 seventh (sometimes also the fourth) left intercostal veins ; (2) the left 

 bronchial vein, as a rule; and (3) some oesophageal veins. 



The right azygos vein and the lower left azygos vein, through 

 their connections with the ascending lumbar veins, establish com- 

 munications with tjie inferior vena cava and with the common iliac 

 veins or some of their tributaries. They therefore form important 

 channels by which a considerable quantity of blood is returned 

 from the lower limbs and abdominal wall in cases of obstruction 

 of the inferior vena cava. 



Development. — The right azygos vein is developed from the thoracic 

 portion of the right cardinal vein. The lower and upper left azygos veins 

 are developed from the thoracic portion of the left cardinal vein. 



Subcostal Veins. — These are two in number, right and left, and 

 they are serially continuous with the intercostal veins. Each 

 vessel enters the thor^^-f fOffiT the' abd>omen by passing behind the 



A M A 



