INTRODUCTION Azygos system of veins taken out for study because its nearness to viscera-like intrathoracic course of esophagus, aorta, heart, lungs and its intimate relationship with contents of posterior mediastinum. Azygos system includes Azygos vein and accessory Azygos vein and Hemiazygos vein. The present study carried out on hundred cadavers, out of which foetus 10, children 8, adults 82, in three years span in RIMS Medical College, Kadapa. Their formation, course, relations, termination, their tributaries and communications are explored. Different variations are observed. Knowledge of such variations and drainage pattern is significant in abdominal surgeries also plays an important role in CT and MRI studies on posterior mediastinum. Variations in Azygos system also useful for thoracic surgeon because its vulnerability to damage during the surgical procedures, injuries, firearm wounds.

ABSTRACT:The cause of venous compromise is multifactorial. The venous system variations are generally explained on the basis of their embryological basis. Variations of azygos venous system is not clearly described in the literature. Multiple variations like mode of formation of azygos vein formed mostly by the union of ascending lumbar and subcostal veins, position of azygos vein which courses normally to the right side forms in the midline and on left side in some cases. Variations in the mode of termination of Azgos vein, in formation of Hemi azygos vein, mode of termination of Hemi azygos vein are explained in view of their embryological development. Venous abnormalities often complicate mediastina surgery with intra operative haemorrhage. Prior knowledge of possible anatomical variations may help the surgeons to reduce the risk of such events. KEYWORDS: Azygos vein (AZV), Hemiazygos vein (HAZV), Accessory hemiazygos vein (AHAZV), Inferior vena cava (IVC). INTRODUCTION:The azygos venous system develops in the basis of multiple transformations of the subcardinal veins, 1 which causes its great variability, especially on the left side.Azygos veins are important cavo-caval and porto caval junctions, thus forming collateral circulation in caval vein occlusion and in portal hypertension. 2 The azygos venous system trnsporting deoxygenated blood from the posterior wall of the thoracic and abdomen into the superior vena cava is expected to arise from the postrior aspect of inferior vena cava at or below the level of renal veins from its development. But it is not constant. It has no symmetrically equivalent vein on the left side of the body.this system varies much in the mode of orign, course, tributaries anastomosis and termination.Azygos vein is formed by the union of the ascending lumbar veins with the right subcostal veins at the level of the 12 th thoracic vertebra, ascending in the posterior mediastinum, and arching over the right main bronchus at the root of the right lung to join the superior venacave. Its tributaries, apart from its main tributary, the hemiazygos vein are the bronchial veins, pericardial veins, and posterior right intercostal veins.The azygos system of veins is considered to be, the azygos vein, along with its left-sided counterparts, the hemiazygos vein and the accessory hemiazygos vein. Together they form an anastomosis between the superior vena cava to the inferior vena cava.The azygos venous system is a main channel of drainage of the thoracic wall and also part of parietals of the abdomen. The large continuous channel draining the right half of the thoracic and abdominal parietals forms the agygos vein. The left complement of this venous channel which drain the upper abdominal parietals and the lower thoracic wall in named as accessory hemoazygos vein or superior hemiazygos vein. Both of them drain into the azygos vein