The accessory hemiazygos vein (superior hemiazygos vein) descends on the left side from the superior portion of the posterior mediastinum to approximately vertebral level TVIII (Fig. 3.102). At this point, it crosses the vertebral column to join the azygos vein, or ends in the hemiazygos vein, or has a connection to both veins. Usually, it also has a connection superiorly to the left superior intercostal vein.
Vessels that drain into the accessory hemiazygos vein include: the fourth to eighth left posterior intercostal veins, and sometimes, the left bronchial veins.
Thoracic duct in the posterior mediastinum
The thoracic duct is the principal channel through which lymph from most of the body is returned to the venous system. It begins as a confluence of lymph trunks in the abdomen, sometimes forming a saccular dilation referred to as the cisterna chyli (chyle cistern), which drains the abdominal viscera and walls, pelvis, perineum, and lower limbs.
The thoracic duct extends from vertebra LII to the root of the neck.
Entering the thorax, posterior to the aorta, through the aortic hiatus of the diaphragm, the thoracic duct ascends through the posterior mediastinum to the right of midline between the thoracic aorta on the left and the azygos vein on the right (Fig. 3.103). It lies posterior to the diaphragm and the esophagus and anterior to the bodies of the vertebrae.
At vertebral level TV, the thoracic duct moves to the left of midline and enters the superior mediastinum. It continues through the superior mediastinum and into the neck.
After being joined, in most cases, by the left jugular trunk, which drains the left side of the head and neck, and the left subclavian trunk, which drains the left upper limb, the thoracic duct empties into the junction of the left subclavian and left internal jugular veins.
The thoracic duct usually receives the contents from: the confluence of lymph trunks in the abdomen, descending thoracic lymph trunks draining the lower six or seven intercostal spaces on both sides, upper intercostal lymph trunks draining the upper left five or six intercostal spaces, ducts from posterior mediastinal nodes, and ducts from posterior diaphragmatic nodes.
The sympathetic trunks are an important component of the sympathetic part of the autonomic division of the peripheral nervous system and are usually considered a component of the posterior mediastinum as they pass through the thorax.
This portion of the sympathetic trunks consists of two parallel cords punctuated by 11 or 12 ganglia (Fig. 3.104). The ganglia are connected to adjacent thoracic spinal nerves by white and gray rami communicantes and are numbered according to the thoracic spinal nerve with which they are associated.
In the superior portion of the posterior mediastinum, the trunks are anterior to the neck of the ribs. Inferiorly, they become more medial in position until they lie on the lateral aspect of the vertebral bodies. The sympathetic trunks leave the thorax by passing posterior to the diaphragm under the medial arcuate ligament or through the crura of the diaphragm. Throughout their course the trunks are covered by parietal pleura.
Branches from the ganglia
Two types of medial branches are given off by the ganglia:
The first type includes branches from the upper five ganglia.
The second type includes branches from the lower seven ganglia.
The first type, which includes branches from the upper five ganglia, consists mainly of postganglionic sympathetic fibers, which supply the various thoracic viscera. These branches are relatively small, and also contain visceral afferent fibers.