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Lymphatic ducts empty lymph fluid into the venous system. The two lymphatic ducts of the body are the right lymphatic duct and the thoracic duct. The thoracic duct is the larger of the two and responsible for lymph drainage from the entire body except for the right sides of the head and neck, the right side of the thorax, and the right upper extremity which are primarily drained by the right lymphatic duct.[1][2] Structure and Course The thoracic duct is 38 to 45 centimeters long and 2 to 5 millimeters in diameter. It runs from the superior aspect of the cisterna chyli, a lymph sac at the L2 vertebral level, to the lower cervical spine. From the cisterna chyli, the duct continues superiorly, running between the aorta and the azygos vein and anterior to the vertebral column. The thoracic duct ascends through the aortic hiatus of the diaphragm entering the posterior mediastinum, still to the right of the vertebral column. It courses posterior to the esophagus at the T7 level and crosses over the midline to the left side of the thorax around the T5 vertebral level. As it continues upward, it runs behind the aorta and to the left of the esophagus ascending 2-3 cm above the clavicle. In the superior mediastinum, it passes behind the left common carotid artery, the vagus nerve, and the internal jugular vein. It then descends to empty into the junction of the left subclavian and internal jugular veins. The wall of the thoracic duct has three layers: the intima, the media, and the adventitia. It also has a basement membrane. The media is composed of smooth muscle and connective tissue. The smooth muscle contracts regularly to move lymph flow forward. The thoracic duct also contains valves which may be unicuspid, bicuspid, or tricuspid, but are usually bicuspid. At the junction of the lymphatic and venous system, a bicuspid valve prevents venous backflow into the lymphatic system.[3]