In human anatomy, the thoracic duct is the larger of the two lymph ducts of the lymphatic system. The thoracic duct usually begins from the upper aspect of the cisterna chyli, passing out of the abdomen through the aortic hiatus into first the posterior mediastinum and then the superior mediastinum, extending as high up as the root of the neck before descending to drain into the systemic circulation at the venous angle.