ABSTRACT
Because of their minute size and unrecognised function, lymph vessels were not observed until 1563, when Eustachius discovered the thoracic duct in a horse, and termed it the 'Vena Alba Thoracis', from its milk-like content. Tumour cells entering the lymphatics of the lung may travel centrally towards the hilar regions, or peripherally towards the sub-pleural plexus, depending on the direction of lymph flow. Becker classified the thoracic lymph nodes into parietal and visceral, distinguishing three groups: lying close to the trachea, at the bifurcation of the trachea, and at the hila of the lungs. At the end of the 19th century, many publications appeared describing the anatomy of thoracic nodes in relation to pulmonary tuberculosis. The thoracic duct arises in the upper abdomen from the cisterna chyli and enters the thorax behind and to the right of the aorta through the aortic opening in the diaphragm. In the lower mediastinum it lies posteriorly between the aorta and the azygos vein.
