Final answer:
The thoracic duct enters the superior mediastinum of the thoracic cavity, moves behind the thoracic cavity structures, and eventually drains into the left subclavian vein.
Step-by-step explanation:
The thoracic duct enters the thoracic cavity in the superior mediastinum and travels alongside the vertebral column. It serves as the main conduit for lymph coming from the lower part of the body to reach the left subclavian vein. The duct begins in the abdomen as the cisterna chyli, receiving lymph from both the left and right lumbar trunks and the intestinal trunk that drains the lower abdomen, pelvis, and lower limbs.
In the thoracic cavity, the duct ascends behind the thoracic cavity structures such as the esophagus and the aorta, moving through the aortic opening of the diaphragm. Eventually, it empties into the venous system at the junction of the left subclavian vein and the left internal jugular vein. If a bullet were to penetrate the lung, it could affect multiple structures: the pleural cavity containing the lung, the pericardial cavity containing the heart, and the superior mediastinum where the thoracic duct resides. The first serous membrane layer it would encounter would be the parietal pleura.