The<unk>  (or<unk>  primary<unk> , or<unk>  principal<unk>  is smaller in caliber but longer than the right, being nearly 5 cm long. It enters the root of the<unk>  lung opposite the sixth thoracic vertebra.
It passes beneath the aortic arch, crosses in front of the esophagus, the thoracic duct, and the descending aorta, and has the<unk>  pulmonary artery lying at first above, and then in front of it.
The<unk><unk>  has no eparterial branch, and therefore it has been supposed by some that there is no upper lobe to the<unk>  lung, but that the so-called upper lobe corresponds to the middle lobe of the right lung.
Left main bronchus
Tracheal bronchus
Secondary bronchus
Hilum of lung
Tertiary bronchus
Costomediastinal recess
Mediastinal pleura
Intercostal nerves