Azygos fissure

Sometimes the azygos vein can be seen clearly on chest x-ray. And sometimes you can even see an "azygos fissure". This fissure delineates a accessory "azygos lobe".

It's actually a congenital failure of normal migration of the azygos vein from the chest wall to its place above the tracheo-bronchial angle. The invaginated visceral and parietal pleural layers persist to form a fissure.

Normally it's just a variant of the normal. But it can cause trouble during thoracotomy(bleeding) and one case of torsion of the azygos lobe has been described.

For further reading:

http://www.ispub.com/journal/the_internet_journal_of_anesthesiology/volume_11_number_1_1/article_printable/the_azygos_lobe_anesthetic_considerations.html