The left lung has two lobes and the right has three
Each lobe has its own pleural covering
The horizontal fissure (right) is often seen on a normal frontal view
The oblique fissures may be seen on a normal lateral view
The visceral pleura covers the lung surfaces and is continuous with the visceral pleura that covers the fissures.
The left lung is divided into two lobes, upper and lower, by the oblique (major) fissure.
The right lung has two fissures, oblique fissure and horizontal fissure, which separate the lung into three lobes - upper, middle, and lower. Each lobe has its own visceral pleural covering.
Only the horizontal fissure is commonly seen on a frontal chest X-ray. The oblique fissures (also known as the major fissures) may be visible on lateral images.
This cut-out of a lateral chest X-ray shows the positions of the three lobes of the right lung (roll over image) separated by the horizontal and oblique (or major) fissures
The azygos fissure is the most common accessory fissure visible on a chest X-ray (1-2% of individuals)
In most people the azygos vein passes horizontally along the right side of the mediastinum
In those with an azygos fissure a small section of the right upper lobe (an azygos lobe) develops medial to the vein which is pushed away from the mediastinum
The azygos vein appears to pass through the lung and is surrounded by both parietal and visceral pleura
The azygos fissure therefore consists of four layers of pleura, two parietal layers and two visceral layers
The appearance resembles a tadpole - the head is the azygos vein and the tail the azygos fissure
Note that the azygos lobe is not a separate lobe - it is part of the right upper lobe
Assessing the fissures
Occasionally lung disease is limited in extent by a fissure. This can help locate a disease process more specifically to a lobe. For most cases this degree of accuracy is not clinically important, unless further action such as biopsy or surgery is required, in which case other imaging such as CT would probably be performed. In most cases it is still best to refer to the location of lung abnormalities seen on a chest X-ray in terms of lung zones rather than lobes.
