Horizontal fissure of right lung

The horizontal fissure of the right lung (Minor fissure; Right horizontal fissure) is a cleft in the visceral pleura of the right lung that separates the right upper lobe from the right middle lobe. It is unique to the right lung, as the left lung does not normally possess a horizontal fissure.

Anatomical course: The horizontal fissure typically originates from the right oblique (major) fissure at approximately its midpoint and extends anteriorly and medially toward the anterior thoracic wall. On the lung surface, it most commonly meets the anterior chest wall at the level of the fourth rib (or fourth costal cartilage) and originates posteriorly from the middle third of the right oblique fissure. On the mediastinal surface it may be traced backward to the hilum.

Orientation: The minor fissure is generally oriented so that its anterior part is lower than its posterior part, and its lateral margin is lower than its medial margin.

Variability: The horizontal fissure is the most variable of the three main pulmonary fissures. A metaanalysis of anatomical studies found it to be complete in approximately 54%, incomplete in 35%, and absent in 11% of individuals.