Horizontal fissure of the lung

The horizontal fissure of the right lung is a deep groove that separates the superior from the middle lobe. It begins in the oblique fissure near the posterior border of the lung and passes horizontally forward. The horizontal fissure cuts the anterior border of the right lung on a level with the sternal end of the fourth costal cartilage. On the mediastinal surface of the right lung, it can be traced posteriorly to the hilum.