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The horizontal fissure of the lung is a recess in the parenchyma of the right superior lobe of the lung. Its surface markings are from the level of the right fourth costal cartilage horizontally to a junction with the oblique fissure at approximately the midaxillary line in the fifth intercostal space. It divides up the right-sided volume of lung tissue above the oblique fissure into:
The cleft extends deeply from lung surface to hilum. Over this area, the superior surface of the middle lobe is in contact with the inferior surface of the superior lobe. Both visceral layers are very smooth and are separated by a film of serous fluid. This facilitates movement of the lobes relative to one another throughout the phases of respiration.
In the majority of individuals, the horizontal fissure fails to completely divide superior from middle lobes.