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The oblique fissures divide the lung on both sides into upper and lower lobes. The upper lobe on the right is further subdivided by the horizontal fissure into definitive upper lobe - above the fissure - and middle lobe - below the fissure. The fissures extend from the surface of the lung to its hilum. Along this route, visceral pleura apposes visceral pleura. Both surfaces are smooth and separated by a layer of lubricant fluid. This allows individual lobes to move freely with respect to one another.
The fissure extends on the left from the tip of the spinous process of the T3 vertebra inferiorly around the thorax to the level of sixth costochondral junction anteriorly. In taking this route, the approximate path of the sixth rib is followed. The posterior origin of the fissure on the right is slightly inferior - the inferior margin of the T4 vertebra.
In roughly half of all lungs, the oblique fissure is incomplete and the lobes may still be connected.
The oblique fissure may be an evolutionary development to permit greater transmission of diaphragmatic excursions to the superior lobe. In this way, the upper lobe can expand to a relatively greater extent for a given increase in the superoinferior diameter of the pleural cavity.