Quadrangular Membrane

The quadrangular membrane (membranaquadrangularis) is a layer of thickened connective tissue membrane, which is the fourth outer layer of the parietal pleura of the lung. In the lower part, a fibrous-fatty mediastinal defect passes through it - the diaphragmatic-cartilaginous suture of the left and right lungs.

The quadrangular cavity is bounded in front by the two-layer Boves diaphragm, behind by the mediastinal part of the diaphragm, below by serrated chords (4 in each lung) and above by a system of connective tissue plates supported by spiral fibrous ligaments (6 in each lung).

In the walls of the cavity there are 3 villous cartilages and pulmonary poles with free segments of the main bronchi and descending branches of their lobar bronchi entering them. Inside the cavity there is visceral pleura, a smooth connective tissue film adherent to its wall.

The walls and intrapleural part of the membrane are abundantly supplied with nerves and lymphatics.