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The cardiac notch is the lateral deflection of the anterior border of the left lung. It is produced to accommodate the space taken up by the heart. It leaves a small anterior area of pericardium in direct contact with the sternum; this area is dull to percussion - the area of 'superficial cardiac dullness' - unlike the surrounding regions where overlying lung makes the percussion note relatively resonant.
On the mediastinal surface of the left lung it in continuity with the anterior margin of the cardiac impression.