The contents of the mediastinum take part in a different degree in its inspiratory longitudinal expansion: Heart, pulmonary root, and the bifurcation of the trachea – all of them connected with the diaphragm by pericardium or membrana bronchopericardiaca -move caudad proportional to the sinking of the diaphragm. Compared with the corona cordis the planum cardiacum moves further caudad according to its distance from the upper apertura thoracis. So the heart necessarily has to compensate for this difference by taking an upright position and by rotation to the right. The arcus aortae, however, fixed in the neck by its three main branches cannot perform such a proportional excursion. A significant distance of the left pulmonary root from the aortic arch is the result. At the same time a subaortal space, the ‘spatium subaortale mediastini’, unfolds into which a bulge of the pleura mediastinalis moves. This bulge is filled with tissue of the upper lobe of the left
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