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When air gains access to the pleural cavity a displacement of the mediastinum frequently takes place. This mobility of the mediastinum depends on the flexibility of its tissues, the lack of pleural or pleuro-pericardial adhesions, the pressure created within the pleural space and the size or volume of the pneumothorax. The entire mediastinum may be shifted or a part of it. Thus, only the upper or only the lower portion of it may be displaced. Besides these variations in the position of the mediastinum, a protrusion of the pneumothorax cavity through the mediastinum into the sound, or untreated, side, occasionally takes place, thus forming a culdesac, or hernia. The most frequent point for a hernia of this nature to occur is in the upper anterior part of the mediastinum between the first and third ribs. Here the right pleura and the left pleura come in closest contact and are separated