The last mentioned fact may perhaps find its explanation in the following statements: the two pleural cavities are separated by the layers of the anterior and posterior mediastinal septa. Between the two lies the heart. In the dog, the posterior seems to be somewhat tougher than the anterior septum, and somewhat more fixed and tense. With violent respiratory movements, it is the anterior septum which more especially flaps to and fro and bulges when an opening in the pleura has been made, and it is the anterior septum which is so apt to rupture and thus cause double pneumothorax and the death of the animal. When the dog is on its back, the heart falls backward and the bulging of the anterior mediastinal septum is made more easy. It is different when the animal is on its belly. The heart falls toward the anterior chest wall and thus supports the anterior septum: hence the flapping of the septum, the interference with the respiration of the lung on the sound side, the bulging on expiration on the open side, can not so readily occur.
The danger of the open pneumothorax is greatly lessened when the animal is in the prone position. In the supine position the danger of the pneumothorax is due to the falling back of the heart and thus facilitating the rupture of the fragile anterior mediastinal septum; the danger is therefore obviated by fixing the pericardium to the anterior wall of the thorax.