6 



Scientific Proceedings (25). 



ing it on its back. Sometimes it is even possible to resuscitate an 

 animal that has stopped breathing by thus turning it on its belly- 



A dog on its back will sometimes stand a double pneumothorax 

 when the opening in each pleura is a very small one, not more 

 than 1-2 mm. in diameter and very slowly made. But with the 

 dog on its belly, an opening almost 1 cm. in diameter can some- 

 times be made in each pleura, if cautiously done, and the animal 

 will often continue to breathe and survive for hours. The amount 

 of inspired and expired air in such an animal is surely small as 

 compared with the normal, but it is apparently sufficient to keep 

 the animal alive. 



The explanation for this great difference between an animal on 

 its belly and on its back with an open pneumothorax, is not an 

 easy one. The following considerations and experiments are 

 offered as a preliminary contribution to a solution of the problem. 



The two pleural cavities are separated by the layers of the an- 

 terior and posterior mediastinal septa. Between the two lies the 

 heart. In the dog, the posterior seems to be somewhat tougher 

 than the anterior, and somewhat more fixed and tense. With vio- 

 lent 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 death of 

 the animal. When the dog is on its back, the heart falls back- 

 ward and the bulging of the anterior mediastinal septum is made 

 more easy. It is different when the animal is on its belly. The 

 heart then falls toward the anterior chest wall and thus supports 

 the anterior septum; hence the flapping of the septum, the inter- 

 ference with the respiration of the lung on the sound side, the 

 bulging on expiration on the open side, cannot so readily occur. 

 The following experiments, which I have repeated several times, 

 are, I think, of importance in this connection. 



After several ribs had been resected and before the pleura was 

 opened, with the dog lying on his back, the pericardium was 

 attached to the anterior chest wall by a suture, so that the heart 

 was pulled forward. A very large opening could now be made 

 in the pleura, almost as large as when the animal was on its 

 belly, without causing the occurrence of serious symptoms. The 



