This has been referred to in connection with phthisis pulmonalis, and the mode of origin there indicated is that in the great majority of cases. Of the remainder there are some in which it is due to empyema - the visceral pleura having softened and the lung having become ulcerated so as to communicate with the pleura, - and others in which it is due to gangrene of the lung, or to metastatic abscesses, or to the bursting of emphysematous vesicles, or to a traumatic cause.

The air in the pleural cavity is usually at a high pressure and the cavity is much distended, so that when the chest is opened the air rushes out with some force. The pleural cavity as exposed presents a remarkably empty appearance, the lung being compressed except where there are adhesions, which may form tense bridges across from parietal to visceral layer or may limit the pneumothorax considerably. If the patient live there is nearly always an acute suppurative pleurisy, so that the condition may be designated Pyo-pneumothorax.

Experimental observation seems to show that the air may be absorbed, and this is confirmed by actual clinical observation.