The absorption, transportation and elimination of gases in the living body have long been subjects of theoretical interest and extensive investigation. They have now become matters of great clinical importance also. This is especially the case in regard to two conditions which appear at first sight to be diametrically opposed to each other in all essential characters. They are the collapse, or atelectasis, of a lung when a bronchus is closed, and the reexpansion of a lung that has been collapsed by pneumothorax. The forces involved can both inflate and deflate a lung. It is therefore timely to set forth the mechanics of gas absorption from a lung or from any cavity in the body in which air or any other gas happens to be occluded. In the development of atelectasis the first stage is the complete shutting off and cessation of ventilation in a part or the whole of