Discussion on pressure, air pressure, volume, and relationship between volume and pressure... Anatomy of the pleural cavity showing lungs, ribs and muscles, and diaphragm... How the diaphragm and the ribs move to increase the pleural cavity, reduce the pleural pressure below atmospheric pressure and have air rush into the lungs due to the decrease in internal pressure... Expiration was shown to be the reverse reaction. The stimulus of these muscles by brain messages, and the control of the the stimulus from the higher regions was discussed.