Breathing
Breathing in = The intercostal muscles between the ribs contract, this raises the ribs up and out. The mucles of the diaphragm contract, the diaghragm flattens. These increase the volume of the thorax, the inside the thorax decreases. Air is sucked into the lungs
Breathing out = The intercostal muscled relax, this lowers the ribs down and in. The muscles of the diaphragm relax, the diaphagm moves up. These decrease the volume of the thorax, the pressure inside the thorax increases. Air is forced out of the lungs.