How does the thoracic cage move during respiration?

The thoracic cage plays a crucial role in the mechanics of breathing, particularly during the processes of inspiration and expiration. During inspiration, the diaphragm contracts and moves downwards, while the intercostal muscles between the ribs contract as well. This contraction of the intercostal muscles causes the rib cage to widen and elevate, increasing the volume of the thoracic cavity. As a result, air is drawn into the lungs.

During expiration, particularly during quiet breathing, the diaphragm relaxes and returns to its dome shape, and the intercostal muscles also relax, allowing the rib cage to return to its resting position. This return to the normal width of the thoracic cage helps expel air from the lungs.

Therefore, the correct understanding is that the thoracic cage widens during inspiration, allowing for increased lung capacity and air intake, and returns to its original position during expiration. This process is dynamic and essential for effective respiration, making the widening during inspiration and the return to normal during expiration the accurate description of how the thoracic cage moves.