What is the primary function of the sternum?

The primary function of the sternum is to connect the ribs together to form a protective cage around vital organs in the thoracic cavity, particularly the heart and lungs. This structure, also known as the breastbone, plays a crucial role in the ribcage's integrity, allowing it to maintain flexibility while still providing strong support and protection.

In addition to its function in forming the ribcage, the sternum serves as an attachment point for several muscles that contribute to respiration. The connection it provides between the ribs enhances the overall stability of the chest wall, enabling efficient movements during breathing.

The other options describe functions that are more associated with different anatomical structures. For instance, while the sternum supports the heart by providing a protective barrier, the heart's support is more indirectly related to the direct role of the sternum. The arms' movement is facilitated primarily by the shoulder girdle, and the protection of the spinal cord is the function of the vertebrae surrounding it. Therefore, the connection of ribs by the sternum stands out as the most defining role of this bone.