The hard outer layer of bone.

The hard outer layer of bone is known as **cortical bone** or **compact bone**. It is dense and forms the protective outer shell of bones, providing strength and structural support. Cortical bone is critical for the stability of the skeleton and is responsible for the majority of the bone's strength and load-bearing capacity. Beneath the cortical bone lies the **trabecular bone** (or spongy bone), which has a more porous structure and is involved in metabolic functions, including the storage of bone marrow and the production of blood cells.