What is the outermost portion of bone called?

The outermost portion of bone is referred to as the periosteum. This fibrous connective tissue layer covers the external surface of bones, serving several critical functions. It provides an attachment point for muscles and tendons, aiding in movement and structural support. Additionally, it contains blood vessels and nerves that are essential for supplying nutrients to the bone and facilitating the repair process in case of injury. The periosteum plays a vital role in bone growth and maintenance as well, being involved in the processes of bone remodeling and healing.

The other choices represent different aspects of bone anatomy. The endosteum is a thin membrane lining the inner surface of the bone, primarily involved in bone growth and repair from the inside. The cortex refers to the dense outer surface layer of the bone itself, while the medulla pertains to the interior space of bones that houses bone marrow. Each of these structures has specific functions, but in the context of the question regarding the outermost layer, the periosteum is the correct and most relevant term.