Which bone is not part of the axial skeleton?

The femur is not part of the axial skeleton; rather, it belongs to the appendicular skeleton. The axial skeleton primarily consists of the bones that form the long axis of the body, which includes the vertebrae, skull, and ribs. These structures support the central axis and protect vital organs such as the brain and spinal cord. In contrast, the femur, which is the thigh bone, is involved in limb movement and support, thus categorizing it under the appendicular skeleton, which includes the bones of the limbs and their attachments to the axial skeleton. This distinction is crucial for understanding the organization of the human skeleton in terms of function and location.