Which part of the bone such as the femur prevents the skeleton from becoming too heavy?

The part of the bone that helps prevent the skeleton from becoming too heavy is the medullary cavity, which is the hollow center of long bones like the femur. This cavity contains bone marrow and is filled with fat, reducing the overall weight of the bone while still maintaining strength and structural integrity. Additionally, the trabecular (or spongy) bone found at the ends of long bones also contributes to a lighter skeleton by providing a porous structure.