The humerus is the only bone in the upper arm. Like many of the long bones in the body, the humerus has a proximal end, shaft, and distal end. Proximal refers to the area closest to the center of the body -- the shoulder. Distal is the end of the bone closest to the elbow.
Long bones like the humerus support a significant load while the arm carries out various functions, and has a specific internal structure. The outside of the bone has a layer of connective tissue -- the periosteum. The outer layer of the bone itself is hard and compact, surrounding an internal layer of spongy cancellous bone. The innermost medullary cavity contains the bone marrow.
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