The two main features of a long bone are the diaphysis and the epiphysis.
The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The walls of the diaphysis are composed of dense and hard compact bone made of numerous osteons — the functional unit of the compact bone. The hollow region in the diaphysis is called the medullary cavity, which harbors the bone marrow. In infants and children, this marrow cavity is filled with red marrow, whereas in adults, it is filled with yellow marrow.
The broad section at each end of the bone, called the epiphysis (plural – epiphyses), is filled with spongy or cancellous bone. Red marrow fills the spaces in the spongy bone. Each epiphysis meets the diaphysis at the metaphysis, the thin zone that contains the epiphyseal or growth plate — a layer of hyaline cartilage in a growing bone. When the bone stops growing in early adulthood, the cartilage is replaced by osseous tissue, and the epiphyseal plate becomes an epiphyseal line.
The outer surface of the bone is covered with a fibrous membrane called the periosteum. It contains blood vessels, nerves, and lymphatic vessels that nourish the compact bone. Tendons and ligaments also attach to bones at the periosteum. The periosteum covers the outer surface except where the epiphyses meet other bones to form joints. This region is covered with articular cartilage, a thin layer of hyaline cartilage that reduces friction and acts as a shock absorber. Unlike the periosteum, the cartilage is avascular and cannot heal as quickly as other bone tissues.
Flat bones, such as those of the cranium, have a slightly different anatomy. They consist of spongy bone lined on either side by a layer of compact bone. The two layers of compact bone and the interior spongy bone work together to protect the internal organs.
Part of this text is adapted from Openstax, Anatomy and Physiology 2e, Section 6.3: Bone Structure
