Bones are hard substances that form the framework, give shape, support, and protect the vital organs of the human body. .
Bones are classified easiest by shape. There are the long bones, short bones, flat bones, irregular, and the sesamoid. .
Long bones typically make up the limbs. They consist of a central, usually hollow, tubular region, and the diaphysis, which is linked to the epiphysis by a metaphysis. Near the ends of the shaft the marrow cavity has a tendency to be broader and it is occupied with trabecular bone, arranged along lines of force which has a skeletal function in its own right and sustains the marrow.
Short bones are found in the wrist and ankle, carpals and tarsals and have no shaft in view of the fact that they do not grow significantly in size during growth. They tend to be cuboidal in shape, and have a cancellous bone in the center and a hard outer shell of compact bone.
Flat bones are found in the cranium or the scapula and are sandwiches of spongy bone amid two layers of compact bone and are usually curved.
Irregular bones are any bones that don't fit these arbitrary categories (bones of the face, vertebrae).
Sesamoid bones occur in tendon, particularly where a tendon turns a corner, and is as a result exposed to friction.
Bones are not constructed of dead tissue, but of cells, nerves, blood vessels and pain receptors. Two forms that make up the bone are compact and cancellous. The compact form of the bone is the solid part on the outside of the bone, and the cancellous form appears spongy and is on the ends of the bone inside the compact bone, supporting the end of the bone and transferring weight from the bone ends to the bone shaft. On the outside of the bone is a fibrous membrane, called the periosteum, and the inside is lined by the endosteum. In the center of the bone is the medullary cavity, and that is filled with red, active marrow in children, and yellow, fatty and inactive marrow in adults.