A long bone is a bone that has a shaft and 2 ends and is longer than it is wide. Long bones have a thick outside layer of compact bone and an inner medullary cavity containing bone marrow. The ends of a long bone contain spongy bone and an epiphyseal plate or line depending on the stage of development. The epiphyseal plate is a hyaline cartilage, where new bone growth takes place, lengthening the bone prior to adulthood. Bone lengthening ceases when this cartilage is lost, leaving a remnant referred to as an epiphyseal line.

Endochondral_ossification is an essential process during the rudimentary formation of long bones, with the exception of the clavicle[Wikipedia:Endochondral_ossification]. The medial and and lateral ends undergo EO, the mid-portion is formed by a process with features of EO & IO (the process is shared by the mandible)[ISBN:9780397517251]