Many bones have epiphyses at both ends, others at one end only.
Long limb bones show the former, while the metacarpals, metatarsals, phalanges, clavicles and ribs have only one epiphysis, though the costal cartilages may represent epiphyses normally devoid of ossification centres.
Epiphyseal ossification is sometimes more complex; for example, the proximal end of the humerus, wholly cartilaginous at birth, develops three centres during childhood, which coalesce into a single mass before fusing with the diaphysis. Only one of these centres forms an articular surface, the others forming the greater and lesser tubercles giving muscular attachments. Similar composite epiphyses occur at the distal end of the humerus and in the femur, ribs and vertebrae.
Ossification centres appear in regions
1. Exposed to articular pressure, [pressure epiphyses, appearing at the articular ends of the bones and transmitting the weight of the body from bone to bone]
and in regions
2. Subject to muscular traction,[traction epiphyses, associated with the insertion of muscles and originally sesamoid structures though not necessarily sesamoid bones;]
and others in regions
3. Considered to represent skeletal elements separate at earlier evolutionary stages. [atavistic epiphyses, representing parts of the skeleton, which at one time formed separate bones, but which have lost their function, and only appear as separate ossifications in early life.]
Comparative morphology suggests that some mammalian bones are composites of separate reptilian or amphibian elements. The skull, clavicle, scapula and innominate bones are examples; a small centre in the human coracoid process and an epiphysis at the medial end of the clavicle may be vestiges of skeletal elements separate in earlier vertebrates, repeated during development as transient features in subsequent mammalian forms. However, the medial end of the clavicle could equally be regarded as a pressure epiphysis