The phylogeny and ontogeny of humeral torsion.

In a series of specimens extending from fossil material through recent vertebrates including man there occurs a gradual phylogenetic increase in the degree of humeral torsion. A further (ontogenetic) torsion is superimposed upon the phylogenetic one in man which increases from birth until the proximal epiphysial cartilage of the humerus disappears and bony fusion occurs. There is a distinct correlation between the calculated strength of humeral rotator muscles inserting above and below the proximal epiphysis; this suggests that they provide the forces involved in the production of humeral torsion. It is shown that ontogenetic or secondary torsion occurs proximally and not along the shaft of the bone. Differences in the degree of humeral torsion in either sex of adult Whites and Negroes are given and discussed.