The Polynesian head: growth and form.

We consider the cranial base to be the primordial determinant of the head form and mandibular shape so common amongst (but not exclusive to) adult Polynesians. The flatness of the cranial base manifests its full influence only when growth of the upper facial skeleton is complete in early adulthood. We argue that during growth and maturation the upper facial skeleton and the maxillary occlusal plane are required to adjust in position to a major extent according to the template set out by the flat cranial base, with consequent obligatory and extreme adjustment in shape and position of the mandible in order that occlusion be maintained. The base is constrained from adjusting its own shape significantly by virtue of its intimate relationship with the brain and the emerging cranial nerves. The structural consequences of these adaptations are seen in the bony profile, which is vertically disposed and orthognathic, and in the large nasopharynx, while functionally the relative inefficiency of the mandible as a lever requires extensive compensatory development of masticatory musculature which influences the shape of the face and vault.