Supraorbital ridge and masticatory apparatus I: Primates

The supraorbital ridge formation in apes is a result of the traction of the masticatory force exerted by the anterior part of the temporalis muscle. This force varies inversely with the ratio of the power arm to the load arm of the mandible. The smaller this ratio, the greater the anterior part of the temporalis muscle and the larger the supraorbital ridge. The size of the ridge is independent of the actual size of the skull. The orang has a larger skull but the smallest brow ridge of the three species studied: gorilla, chimpanzee and orang. Pearson’s correlation coefficient shows a positive correlation between the anterior temporalis muscle and the ridge as well as between the ridge/sinus and anterior/whole temporalis muscle. The correlation is negative between the power/load arms and anterior/whole muscle and also between the power/load arms and the ridge/sinus ratio.