Developmental change in a basicranial line and its relationship to the upper respiratory system in living primates

This study investigated the relationship between alterations in a basicranial line and changes in the upper respiratory system of selected non‐human primates and of man. This was done through multivariate analysis of craniometric measurements which describe midline exocranial flexion, and compares the analytical results with previous postmortem findings on upper respiratory structure and function. Craniometric analysis has shown that the skulls of the non‐human primate species studied and those of newborn human infants are relatively non‐flexed exocranially between the posterior border of the hard palate and foramen magnum. This finding corresponds to the relatively high position of their upper respiratory structures. In this group the tongue lies entirely within the oral cavity, and the epiglottis is found intranarial. After approximately the second year, humans exhibit marked exocranial flexion between the hard palate and foramen magnum. These basicranial changes coincide with concomitant changes occurring in the positional relationships of the upper respiratory system. After the second year the tongue and larynx have descended considerably into the neck, greatly altering their functional relationships. There thus appears to be a relationship between the exocranial orientation of the basicranium and the positioning of upper respiratory structures. There also appears to be a direct structural, and possible functional, relationship between (1) the position of the larynx, (2) the orientation of the pharyngeal constrictor muscles, and (3) the orientation of the basiocciput.