The adaptive mandible: a product of the relative osteo-neural growth

The growth interrelation existing between the developing brain and its bony case (with the brain representing the morphogenetically determining structure to the shape and size of which the neurocranial bony envelope is adapted) is to be conceived just as a special case of a general phenomenon, the osteo-neural relation, which works throughout the entire vertebrate body. The morphological manifestations of this relation may be disclosed in the axial as well as in the appendicular skeleton. In the present communication the osteo-neural concept is applied to the morphogenesis of the facial skeleton with special reference to the mandible. The more or less elongated shape of the mandible in various species of vertebrates depends on the phylogenetically established higher or lesser degree of growth potentiality of the mandibular nerve. With the increasing size of the brain in the course of hominization the growth-in-length capability of the mandibular nerve is (compensatorily?) decreased with corresponding shortening of the mandible and with appearance of the chin. The proposed interpretation is supported by a model experiment. The highly characteristic shortening and angulation of the lower beak which may be produced experimentally in the chick embryo by a great variety of teratogens, is related most probably to the same underlying mechanism, viz., to the primary inhibition of the highly susceptible neural growth with the secondary adaptive deformity of the beak. Pierre-Robin-Syndrome (cleft palate and micrognathia) may be readily explained by a growth insufficiency of the palatine and mandibular nerves.