[Morphology of the basi-exoccipital synchondrosis during human growth]

The morphology of the basi-exoccipital synchondrosis has been studied in a series of 150 human skulls. This series consisted of 12 skulls of fetuses, 12 skulls of new-born children, 86 skulls of children and 40 skulls of adult subjects. The synchondrosis was composed of two plates: a principal vertical plate or basi-exoccipital plate, and a accessory horizontal plate or hypoglossal prolongation. The exoccipital bone alone participated in the formation of the walls of the hypoglossal nerve's canal. In the first stages of development, the anterior wall of this canal was cartilaginous, formed by the posterior edge of the hypoglossal prolongation of the synchondrosis. The occipital condyle derived from the basioccipital and the exoccipital bones; the inferior edge of the basi-exoccipital plate separated these two parts. The relative proportions of the anterior basioccipital part compared with the posterior exoccipital part varied from the fifth to the seventh of the total condyle area. The observations could have been arranged in six age groups presenting morphological similarities of development. At first, the hypoglossal processes of the exoccipital bone drew together (VI intra-uterine months to 1 year 6 months), then came into contact (1 year 6 months to 2 years 6 months), and finally fused together (2 years 6 months to 4 years) realizing the bony continuity of the walls of the hypoglossal nerve's canal. The contact and finally the fusion between the basioccipital and exoccipital bones were then realized (4 years to 8 years). The entire fusion of the synchondrosis was completed between the ages of 6 and 8 years. The adult morphology was established at around 8 years of age.