At birth the skull is large in proportion to the other parts of the skeleton, but the base is short and narrow in proportion to the vault ; the facial portion is small and equals only about one-eighth of the bulk of the cranium as compared with one-half in the adult. The frontal and parietal eminences are prominent, and when the skull is viewed from above it presents a pentagonal outline with its greatest width at the parietal eminences (fig. 397) on the other hand, the glabella, superciliary arches, and mastoid processes are not developed. Ossification of the skull bones is not completed, and many of them – e.g, the occipital, temporals, sphenoid, frontal, and mandible-consist of more than one piece. Unossified membranous intervals, termed fontanelles (fonticuli), are seen at the angles of the parietal bones ; these fontanelles are six in number; two, the anterior (frontal) and posterior (occipital), are situated in the median plane, and two, the anterolateral (sphenoid) and posterolateral (mastoid), on each side.
The fontanelles are usually closed by the growth and extension of the bones which surround them, but sometimes they are the sites of separate centres of ossification which develop into sutural bones. The posterior and anterolateral fontanelles are obliterated within two or three months after birth ; the posterolateral fontanelle is usually closed about the end of the first year, and the anterior fontanelle about the middle of the second year.
The smallness of the face at birth is mainly accounted for by the rudimentary condition of the mandible and maxillae, the non-eruption of the teeth, and the small size of the maxillary sinuses and nasal cavities. At birth the nasal cavities lie almost entirely between the orbits, and the lower border of the anterior nasal aperture is only a little below the level of the orbital floor. With the eruption of the deciduous teeth, there is an enlargement of the face and jaws, and these changes are still more marked after the second dentition.
The skull grows rapidly from birth to the seventh year, but the greater part of the increase of its cranial part occurs during the first year owing to the rapidity of the growth of the brain in that period. Growth in length occurs at the coronal suture and at the other sutures which are parallel to it ; growth in breadth, at the sagittal and parallel sutures ; growth in height, at the parietotemporal and at the other sutures which lie in a transverse plane. Towards the end of the second year the mastoid process makes its appearance, and the tympanic ring lengthens to form the bony part of the external auditory meatus. As a result the styloid process and the stylomastoid foramen, which are relatively superficial in the newly-born child, become more deeply situated. At the seventh year the cribriform plate of the ethmoid bone, the foramen magnum, and the petrous parts of the temporal bones have reached their full size, and the orbital cavities are only a little smaller than those of the adult. Growth is slow from the seventh year until the approach of puberty, when a second period of increased activity occurs : this results in an enlargement in all directions, but especially in the frontal and facial regions, where it is associated with the development of the air-sinuses. The enlargement of the cranium is effected, in its latter stages, by absorption from the inner surface and by bone deposition on the outer surface. This process chiefly affects the bones of the vault and of the lateral aspects of the skull.
In old age the skull generally becomes thinner and lighter, but in a small proportion of cases it increases in thickness and weight. The most striking feature of the senile skull is the diminution in the size of the mandible and maxilla consequent on the loss of the teeth and the absorption of the alveolar processes. This is associated with a marked reduction in the vertical measurement of the face and with an alteration in the angles of the mandible.
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