skull, the skeletal structure of the head, composed of the facial and cranial bones. The skull houses and protects the brain and most of the chief sense organs; i.e., the eyes, ears, nose, and tongue. Among humans, some 14 bones shape the face, most occurring in symmetrical pairs. They are the lacrimals at the inner sides of the eyes, the nasals and nasal conchae of the nose, the palatines (palate), the zygomatics, or malars at the cheeks, the vomer (nasal septum), and the maxillae, or upper jaw. The mandible, or lower jaw, is not technically part of the skull. The adult human cranium, or braincase, is formed of fused skull bones: the parietals, temporals, ethmoid, sphenoid, frontal, and occipital. These are separate plates of bone in the fetus, but by birth they have generally grown sufficiently for most of their edges to meet. The remaining separations are known as fontanels, the most prominent being the soft spot atop a newborn's head. By the age of two years, all of these fontanels have been closed over by the growing cranial bones. However, the seams, or sutures, between the bones do not completely knit until the age of 20. The occipital bone at the base of the skull forms a complex joint with the first vertebra of the neck, known as the atlas, permitting rotation and bending of the head (see spinal column). Study of the fossil skulls of humans and their precursors has made important contributions to evolutionary theory, and to the science of physical anthropology. Earlier skulls of human ancestors, for instance, have been shown to have markedly smaller cranial capacities, as well as more powerful jaws, than do the Homo sapiens species which exist today.