In vertebrates, the collection of flat and irregularly shaped bones (or cartilage) that enclose the brain and the organs of sight, hearing, and smell, and provide support for the jaws. In most mammals, the skull consists of 22 bones joined by fibrous immobile joints called sutures. The floor of the skull is pierced by a large hole (foramen magnum) for the spinal cord and a number of smaller apertures through which other nerves and blood vessels pass.
The skull comprises the cranium (brain case) and the bones of the face, which include the upper jaw, enclosing the sinuses, and form the framework for the nose, eyes, and the roof of the mouth cavity. The lower jaw is hinged to the middle of the skull at its lower edge. The opening to the middle ear is located near the jaw hinge. The plate at the back of the head is jointed at its lower edge with the upper section of the spine. Inside, the skull has various shallow cavities into which fit different parts of the brain.
Development of the skull
The bones of the skull base develop in cartilage whereas the skull vault develops in membrane. At birth ossification of the skull vault is incomplete, leaving small membranous areas, notably the anterior and posterior fontanelles, which close during the second year of life. This incomplete ossification and imperfect interlocking of the bones means that the skull is less rigid at birth. The slight mobility facilitates delivery of the baby. The face at birth is smaller in proportion to the cranium than in the adult because the teeth are not fully formed and paranasal sinuses are undeveloped. The shape and size of the skull are used as means of classification by anthropologists and biologists. Thus the pre-human Homo habilis who lived about 2 million years ago had a skull (and therefore brain) volume of about 510 ml, whilst that of modern Homo sapiens has an average volume of 1,400 ml, reflecting an increase in intellectual capacity. The notable features of the human skull are the large size of the cranium, the small size of the face (even in the adult the apparatus of mastication is reduced), and the way the skull is poised on the vertebral column.
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