The skull forms the skeleton of the head. It consists of many bones interconnected with each other by immobile fibrous joints called sutures. These joints fuse together in adulthood, permitting brain growth throughout development.
The skull supports the face, and provides a protective casing for the brain. Its bones can be divided into two groups, those of the cranium, and those of the face.
The cranium, also known as the neurocranium, is the bony case enclosing the brain, meninges, cranial nerves, and cerebral vasculature. In adults, it is formed by four singular bones: the frontal, ethmoidal, sphenoidal, and occipital bones, and two sets of temporal and parietal bones. Anatomically, the cranium can be further divided into the calvarium and cranial base.
The calvarium or skull cap is a dome like roof that surrounds and shields the brain. It is primarily comprised of flat bones, which are formed by intramembranous ossification of the head mesenchyme from the neural crest. These bones are the frontal bone, the occipital bone, and the two parietal bones.
The cranial base, also termed the basicranium, can be divided into anterior, middle, and posterior cranial fossae. These fossae contain the frontal and temporal lobes of the cerebral hemispheres and the cerebellum respectively.
The bones contributing to the cranial base are primarily irregular bones with some flat portions and they are formed by endochondral ossification of cartilage or from more than one type of ossification. The cranial base is made up of six bones including: the frontal, sphenoidal, ethmoidal, occipital, parietal and temporal bones.
The bones of the cranial base are important because they provide articulation points for the first cervical vertebra, the facial bones, and the mandible. The cranial base also has a large opening, the foramen magnum, which permits the spinal cord to be continuous with the brain.
The facial skeleton or viscerocranium forms the anterior part of the skull. It consists of bones surrounding the mouth, nose, and orbits, as well as the sinuses. The facial skeleton is comprised of fifteen irregular bones, most of which develop from the mesenchyme of the embryonic pharyngeal arches.
There are three singular bones that lie in the midline: the mandible, the ethmoid, and the vomer; and six bones occurring in bilateral pairs: the maxillae; the inferior nasal conchae; and the zygomatic, palatine, nasal, and lacrimal bones. The maxillae comprise most of the upper facial skeleton, forming the upper jaw that’s fixed to the cranial base. The mandible forms the lower jaw, which is movable due to its articulation with the cranial base at the temporomandibular joints. Both the maxillae and the mandible provide sockets and supporting bone for housing teeth.
Sutures are fibrous joints that are unique to the skull. They are immovable, and fuse completely around the age of twenty. The main sutures in adulthood are the coronal suture, which fuses the frontal bone with the two parietal bones; the sagittal suture, which fuses the two parietal bones together; and the lambdoid suture, which fuses the occipital bone with the two parietal bones.
In neonates, the incompletely fused suture joints give rise to membranous gaps called fontanelles. This allows for movement of the bones of the skull, facilitating childbirth.