Skull
Cranium
Also known as: cranium, neurocranium, viscerocranium
The skeleton of the head — 22 bones (8 cranial, 14 facial) joined mostly by immovable sutures, with the mandible as the only freely movable bone. It encloses the brain, special sense organs and the openings of the respiratory and digestive tracts.
Overview
The skull is divided into the neurocranium (cranium) — the calvaria (skullcap) and the cranial base that enclose the brain — and the viscerocranium (facial skeleton), which forms the orbits, nasal cavity, paranasal sinuses and jaws. The 8 cranial bones are the frontal, two parietals, occipital, two temporals, sphenoid and ethmoid; the 14 facial bones are two each of nasal, maxilla, zygomatic, lacrimal, palatine and inferior nasal concha, plus the vomer and mandible. Associated bones are the 6 auditory ossicles and the hyoid.
All skull bones except the mandible are united by sutures (fibrous joints); the mandible articulates with the temporal bones at the temporomandibular joints. The skull is described from several views (norma): frontalis, lateralis, verticalis, occipitalis and basalis. Internally, the cranial base is stepped into anterior, middle and posterior cranial fossae perforated by foramina transmitting the cranial nerves and vessels. The calvaria consists of an outer and inner table of compact bone separated by spongy diploë containing diploic veins.
At birth the face is small relative to the cranium (≈1:8, versus ≈1:2 in adults); the bones of the vault are separated by membranous fontanelles that allow moulding during birth and brain growth in infancy. The mastoid processes, paranasal sinuses and alveolar processes develop postnatally.
- Location
Head, superior to the vertebral column.
Parts & subdivisions
- Forehead, roof of orbits.
- Roof and sides of the vault.
- Back and base; foramen magnum.
- Sides and base; house the ear.
- Central keystone of the cranial base; sella turcica.
- Cribriform plate, nasal septum, ethmoidal air cells.
- Nasal, maxilla, zygomatic, lacrimal, palatine, inferior nasal concha (paired); vomer, mandible (single).
- Lower jaw — the only movable skull bone.
- Fibrous joints and membranous gaps of the vault.
- Three stepped floors of the cranial cavity with their openings.
- Pyramidal bony socket formed by seven bones.
Anatomy
- Articulations
Sutures between cranial and facial bones; temporomandibular joints (mandible–temporal); atlanto-occipital joints (occipital condyles–atlas); gomphoses (teeth in alveoli).
- Ossification
Calvaria and most facial bones: intramembranous. Cranial base (occipital except upper squama, sphenoid body, petrous temporal, ethmoid): endochondral (chondrocranium).
- Development
Neurocranium from paraxial mesoderm (occipital and posterior skull) and neural crest (frontal, anterior vault); viscerocranium chiefly from neural crest of the 1st and 2nd pharyngeal arches.
Function
- Encloses and protects the brain and its meninges
- Houses and protects the organs of sight, hearing, balance, smell and taste
- Provides openings for air and food and supports the teeth
- Gives attachment to muscles of mastication, facial expression, the neck and the tongue
- Transmits cranial nerves and vessels through its foramina
Clinical significance
Skull fractures
Linear vault fractures are commonest; depressed fractures may tear dura and brain. Fractures crossing the pterion risk the middle meningeal artery (extradural haematoma). Base-of-skull fractures cause periorbital ecchymosis ("raccoon eyes"), Battle sign (mastoid bruising), haemotympanum and CSF rhinorrhoea or otorrhoea.
Craniosynostosis
Premature fusion of one or more sutures produces a predictable skull shape (e.g. sagittal → long narrow scaphocephaly) and may raise intracranial pressure.
Mnemonics
Cranial bones
"PEST OF 6": Parietal, Ethmoid, Sphenoid, Temporal, Occipital, Frontal.
Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.