Components & Features
The human skull consists of about 22 to 30 single bones which are mostly connected together by ossified joints, so called sutures. The skull is divided into the braincase (cerebral cranium) and the face (visceral cranium). Its main task is the protection of the most important organ in the human body: the brain. The brain is almost entirely enclosed by the cerebral cranium with the exception of the foramen magnum and other foramina at the skull base which serve as entry and exit point for blood vessels and cranial nerves.
The braincase consists of the skullcap (calvarium) and the skull base. The skull cap is made up of the pairs of parietal bones and parts of the frontal bone as well as the occipital bone. The most important sutures in the human skull are:
- the coronal suture (between the frontal and parietal bone)
- the sagittal suture (dividing both the parietal bones)
- the lambdoidal suture (running horizontally between the occipital bone and both parietal bones)
These are the three most significant of all 33 sutures which are formed by the human skull bones.
The skull base is the caudal boundary of the cerebral cranium. Looking at it from the inside it can be subdivided into the anterior, middle and posterior cranial fossae. The skull base comprises parts of the frontal, ethmoid, sphenoid, occipital and temporal bones.
The face is referred to as all skull bones fronto-caudally connected to the cerebral cranium. Prominent representatives are the maxilla (upper jaw) and the mandible (lower jaw). The orbita and the nasal cavity are formed by the zygomatic, nasal, palatine, lacrimal bones, the vomer and the inferior nasal concha (lower turbinate).
Foramina & Contents
Most foramina in which relevant nerves and blood vessels pass through are located at the base of the skull. In the following, the most important structures are discussed ordered by their location in the three cranial fossae.
Anterior Cranial Fossa
The anterior cranial fossa comprises a holey plate at the center, the so called cribriform plate (lamina cribrosa). The approximately 20 cribriform foramina serve as a passageway for the olfactory nerves to the olfactory mucosa in the nasal cavity.
Both the optic nerve and the ophthalmic artery pass through the optic canal which is centrally located on the sphenoid bone. The lesser wing of the sphenoid bone (ala minor) forms the dorsal boundary of the anterior cranial fossa.
Middle Cranial Fossa
The middle cranial fossa lies slightly deeper than the anterior cranial fossa. The superior orbital fissure which is bounded by the greater and lesser wings of the sphenoid bone contains the trochlear nerve, abducencs nerve, oculomotor nerve and ophthalmic nerve. The sella turcica is a depression in the sphenoid bone. In the center of the middle cranial fossa it forms the pituitary fossa in which the pituitary gland sits.
Further important foramina are the:
- foramen rotundum (maxillary nerve)
- foramen ovale (mandibular nerve)
- carotid canal (internal carotid artery)
Posterior Cranial Fossa
The largest opening in the skull is the foramen magnum. Here the brainstem leaves the skull and becomes the spinal cord. The foramen magnum is situated in the center of the posterior cranial fossa. It is separated from the middle cranial fossa by the dorsum sellae and the upper edge of the petrous bone.
Further important structures are the:
- internal acoustic meatus (facial nerve, vestibulocochlear nerve),
- jugular foramen (internal jugular vein, glossopharyngeal nerve, vagus nerve, accessory nerve)
- hypoglossal canal (hypoglossal nerve)
- Cribriform plate - olfactory nerves
- Optic canal - optic nerve, opthalmic artery
- Superior orbital fissure - trochlear, abducens, oculomotor, and ophthalmic nerves
- Foramen rotundum - maxillary nerve
- Foramen ovale - mandibular nerve
- Carotid canal - internal carotid artery
- Foramen magnum - brainstem
- Internal acoustic meatus - facial and vestibulocochlear nerves
- Jugular foramen - internal jugular vein, glossopharyngeal nerve, vagus nerve, accessory nerve
- Hypoglossal canal - hypoglossal nerve)