Skull

A skull, or cranium, is a bony structure of vertebrates which serves as the general framework for a head. The skull supports the structures of the face and protects the brain against injury.

Humans

In humans, the adult skull is normally made up of 28 bones. Except for the mandible, all of the bones of the skull are joined together by sutures, rigid articulations permitting very little movement.

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Eight bones form the neurocranium (braincase), a protective vault of bone surrounding the brain and medulla oblongata. Fourteen bones form the splanchnocranium, the bones supporting the face. Encased within the temporal bones are the six ear ossicles of the middle ear. The hyoid bone, supporting the larynx, is usually not considered as part of the skull, as it does not articulate with any other bones.

Related Topics:
Medulla oblongata - Temporal bone - Middle ear - Hyoid bone - Larynx

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Development of the skull

The skull is a complex structure; its bones are formed both by intramembranous and endochondral ossification. The bones of the splanchnocranium and the sides and roof of the neurocranium are formed by intramembranous (or dermal) ossification, while the bones supporting the brain (the occipital, sphenoid, temporal, and ethmoid) are largely formed by endochondral ossification.

Related Topics:
Intramembranous - Endochondral ossification - Occipital - Sphenoid - Temporal - Ethmoid

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At birth, the human skull is made up of 45 separate bony elements. As growth occurs, many of these bony elements gradually fuse together into solid bone (for example, the frontal bones).

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The bones of the roof of the skull are initially separated by regions of dense connective tissue.

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At birth these regions are fibrous and moveable, necessary for birth and later growth. Larger regions of connective tissue, called fontanelles, occur where certain bony elements meet. As growth and ossification progress, the connective tissue of the fontanelles is invaded and replaced by bone. The posterior fontanelle usually closes by eight weeks, but the anterior fontanelle can remain up to eighteen months. The anterior fontanelle is located at the junction of the frontal and parietal bones; it is a "soft spot" on a baby's forehead. Careful observation will show that you can count a baby's heartrate by observing his or her pulse pulsing softly through the anterior fontanelle.

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Pathology

If the brain is bruised or injured it can be extremely serious. Normally the skull protects the brain from damage through its hard unyieldingness, but in some cases of head injury, there can be raised intracranial pressure through mechanisms such as a subdural haematoma. In these cases the raised intracranial pressure can cause herniation of the brain out of the foramen magnum ('coning') because there is no space for the brain to expand to—this can result in significant brain damage or death unless an urgent operation is performed to relieve the pressure. This is why patients with concussion must be watched extremely carefully.

Related Topics:
Head injury - Intracranial pressure - Subdural haematoma - Foramen magnum - Brain damage - Concussion

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In earlier times, a skull operation called trepanation was often performed for semi-mystical reasons and not only as an attempted life-saving technique.

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The skull also contains the sinus cavities. The meninges are the membranes that separate the brain from the skull.

Related Topics:
Sinus cavities - Meninges

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Craniometry, morphology

Like the face of a living individual, a human skull also can tell to a certain degree his or her life history and origin. Forensic scientists and archaeologist uses metric and nommetric traits to estimate what the bearer of the skull looked like. When a good amount of bones are found, such as Spital Field in UK and Jomon shell mounds in Japan, osteologists can use such traits, such as proportions of length, height, width, to know the relationships of population of the study, with living or extinct ones.

Related Topics:
Forensic - Archaeologist - Metric - Nommetric - Spital Field - Jomon - Shell mounds

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Sexual descrimination

Age at death

Ancestry

Although persons' descents are occasionally stereotyped as different from other ethnic groups on the basis of a variety of traits like eye, hair and skin color, all such characters are not discrete nor preserved in bones.

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Among archaeologists and forensic scientists, it is well-known that the most consistent and unique trait of ancestry in skeleton is skull shape.

Related Topics:
Archaeologists - Forensic scientists

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Bones of the human skull

Cranial bones

• frontal bone
• parietal bone (2)
• temporal bone (2)
• occipital bone
• sphenoid bone
• ethmoid bone

Facial bones

• mandible
• maxilla (2)
• palatine bone (2)
• zygomatic bone (2)
• nasal bone (2)
• lacrimal bone (2)
• vomer bone
• inferior nasal conchae (2)

Ear ossicles

• malleus (2)
• incus (2)
• stapes (2)

Wormian bones

In addition to the usual centers of ossification of the cranium, others may occur, giving rise to irregular isolated bones termed sutural or Wormian bones. They occur most frequently in the course of the lambdoidal suture, but are occasionally seen at the fontanelles, especially the posterior. One, the pterion ossicle, sometimes exists between the sphenoidal angle of the parietal bone and the great wing of the sphenoid bone. They have a tendency to be more or less symmetrical on the two sides of the skull, and vary in size. Their number is generally limited to two or three; but more than a hundred have been found in the skull of an adult hydrocephalic subject.

Related Topics:
Parietal bone - Sphenoid bone

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Note: Ole Worm, Professor of Anatomy at Copenhagen, 1624–1639, was erroneously supposed to have given the first detailed description of these bones.

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Other features of the skull

Foramina of skull base

The following is a list of holes, or foramina, in the base of the skull and what goes through each of them. Arranged from anterior to posterior:

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• foramen caecum - emissary veins to superior sagittal sinus
• foramina of cribriform plate - olfactory nerve bundles
• posterior ethmoidal foramen - posterior ethmoidal artery, vein and nerve
• optic canal - optic nerve (II), ophthalmic artery
• superior orbital fissure
• oculomotor nerve (III)
• trochlear nerve (IV)
• lacrimal, frontal and nasociliary branches of ophthalmic nerve (V₁)
• abducens nerve (VI)
• superior ophthalmic vein
• foramen rotundum - maxillary nerve (V₂)
• foramen ovale
• mandibular nerve (V₃)
• accessory meningeal artery
• lesser petrosal nerve (occasionally)
• foramen spinosum
• middle meningeal artery and vein
• meningeal branch of mandibular nerve
• foramen lacerum
• internal carotid artery
• internal carotid nerve plexus
• hiatus of canal of lesser petrosal nerve
• hiatus of canal of greater petrosal nerve
• internal acoustic meatus
• facial nerve (VII)
• vestibulocochlear nerve (VIII)
• labyrinthine artery
• jugular foramen
• inferior petrosal sinus
• glossopharyngeal nerve (IX)
• vagus nerve (X)
• accessory nerve (XI)
• sigmoid sinus
• posterior meningeal artery
• internal jugular vein
• hypoglossal canal - hypoglossal nerve (XII)
• foramen magnum
• medulla oblongata
• vertebral arteries
• meningeal branches of vertebral arteries
• spinal roots of accessory nerves

Notable sutures

Most sutures are named for the bones they articulate, but some have special names of their own.

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• Sagittal - along the midline, between parietal bones
• Coronal - between the frontal and parietal bones
• Lambdoidal - between the parietals and the occipital bone
• Squamosal - between the parietal and the temporal bone
• Metopic - between the two frontal bones, prior to the fusion of the two into a single bone