A skull, or cranium, is a bony structure of vertebrates which serves as the general framework for a head. The skull functions to protect the brain, acting as a form of natural helmet, and to support the structures of the face.
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.
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.
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.
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). The bones of the roof of the skull are initially separated by regions of dense connective tissue. 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.
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.
In earlier times, a skull operation called trepanation was often performed for semi-mystical reasons and not only as an attempted life-saving technique.
The skull also contains the sinus cavities. The meninges are the membranes that separate the brain from the skull.
Although persons of East Asian descent are occasionally stereotyped as different from other ethnic groups on the basis of a variety of traits like eye shape, nose shape, hair color, and skin color, nearly all such stereotypes are incorrect and highly flawed. Among neurologists and pathologists, it is well-known that the most consistent and unique trait in East Asians is skull shape. However, this bit of knowledge is rarely discussed in public because of the need to avoid encouraging pseudoscientific theories like phrenology which attempt to connect skull shape to intelligence.
Bones of the human skull
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.
Note: Ole Worm, Professor of Anatomy at Copenhagen, 1624–1639, was erroneously supposed to have given the first detailed description of these bones.
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:
- foramen caecum - emissary vein 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
- foramen rotundum - maxillary nerve (V2)
mandibular nerve (V3)
- accessory meningeal artery
- lesser petrosal nerve (occasionally)
- foramen spinosum
- middle meningeal artery and vein
- meningeal branch of mandibular nerve
hiatus of canal of lesser petrosal nerve
hiatus of canal of greater petrosal nerve
- internal acoustic meatus
- jugular foramen
- hypoglossal canal - hypoglossal nerve (XII)
Most sutures are named for the bones they articulate, but some have special names of their own.
- 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
This article is based on an entry from the 1918 edition of Gray's Anatomy, which is in the public domain. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.
Last updated: 05-13-2005 07:56:04