Skull

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For symbolic or mythic uses of the human skull, see skull (symbolism). For the professional wrestler, see Ron Harris (wrestler).
Image:Hippo skull dark.jpg

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

Contents

Humans

Image:Gray 190 - The skull from the front.png Image:Gray 188 - Side view of the skull.png

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 called "sutures". There are five sutures: the frontal suture, sagittal suture, lambdoid suture, coronal suture, and squamosal suture. At birth these regions are fibrous and moveable, necessary for birth and later growth. This growth can put a large amount of tension on the "obstetrical hinge," which is where the squamous and lateral parts of the occipital bone meet. A possible complication of this tension is rupture of the great cerebral vein of Galen. Larger regions of connective tissue where multiple sutures meet are called fontanelles. The six fontanelles are: the anterior fontanelle, the posterior fontanelle, the two sphenoid fontanelles, and the two mastoid fontanelles. 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 heart rate by observing his or her pulse pulsing softly through the anterior fontanelle.

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.

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.

Craniometry and morphology of human skulls

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 nonmetric traits to estimate what the bearer of the skull looked like. When a good amount of bones are found, such as at Spitalfields in the 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.

Sexual differences

In general, male skulls tend to be larger and more robust than female skulls. Male skulls typically have more prominent supraorbital ridges, a more prominent glabella, and more prominent temporal lines. Male skulls typically have larger, broader palates, squarer orbits, larger mastoid processes, larger sinuses, and larger occipital condyles than do females. Male mandibles typically have squarer chins and thicker, rougher muscle attachments than female mandibles.

All of these features vary considerably within human populations, making it difficult to identify the sex of a skull without knowledge of the population it came from. The pelvis is considered the best skeletal indicator of sex.

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

Bones of the human skull

Cranial bones

Facial bones

Ear ossicles

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.

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:

Notable sutures

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

See also

Image:Punuk.Alaska.skulls.jpg

External links

References

  • White, T.D. 1991. Human osteology. Academic Press, Inc. San Diego, CA.
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