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Skull

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For symbolic or mythic uses of the human skull, see skull (symbolism). For other uses of the word, see skull (disambiguation).
A hippopotamus' skull
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A hippopotamus' skull

The skull or cranium is a bony structure found in many animals which serves as the general framework for the head. Those animals having skulls are called Craniates. The skull supports the structures of the face and protects the brain against injury.

Protection of the brain is only one part of the function of a bony skull. For example, a fixed distance between the eyes is essential for stereoscopic vision, and a fixed position for the ears helps the brain to use auditory cues to judge direction and distance of sounds. The skull also serves as a mount for the teeth and jaws, to permit eating, and in some animals (e.g. horned ungulates) it has a defensive function: the frontal bone is where horns are mounted.

Contents

Humans

Human skull (front)
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Human skull (front)

Human skull (side)
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Human skull (side)

In humans, the adult skull is normally made up of 29 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.

The skull also contains the sinus cavities, which are air-filled cavities lined with respiratory epithelium, which also lines the large airways. The exact functions of the sinuses are unclear; they may contribute to lessening the weight of the skull with a minimal reduction in strength, or they may be important in improving the resonance of the voice. In some animals, such as the elephant, the sinuses are extensive. The elephant skull needs to be very large, to form an attachment for muscles of the neck and trunk, but is also unexpectedly light; the comparatively small brain-case is surrounded by large sinuses which reduce the weight.

The meninges are the three layers, or membranes, which surround the structures of the central nervous system. They are known as the dura mater, the arachnoid mater and the pia mater. Other than being classified together, they have little in common with each other.

In humans, the anatomical position for the skull is the Frankfurt plane, where the lower margins of the orbits and the upper borders of the ear canals are all in a horizontal plane. This is the position where the subject is standing and looking directly forward. For comparison, the skulls of other species, notably primates and hominids, may sometimes be studied in the Frankfurt plane. However, this does not always equate to a natural posture in life.

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 "cranial 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 life-threatening. 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; 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.

Dating back to Neolithic times, a skull operation called trepanation was sometimes performed. This involved drilling holes in the cranium. Examination of skulls from this period reveals that the "patients" sometimes survived for many years afterward. It seems likely that trepanation was performed for ritualistic or religious reasons and not only as an attempted life-saving technique.

An image of a skull by Leonardo da Vinci
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An image of a skull by Leonardo da Vinci

Craniometry and morphology of human skulls

Like the face of a living individual, a human skull and teeth can also tell, to a certain degree, the life history and origin of its owner. Forensic scientists and archaeologists use metric and nonmetric traits to estimate what the bearer of the skull looked like. When a significant amount of bones is found, such as at Spitalfields in the UK and Jomon shell mounds in Japan, osteologists can use traits, such as proportions of length, height, width, to know the relationships of population of the study, with living or extinct populations.

The German physician Franz Joseph Gall in around 1800 formulated the theory of phrenology, which attempted to show that specific features of the skull are associated with certain personality traits or intellectual capabilities of its owner. This theory is now considered to be obsolete.

The practice of craniometry has occasionally purported to reliably demonstrate racial or ethnic differences between skulls of different people. Occasionally this been used as justification for ideas of racial supremacy. However, this theory is again obsolete.

Sexual dimorphism

In general, male skulls tend to be larger and more robust than female skulls, which are more gracile. 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 those of 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, due to the fact that the female pelvis must allow for a baby to exit.

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 still sometimes stated that the most consistent and unique trait of ancestry in skeleton is skull shape (see craniometry).

Bones of the human skull

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Cranial bones

Facial bones

Ear ossicles

Wormian bones

See Wormian bones

Other features of the skull

See also

Human skulls
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Human skulls

References

External links

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