- Human skull
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Caption = Human skulls side simplified
Caption2 = Human skull front bones
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In humans, the adult skull is normally made up of 22 bones. [ [http://www.hopkinsmedicine.org/craniofacial/LynmProject/BSC/BSC2.HTM Skull Basics ] ] Except for the
mandible, all of the bones of the skullare joined together by sutures, synarthrodial (immovable) jointsformed by bony ossification, with Sharpey's fibrespermitting some flexibility.
Eight bones form the
neurocranium(brain case), a protective vault of bone surrounding the brain and brain stem. Fourteen bones form the splanchnocranium, which comprises the bones supporting the face. Encased within the temporal bones are the six auditory ossiclesof the middle ear. The hyoid bone, supporting the larynx, is usually not considered as part of the skull, as it is the only bone that does not articulatewith other bones of the skull.
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 debatable; they contribute to lessening the weight of the skull with a minimal reduction in strength, they contribute to resonance of the voice, and assist in the warming and moistening of air drawn in through the nasal cavities.
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 (face) 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 404 separate bony elements. As growth occurs, many of these bony elements gradually fuse together into solid bone (for example, the
frontal bone). The bones of the roof of the skull are initially separated by regions of dense connective tissuecalled " 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 open 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.
If the brain is bruised or injured it can be life-threatening. Normally the skull protects the brain from damage through its hard unyieldingness; the skull is one of the most durable substances found in nature. In some cases, however, of
head injury, there can be raised intracranial pressurethrough mechanisms such as a subdural haematoma. In these cases the raised intracranial pressurecan 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 damageor death unless an urgent operation is performed to relieve the pressure. This is why patients with concussionmust be watched extremely carefully.
Dating back to
Neolithictimes, a skull operation called trepanationwas 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.
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 are found, such as at
Spitalfieldsin the UK and Jōmon 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 Gallin 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
craniometryhas occasionally purported to reliably demonstrate racial or ethnic differences between skulls of different people. Occasionally this has been used as justification for ideas of racial supremacy. However, this theory is again obsolete.
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 on average have larger, broader palates, squarer orbits, larger mastoid processes, larger sinuses, and larger occipital condyles than those of females. Male mandiblestypically 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
populationfrom which it came.
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).
Skull, a general article on skulls other than the human skull
Foramina of the skull, list of holes (foramina) in the base of the skull
Base of the skull, detailed list of the anatomical structures found at the base of the skull
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