Skull

The skull is a

The human skull is the bone structure that forms the head in the human skeleton. It supports the structures of the face and forms a cavity for the brain. Like the skulls of other vertebrates, it protects the brain from injury.^[5]

The skull consists of three parts, of different embryological origin—the neurocranium, the sutures, and the facial skeleton. The neurocranium (or braincase) forms the protective cranial cavity that surrounds and houses the brain and brainstem.^[6] The upper areas of the cranial bones form the calvaria (skullcap). The membranous viscerocranium includes the mandible.

The sutures are fairly rigid joints between bones of the neurocranium.

The facial skeleton is formed by the bones supporting the face.

Bones

Except for the mandible, all of the bones of the skull are joined by

The human skull is generally considered to consist of 22 bones—eight cranial bones and fourteen facial skeleton bones. In the neurocranium these are the occipital bone, two temporal bones, two parietal bones, the sphenoid, ethmoid and frontal bones.

The bones of the facial skeleton (14) are the vomer, two inferior nasal conchae, two nasal bones, two maxilla, the mandible, two palatine bones, two zygomatic bones, and two lacrimal bones. Some sources count a paired bone as one, or the maxilla as having two bones (as its parts); some sources include the hyoid bone or the three ossicles of the middle ear, the malleus, incus, and stapes, but the overall general consensus of the number of bones in the human skull is the stated twenty-two.

Some of these bones—the occipital, parietal, frontal, in the neurocranium, and the nasal, lacrimal, and vomer, in the facial skeleton are flat bones.

Cavities and foramina

The skull also contains

The foramina are openings in the skull. The largest of these is the foramen magnum, of the occipital bone, that allows the passage of the spinal cord as well as nerves and blood vessels.

Processes

The many

Bones

The

The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls.

Fish

The skull of fishes is formed from a series of only loosely connected bones.

The simpler structure is found in

In

Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the pterygoids and vomers alone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced.^[9]

Tetrapods

The skulls of the earliest

Birds have a diapsid skull, as in reptiles, with a prelacrimal fossa (present in some reptiles). The skull has a single occipital condyle.^[10] The skull consists of five major bones: the frontal (top of head), parietal (back of head), premaxillary and nasal (top beak), and the mandible (bottom beak). The skull of a normal bird usually weighs about 1% of the bird's total bodyweight. The eye occupies a considerable amount of the skull and is surrounded by a sclerotic eye-ring, a ring of tiny bones. This characteristic is also seen in reptiles.

Amphibians

Living amphibians typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage.^[9] In mammals and birds, in particular, modifications of the skull occurred to allow for the expansion of the brain. The fusion between the various bones is especially notable in birds, in which the individual structures may be difficult to identify.

Development

The skull is a complex structure; its bones are formed both by intramembranous and endochondral ossification. The skull roof bones, comprising the bones of the facial skeleton and the sides and roof of the neurocranium, are dermal bones formed by intramembranous ossification, though the temporal bones are formed by endochondral ossification. The endocranium, the bones supporting the brain (the occipital, sphenoid, and ethmoid) are largely formed by endochondral ossification. Thus frontal and parietal bones are purely membranous.^[11] The geometry of the skull base and its fossae, the anterior, middle and posterior cranial fossae changes rapidly. The anterior cranial fossa changes especially during the first trimester of pregnancy and skull defects can often develop during this time.^[12]

At birth, the human skull is made up of 44 separate bony elements. During development, many of these bony elements gradually fuse together into solid bone (for example, the

A copper beaten skull is a phenomenon wherein intense intracranial pressure disfigures the internal surface of the skull.^[17] The name comes from the fact that the inner skull has the appearance of having been beaten with a ball-peen hammer, such as is often used by coppersmiths. The condition is most common in children.

Injuries and treatment

Injuries to the brain can be life-threatening. Normally the skull protects the brain from damage through its high resistance to deformation; the skull is one of the least deformable structures found in nature, needing the force of about 1 ton to reduce its diameter by 1 cm.

In March 2013, for the first time in the U.S., researchers replaced a large percentage of a patient's skull with a precision, 3D-printed polymer implant.^[20] About 9 months later, the first complete cranium replacement with a 3D-printed plastic insert was performed on a Dutch woman. She had been suffering from hyperostosis, which increased the thickness of her skull and compressed her brain.^[21]

A study conducted in 2018 by the researchers of Harvard Medical School in Boston, funded by National Institutes of Health (NIH), suggested that instead of travelling via blood, there are "tiny channels" in the skull through which the immune cells combined with the bone marrow reach the areas of inflammation after an injury to the brain tissues.^[22]

Transgender procedures

Surgical alteration of sexually dimorphic skull features may be carried out as a part of facial feminization surgery, a set of reconstructive surgical procedures that can alter male facial features to bring them closer in shape and size to typical female facial features.^[23][24] These procedures can be an important part of the treatment of transgender people for gender dysphoria.^[25][26]

Society and culture

Artificial cranial deformation is a largely historical practice of some cultures. Cords and wooden boards would be used to apply pressure to an infant's skull and alter its shape, sometimes quite significantly. This procedure would begin just after birth and would be carried on for several years.^{[citation needed]}

Osteology

Like the face, the skull and teeth can also indicate a person's life history and origin.

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. His theory is now considered to be pseudoscientific.^{[citation needed]}

Sexual dimorphism

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In the mid-nineteenth century,

Research has shown that while in early life there is little difference between male and female skulls, in adulthood male skulls tend to be larger and more robust than female skulls, which are lighter and smaller, with a cranial capacity about 10 percent less than that of the male.[28] However, later studies show that women's skulls are slightly thicker and thus men may be more susceptible to head injury than women.^[29] However, other studies shows that men's skulls are slightly thicker in certain areas.^[30] As well as some studies showing that females are more susceptible to head injury (concussion) than males.^[31] Men's skulls have also been shown to maintain density with age, which may aid in preventing head injury, while women's skull density slightly decreases with age.^[32][33]

Male skulls can all have more prominent

The cephalic index is the ratio of the width of the head, multiplied by 100 and divided by its length (front to back). The index is also used to categorize animals, especially dogs and cats. The width is usually measured just below the parietal eminence, and the length from the glabella to the occipital point.

Humans may be:

• Dolichocephalic — long-headed
• Mesaticephalic — medium-headed
• Brachycephalic — short-headed^[13]

The vertical cephalic index refers to the ratio between the height of the head multiplied by 100 and divided by the length of the head.

Humans may be:

• Chamaecranic — low-skulled
• Orthocranic — medium high-skulled
• Hypsicranic — high-skulled

Terminology

• Chondrocranium, a primitive cartilaginous skeletal structure
• Endocranium
• Epicranium
• Pericranium
  , a membrane that lines the outer surface of the cranium

History

External links

Wikimedia Commons has media related to Animal skulls and Human skulls.

Look up skull in Wiktionary, the free dictionary.

• Skull Module (California State University Department of Anthology)
• Skull Anatomy Tutorial. (GateWay Community College)
• Bird Skull Collection Bird skull database with very large collection of skulls (Agricultural University of Wageningen)
• Human skull base (in German)
• Human Skulls / Anthropological Skulls / Comparison of Skulls of Vertebrates (PDF; 502 kB)