Skull in situHuman head skull from sideAnatomy of a flat bone – the periosteum of the neurocranium is known as thepericraniumHuman skull from the frontSide bones of skull
Thehuman skull is the bone structure that forms thehead in thehuman skeleton. It supports the structures of theface and forms a cavity for thebrain. Like the skulls of other vertebrates, it protects the brain from injury.[6]
The skull consists of three parts, of differentembryological origin—theneurocranium, thesutures, and thefacial skeleton. The neurocranium (orbraincase) forms the protectivecranial cavity that surrounds and houses the brain andbrainstem.[7] The upper areas of thecranial bones form thecalvaria (skullcap). The facial skeleton (membranous viscerocranium) is formed by the bones supporting the face, and includes themandible.
The skull also containssinuses, air-filled cavities known asparanasal sinuses, and numerousforamina. The sinuses are lined withrespiratory epithelium. Their known functions are the lessening of the weight of the skull, the aiding of resonance to the voice and the warming and moistening of the air drawn into thenasal cavity.
The foramina are openings in the skull. The largest of these is theforamen magnum, of the occipital bone, that allows the passage of thespinal cord as well asnerves andblood vessels.
Thejugal is a skull bone that found in most of the reptiles, amphibians and birds. In mammals, the jugal is often called the zygomatic bone or malar bone.[8]
Theprefrontal bone is a bone that separates the lacrimal and frontal bones in many tetrapod skulls.
Fish head parts, 1889, Fauna of British India, Sir Francis Day
The skull of fish is formed from a series of only loosely connected bones.Lampreys andsharks only possess a cartilaginous endocranium, with both the upper jaw and the lowerjaws being separate elements. Bony fishes have additionaldermal bone, forming a more or less coherentskull roof inlungfish andholost fish. The lower jaw defines the chin.
The simpler structure is found injawless fish, in which the cranium is normally represented by a trough-like basket of cartilaginous elements only partially enclosing the brain, and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws.[9]
Cartilaginous fish, such as sharks and rays, have also simple, and presumably primitive, skull structures. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a largefontanelle. The most anterior part of the cranium includes a forward plate of cartilage, therostrum, and capsules to enclose theolfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of theinner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a singlecondyle, articulating with the firstvertebra. There are, in addition, at various points throughout the cranium, smallerforamina for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.[9]
Inray-finned fish, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced; there is little cheek region behind the enlarged orbits, and little, if any bone in between them. The upper jaw is often formed largely from thepremaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.[10]
Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the livinglungfishes. Theskull 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 thepterygoids andvomers alone, all of which bear teeth. Much of the skull is formed fromcartilage, and its overall structure is reduced.[10]
The skulls of the earliesttetrapods closely resembled those of theirancestors amongst thelobe-finned fishes. Theskull roof is formed of a series of plate-like bones, including the maxilla,frontals,parietals, andlacrimals, among others. It is overlaying theendocranium, corresponding to the cartilaginous skull in sharks andrays. The various separate bones that compose the temporal bone of humans are also part of the skull roof series. A further plate composed of four pairs of bones forms the roof of the mouth; these include thevomer andpalatine bones. The base of the cranium is formed from a ring of bones surrounding the foramen magnum and a median bone lying further forward; these arehomologous with the occipital bone and parts of the sphenoid in mammals. Finally, the lower jaw is composed of multiple bones, only the most anterior of which (the dentary) is homologous with the mammalian mandible.[10]
In living tetrapods, a great many of the original bones have either disappeared or fused into one another in various arrangements.
Birds have adiapsid skull, as in reptiles, with a prelacrimal fossa (present in some reptiles). The skull has a single occipital condyle.[11] 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' skulls, Hans Gadow, 1909Amphibia and Reptiles
Livingamphibians typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage.[10] 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.
The skull is a complex structure; its bones are formed both byintramembranous andendochondral ossification. Theskull roof bones, comprising the bones of the facial skeleton and the sides and roof of the neurocranium, aredermal bones formed by intramembranous ossification, though the temporal bones are formed by endochondral ossification. Theendocranium, the bones supporting the brain (the occipital,sphenoid, andethmoid) are largely formed by endochondral ossification. Thus frontal and parietal bones are purely membranous.[12] The geometry of theskull base and itsfossae, theanterior,middle andposterior cranial fossae changes rapidly. The anterior cranial fossa changes especially during thefirst trimester of pregnancy and skull defects can often develop during this time.[13]
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, thefrontal bone). The bones of theroof of the skull are initially separated by regions of denseconnective tissue calledfontanelles. There are six fontanelles: one anterior (or frontal), one posterior (or occipital), two sphenoid (or anterolateral), and two mastoid (or posterolateral). 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 thesquamous andlateral parts of the occipital bone meet. A possible complication of this tension is rupture of thegreat cerebral vein. As growth and ossification progress, the connective tissue of the fontanelles is invaded and replaced by bone creatingsutures. The five sutures are the twosquamous sutures, onecoronal, onelambdoid, and onesagittal suture. The posterior fontanelle usually closes by eight weeks, but the anterior fontanel 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 the pulse pulsing softly through the anterior fontanelle.
The skull in theneonate is large in proportion to other parts of the body. The facial skeleton is one seventh of the size of thecalvaria. (In the adult it is half the size). Thebase of the skull is short and narrow, though theinner ear is almost adult size.[14]
Craniosynostosis is a condition in which one or more of the fibroussutures in an infant skull prematurely fuses,[15] and changes the growth pattern of the skull.[16] Because the skull cannot expand perpendicular to the fused suture, it grows more in the parallel direction.[16] Sometimes the resulting growth pattern provides the necessary space for the growing brain, but results in an abnormal head shape and abnormal facial features.[16] In cases in which the compensation does not effectively provide enough space for the growing brain, craniosynostosis results in increasedintracranial pressure leading possibly to visual impairment, sleeping impairment, eating difficulties, or an impairment of mental development.[17]
Acopper beaten skull is a phenomenon wherein intense intracranial pressure disfigures the internal surface of the skull.[18] The name comes from the fact that the inner skull has the appearance of having been beaten with aball-peen hammer, such as is often used bycoppersmiths. The condition is most common in children.
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.[19] In some cases ofhead injury, however, there can be raisedintracranial pressure through mechanisms such as asubdural 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 significantbrain damage or death unless an urgent operation is performed to relieve the pressure. This is why patients withconcussion must be watched extremely carefully. Repeated concussions can activate the structure of skull bones as the brain's protective covering.[20]
Dating back toNeolithic times, a skull operation calledtrepanning was sometimes performed. This involved drilling aburr hole in the cranium. Examination of skulls from this period reveals that the patients sometimes survived for many years afterward. It seems likely that trepanning was also performed purely for ritualistic or religious reasons. Nowadays this procedure is still used but is normally called acraniectomy.
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-printedpolymerimplant.[21] 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 fromhyperostosis, which increased the thickness of her skull and compressed her brain.[22]
Adam was believed to have been buried onMount Calvary. Silk embroidery (17th century).
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]
Like the face, the skull and teeth can also indicate a person's life history and origin.Forensic scientists andarchaeologists use quantitative and qualitative traits to estimate what the bearer of the skull looked like. When a significant amount of bones are found, such as atSpitalfields in the UK andJōmonshell mounds in Japan,osteologists can use traits, such as the proportions of length, height and width, to know the relationships of the population of the study with other living or extinct populations.[citation needed]
The German physicianFranz Joseph Gall in around 1800 formulated the theory ofphrenology, 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 bepseudoscientific.[citation needed]
In the mid-nineteenth century,anthropologists found it crucial to distinguish between male and female skulls. An anthropologist of the time,James McGrigor Allan, argued that the female brain was similar to that of an animal.[28] This allowed anthropologists to declare that women were in fact more emotional and less rational than men. McGrigor then concluded that women's brains were more analogous to infants, thus deeming them inferior at the time.[28] To further these claims of female inferiority and silence the feminists of the time, other anthropologists joined in on the studies of the female skull. These cranial measurements are the basis of what is known ascraniology. These cranial measurements were also used to draw a connection between women and black people.[28]
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.[29] However, later studies show that women's skulls are slightly thicker and thus men may be more susceptible to head injury than women.[30] However, other studies shows that men's skulls are slightly thicker in certain areas.[31] Some studies show that females are more susceptible to concussion than males.[32] 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.[33][34]
Male skulls can all have more prominentsupraorbital ridges,glabella, andtemporal lines. Female skulls generally have rounderorbits and narrower jaws. Male skulls on average have larger, broaderpalates, squarer orbits, largermastoid processes, largersinuses, and largeroccipital condyles than those of females. Male mandibles typically have squarer chins and thicker, rougher muscle attachments than female mandibles.[35]
Thecephalic 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 theparietal eminence, and the length from theglabella to the occipital point.
Trepanning, a practice in which a hole is created in the skull, has been described as the oldest surgical procedure for which there isarchaeological evidence,[36] found in the forms of cave paintings and human remains. At one burial site inFrance dated to 6500 BCE, 40 out of 120prehistoric skulls found had trepanation holes.[37]
^Dechow, Paul C.; Wang, Qian (January 2017). "Evolution of the Jugal/Zygomatic Bones".The Anatomical Record.300 (1):12–15.doi:10.1002/ar.23519.
^abRomer, Alfred Sherwood; Thomas S., Parsons (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 173–177.ISBN0-03-910284-X.
^abcdRomer, Alfred Sherwood; Parsons, Thomas S. (1977).The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 216–247.ISBN0-03-910284-X.
^Wing, Leonard W. (1956). "The Place of Birds in Nature".Natural History of Birds. The Ronald Press Company. pp. 22–23.
^Carlson, Bruce M. (1999).Human Embryology & Developmental Biology (Second ed.). Mosby. pp. 166–170.ISBN0-8151-1458-3.
^abChaurasia, B. D. (2013).BD Chaurasia's Human Anatomy: Regional and Applied Dissection and Clinical. Vol. 3: Head–Neck Brain (Sixth ed.). CBS Publishers & Distributors. pp. 29–30.ISBN978-81-239-2332-1.
Li, Haiyan; Ruan, Jesse; Xie, Zhonghua; Wang, Hao; Liu, Wengling (2007). "Investigation of the critical geometric characteristics of living human skulls utilising medical image analysis techniques".International Journal of Vehicle Safety.2 (4): 345.doi:10.1504/IJVS.2007.016747.
Adeloye, Adelola; Kattan, Kenneth R.; Silverman, Frederic N. (July 1975). "Thickness of the normal skull in the American blacks and whites".American Journal of Physical Anthropology.43 (1):23–30.doi:10.1002/ajpa.1330430105.PMID1155589.
^Schulte-Geers, Christina; Obert, Martin; Schilling, René L.; Harth, Sebastian; Traupe, Horst; Gizewski, Elke R.; Verhoff, Marcel A. (2011). "Age and gender-dependent bone density changes of the human skull disclosed by high-resolution flat-panel computed tomography".International Journal of Legal Medicine.125 (3):417–425.doi:10.1007/s00414-010-0544-3.PMID21234583.S2CID39294670.
^Capasso, Luigi (2002).Principi di storia della patologia umana: corso di storia della medicina per gli studenti della Facoltà di medicina e chirurgia e della Facoltà di scienze infermieristiche (in Italian). Rome: SEU.ISBN978-88-87753-65-3.OCLC50485765.
^Restak, Richard (2000). "Fixing the Brain".Mysteries of the Mind. Washington, D.C.: National Geographic Society.ISBN978-0-7922-7941-9.OCLC43662032.
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