Skull

Skull

The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.

The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates. The skull is a part of the skeleton.

Functions of the skull include protection of the brain, fixing the distance between the eyes to allow stereoscopic vision, and fixing the position of the ears to help the brain use auditory cues to judge direction and distance of sounds. In some animals, the skull also has a defensive function (e.g. horned ungulates); the frontal bone is where horns are mounted.

The English word "Skull" is probably derived from Old Norse "skalli" meaning bald, while the word cranium comes from the Greek root κρανίον (kranion).

Contents

Human skull

Model of a male human skull in the collections of the Museum of Osteology

In humans, the adult skull is normally made up of 22 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 —including one frontal, two parietals, one occipital bone, one sphenoid, two temporals and one ethmoid— form the neurocranium (braincase), a protective vault surrounding the brain. Fourteen bones form the splanchnocranium, the bones supporting the face. Encased within the temporal bones are the six ear ossicles of the middle ears, though these are not part of the skull. The hyoid bone, supporting the tongue, is usually not considered as part of the skull either, as it does not articulate with any other bones. The skull is a protector of the brain.

Other skulls

Tetrapod skulls

The skull of the earliest tetrapods closely resembles that of their ancestors amongst the lobe-finned fishes. The skull roof is formed of a series of plate-like bones, including the maxilla, frontals, parietals, and lacrimals, among others. It is overlaying the endocranium, corresponding to the cartilaginous skull in sharks and rays. 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 the vomer and palatine 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 are homologous 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.[1]

In living tetrapods, a great many of the original bones have either disappeared, or fused into one another in various arrangements. In mammals and birds, in particular, there have been modifications of the skull 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. Living amphibians typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage.[1]

Temporal fenestrae

This Massospondylus skull shows the two temporal fenestrae typical of diapsids.

The temporal fenestrae are anatomical features of the skulls of several types of amniotes, characterised by bilaterally symmetrical holes (fenestrae) in the temporal bone. Depending on the lineage of a given animal, two, one, or no pairs of temporal fenestrae may be present, above or below the postorbital and squamosal bones. The upper temporal fenestrae are also known as the supratemporal fenestrae, and the lower temporal fenestrae are also known as the infratemporal fenestrae. The presence and morphology of the temporal fenestra are critical for taxonomic classification of the synapsids, of which mammals are part.

Physiological speculation associates it with a rise in metabolic rates and an increase in jaw musculature. The earlier amniotes of the Carboniferous did not have temporal fenestrae but two more advanced lines did: the Synapsids (mammal-like reptiles) and the Diapsids (most reptiles and later birds). As time progressed, diapsids' and synapsids' temporal fenestrae became more modified and larger to make stronger bites and more jaw muscles. Dinosaurs, which are sauropsids, have large advanced openings and their descendants, the birds, have temporal fenestrae which have been modified. Mammals, which are synapsids, possess no fenestral openings in the skull, as the trait has been modified. They do, though, still have the temporal orbit (which resembles an opening) and the temporal muscles. It is a hole in the head and is situated to the rear of the orbit behind the eye.

Classification

Chimpanzee skull

There are four types of amniote skull, classified by the number and location of their fenestra. These are:

  • Anapsida - no openings
  • Synapsida - one low opening (beneath the postorbital and squamosal bones)
  • Euryapsida - one high opening (above the postorbital and squamosal bones); euryapsids actually evolved from a diapsid configuration, losing their lower temporal fenestra.
  • Diapsida - two openings

Evolutionarily, they are related as follows:

Skulls in fish

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.[1]

In the ray-finned fishes, 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 the premaxilla, with the maxilla itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.[1]

Cartilaginous fish, such as sharks, have a much simpler, and presumably more primitive, skull structure. 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 large fontanelle. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. There are, in addition, at various points throughout the cranium, smaller foramina for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.[2]

The structure is simpler still in lampreys, in which the cranium is represented by a trough-like basket of cartilagenous 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.[2]

Gallery

Paracyclotosaurus davidi skull, a prehistoric amphibian species  
Tyrannosaurus rex skull, a dinosaur species  
A centrosaurus skull  
Alligator skull, a reptile species  
An elephant skull, a mammal species  
A lion's skull, a typical carnivore  
A hippopotamus' skull  
Killer whale (Orcinus orca) skull  
A bulldog skull  
A Grizzly bear skull  
A coypu skull, a typical rodent  
A gerbil skull, another typical rodent  
Cervocerus novorossiae skull  
A Four-horned antelope skull drawing  
Skull of a multi-horned Jacob sheep  
A Vulture skull, a typical bird species  
Anarhichas lupus skull, a fish species  
Skull of Tiktaalik, a genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period  
Bones of human skull.  

Bones

The jugal is a skull bone found in most reptiles, amphibians, and birds. In mammals, the jugal is often called the malar or zygomatic. The prefrontal bone is askulls.

See also

  • Bucranium, the Greek word for the skull of an ox
  • Chondrocranium, a primitive cartilagionous skeletal structure
  • Endocranium
  • Epicranium
  • Pericranium, a membrane that lines the outer surface of the cranium

Notes

  1. ^ a b c d Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 216–247. ISBN 0-03-910284-X. 
  2. ^ a b Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 173–177. ISBN 0-03-910284-X. 

References

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

External links


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Synonyms:

Look at other dictionaries:

  • skull — less, adj. skull like, adj. /skul/, n. 1. the bony framework of the head, enclosing the brain and supporting the face; the skeleton of the head. 2. the head as the center of knowledge and understanding; mind: to get literature s great ideas… …   Universalium

  • Skull — Skull, n. [OE. skulle, sculle, scolle; akin to Scot. skull, skoll, a bowl, Sw. skalle skull, skal a shell, and E. scale; cf. G. hirnschale, Dan. hierneskal. Cf. {Scale} of a balance.] 1. (Anat.) The skeleton of the head of a vertebrate animal,… …   The Collaborative International Dictionary of English

  • skull — [ skʌl ] noun count ** 1. ) the bones of the head: a sheep s skull fracture your skull: His skull was fractured in the accident. 2. ) INFORMAL a person s head or mind: The sound of the alarm clock pierced his skull. get something into/through… …   Usage of the words and phrases in modern English

  • skull — ► NOUN 1) a bone framework enclosing the brain of a person or animal. 2) informal a person s head or brain. ● out of one s skull Cf. ↑out of one s skull ● skull and crossbones Cf. ↑skull and crossbones …   English terms dictionary

  • skull — [skul] n. [ME scolle < Scand, as in Swed skulle, skull, akin to SCALE3, SHELL] 1. the entire bony or cartilaginous framework of the head of a vertebrate, enclosing and protecting the brain and sense organs, including the bones of the face and… …   English World dictionary

  • Skull — Skull, n. [See {School} a multitude.] A school, company, or shoal. [Obs.] [1913 Webster] A knavish skull of boys and girls did pelt at him. Warner. [1913 Webster] These fishes enter in great flotes and skulls. Holland. [1913 Webster] …   The Collaborative International Dictionary of English

  • skull — [skʌl] n [Date: 1200 1300; Origin: From a Scandinavian language] 1.) the bones of a person s or animal s head 2.) sb can t get it into their (thick) skull spoken someone is unable to understand something very simple ▪ He can t seem to get it into …   Dictionary of contemporary English

  • skull — early 13c., probably from O.N. skalli bald head, skull, a general Scandinavian word (Cf. Swedish skulle, Norw. skult), probably related to O.E. scealu husk (see SHELL (Cf. shell)). But early prominence in southwestern texts suggests rather origin …   Etymology dictionary

  • Skull — Skull. См. Гарнисаж. (Источник: «Металлы и сплавы. Справочник.» Под редакцией Ю.П. Солнцева; НПО Профессионал , НПО Мир и семья ; Санкт Петербург, 2003 г.) …   Словарь металлургических терминов

  • Skull — (engl. scull), Doppelruder, s. Riemen; Skuller (sculler), s. Rudersport …   Meyers Großes Konversations-Lexikon

  • Skull — (engl. scull, spr. ßköll), Riemen, der an jedem Ende ein Blatt hat; Skuller, jemand der mit S.s rudert; auch Bezeichnung eines leichten Bootes, das von einer Person mit S. gehandhabt wird …   Kleines Konversations-Lexikon

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