Skull
Overview
The skull is a bony
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 structure in the head
Head
In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....

 of many animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s that supports the structures of the face
Face
The face is a central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can, depending on the definition in the human case, include the hair, forehead, eyebrow, eyelashes, eyes, nose, ears, cheeks, mouth, lips, philtrum, temple, teeth, skin, and...

 and forms a cavity
Cranial cavity
The cranial cavity, or intracranial space, is the space formed inside the skull. The brain occupies the cranial cavity, which is lined by the meninges and which contains cerebrospinal fluid to cushion blows....

 for the brain.

The skull is composed of two parts: the cranium and the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

. A skull without a mandible is only a cranium. Animals that have skulls are called craniates
Craniata
Craniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives...

. The skull is a part of the skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

.

Functions of the skull include protection of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, 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.
Encyclopedia
The skull is a bony
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 structure in the head
Head
In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....

 of many animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s that supports the structures of the face
Face
The face is a central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can, depending on the definition in the human case, include the hair, forehead, eyebrow, eyelashes, eyes, nose, ears, cheeks, mouth, lips, philtrum, temple, teeth, skin, and...

 and forms a cavity
Cranial cavity
The cranial cavity, or intracranial space, is the space formed inside the skull. The brain occupies the cranial cavity, which is lined by the meninges and which contains cerebrospinal fluid to cushion blows....

 for the brain.

The skull is composed of two parts: the cranium and the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

. A skull without a mandible is only a cranium. Animals that have skulls are called craniates
Craniata
Craniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives...

. The skull is a part of the skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

.

Functions of the skull include protection of the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

, 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
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....

 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).

Human skull

In humans, the adult skull is normally made up of 22 bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

s. Except for the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

, 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
Occipital bone
The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...

, one sphenoid
Sphenoid bone
The sphenoid bone is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit...

, two temporals and one ethmoid— form the neurocranium (braincase), a protective vault surrounding the brain
Cranial vault
The cranial vault is the space in the skull within the neurocranium, occupied by the brain. In humans, the size and shape of the brain, may be affected by the size of the vault as shown in craniometry, but studies relating it to intelligence have been ambivalent...

. Fourteen bones form the splanchnocranium, the bones supporting the face. Encased within the temporal bone
Temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...

s are the six ear ossicles
Ossicles
The ossicles are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid-filled labyrinth . The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss...

of the middle ear
Middle ear
The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...

s, though these are not part of the skull. The hyoid bone
Hyoid bone
The hyoid bone is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies at the level of the base of the mandible in the front and the third cervical vertebra behind.Unlike other bones, the hyoid is only distantly...

, supporting the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

, is usually not considered as part of the skull either, as it does not articulate with any other bones.

Tetrapod skulls

The skull of the earliest tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

s closely resembles that of their ancestors amongst the lobe-finned fishes. The skull roof
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...

 is formed of a series of plate-like bones, including the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

, frontals
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....

, parietal
Parietal bone
The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....

s, and lacrimal
Lacrimal bone
The lacrimal bone, the smallest and most fragile bone of the face, is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders.-Lateral or orbital surface:...

s, among others. It is overlaying the endocranium
Endocranium
For internal cast of the cranium, see Endocast.The endocranium in comparative anatomy is a part of the skull base in vertebrates and represent the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy.-Basic structure:Structurally, the...

, corresponding to the cartilaginous skull in sharks and rays
Batoidea
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

. The various separate bones that compose the temporal bone
Temporal bone
The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebrum.The temporal bone supports that part of the face known as the temple.-Parts:The temporal bone consists of four parts:* Squama temporalis...

 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
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...

 and palatine bone
Palatine bone
The palatine bone is a bone in many species of the animal kingdom, commonly termed the palatum .-Human anatomy:...

s. 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
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 with the occipital bone
Occipital bone
The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...

 and parts of the sphenoid
Sphenoid bone
The sphenoid bone is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit...

 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
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

.

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 amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s typically have greatly reduced skulls, with many of the bones either absent or wholly or partly replaced by cartilage.

Temporal fenestrae

The temporal fenestrae are anatomical features of the skulls of several types of amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s, 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
Postorbital
The postorbital is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some vertebrates, the postorbital is fused with the postfrontal...

 and squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...

 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 Synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

s (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

There are four types of amniote skull, classified by the number and location of their fenestra. These are:
  • Anapsid
    Anapsid
    An anapsid is an amniote whose skull does not have openings near the temples.While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related...

    a - no openings
  • Synapsid
    Synapsid
    Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

    a - one low opening (beneath the postorbital and squamosal bones)
  • Euryapsida
    Euryapsida
    Euryapsida is a polyphyletic group of reptiles that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of...

     - one high opening (above the postorbital and squamosal bones); euryapsids actually evolved from a diapsid configuration, losing their lower temporal fenestra.
  • Diapsid
    Diapsid
    Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

    a - two openings


Evolutionarily, they are related as follows:
  • Amniota
    • Class Synapsid
      Synapsid
      Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

      a
      • Order Therapsida
        Therapsida
        Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be...

        • Class Mammal
          Mammal
          Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

          ia - mammals
    • (Unranked)Sauropsida - reptiles
      • Subclass Anapsid
        Anapsid
        An anapsid is an amniote whose skull does not have openings near the temples.While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related...

        a
      • (unranked) Eureptilia
        Eureptilia
        Eureptilia is one of the two major clades of the Sauropsida, the other being Anapsida . Eureptilia includes not only all Diapsids, but also a number of primitive Permo-Carboniferous forms previously classified under the Anapsida, in the old order "Cotylosauria".Primitive eureptilians were all...

        • Subclass Diapsid
          Diapsid
          Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

          a
          • (unranked) Euryapsida
            Euryapsida
            Euryapsida is a polyphyletic group of reptiles that are distinguished by a single temporal fenestra, an opening behind the orbit, under which the post-orbital and squamosal bones articulate. They are different from Synapsida, which also have a single opening behind the orbit, by the placement of...

          • Class Aves
            Bird
            Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

             - birds

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 lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...

es. The skull roof
Skull roof
The skull roof , or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land living vertebrates. The bones are derived from dermal bone, hence the alternative name dermatocranium...

 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 pterygoid
Pterygoid
Pterygoid can refer to:* Pterygoid processes of the sphenoid bone** The Lateral pterygoid plate by it* a muscle such as Lateral pterygoid muscle or Medial pterygoid muscle* a branch of the Mandibular nerve...

s and vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...

s alone, all of which bear teeth. Much of the skull is formed from cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

, and its overall structure is reduced.

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
Premaxilla
The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

, with the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...

 itself located further back, and an additional bone, the symplectic, linking the jaw to the rest of the cranium.

Cartilaginous fish, such as shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, 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
Fontanelle
A fontanelle is an anatomical feature on an infant's skull.-Anatomy:Fontanelles are soft spots on a baby's head which, during birth, enable the bony plates of the skull to flex, allowing the child's head to pass through the birth canal. The ossification of the bones of the skull causes the...

. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....

, 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
Inner ear
The inner ear is the innermost part of the vertebrate ear. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:...

. 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
Foramina of skull
The human skull has numerous holes through which cranial nerves, arteries, veins and other structures pass.-List of foramina and the structures that pass through them:...

 for the cranial nerves. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper.

The structure is simpler still in lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s, 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.

Gallery

Bones

The jugal
Jugal
The jugal is a skull bone found in most reptiles, amphibians, and birds. In mammals, the jugal is often called the malar or Zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species....

 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
Prefrontal bone
The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and extinct lungfish, amphibians and reptiles...

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

See also

  • Bucranium
    Bucranium
    Bucranium, plural bucrania , is the word for the skull of an ox. It is also an architectural term used to describe a common form of carved decoration in Classical architecture, used to fill the metopes between the triglyphs of the frieze of Doric temples...

    , the Greek word for the skull of an ox
  • Chondrocranium
    Chondrocranium
    The chondrocranium is the primitive cartilaginous skeletal structure of the fetal skull that grows to envelop the rapidly growing embryonic brain....

    , a primitive cartilagionous skeletal structure
  • Endocranium
    Endocranium
    For internal cast of the cranium, see Endocast.The endocranium in comparative anatomy is a part of the skull base in vertebrates and represent the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy.-Basic structure:Structurally, the...

  • Epicranium
    Epicranium
    The Epicranium is the medical term for the collection of structures covering the cranium. It consists of the muscles, aponeurosis, and skin....

  • Pericranium, a membrane that lines the outer surface of the cranium

External links

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