Skull

''"Cranium" and "crania" redirect here. For other uses, see Cranium (disambiguation).''

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 Latin word cranium comes from the Greek root κρανίον (kranion).

The skull is made of flat bones. The human skull is usually 7 mm thick

Fish skulls
The skull of fishes is formed from a series of only loosely connected bones. Lampreys and sharks only possess a cartilaginous endocranium, with both the upper and lower jaws being separate elements. Bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish. The lower jaw defines a chin.

The simpler structure is found in jawless fish, in which the cranium is 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.

Cartilaginous fish, such as sharks, 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 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.

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.

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.

Tetrapod skulls
The skulls of the earliest tetrapods closely resembled those 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.

In living tetrapods, a great many of the original bones have either disappeared, or fused into one another in various arrangements.

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

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.

Fenestrae
The fenestrae (from Latin, meaning windows) are openings in the skull.
 * Antorbital fenestra
 * Mandibular fenestra
 * Palatal fenestra, as found in Marsupials
 * Quadratojugal fenestra
 * Subsquamosal fenestra, an opening between two parts of the squamosal bone in some rodents
 * Temporal fenestra

Ceratopsia dinosaurs may have fenestrae in their frills.

Temporal fenestrae
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 based on fenestrae
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:
 * Amniota
 * Class Synapsida
 * Order Therapsida
 * Class Mammalia - mammals
 * (Unranked)Sauropsida - reptiles
 * Subclass Anapsida
 * (unranked) Eureptilia
 * Subclass Diapsida
 * (unranked) Euryapsida
 * Class Aves - birds

Human skull

 * For details and the constituent bones, see human skull, neurocranium and viscerocranium.

In humans, as in other mammals, the aforementioned division of the skull into the cranium and mandible is not usually followed. Instead, for the purposes of describing their anatomy and enumerating their bones, mammalian and human skulls are divided differently: They are deemed to consist of two categorical parts, the neurocranium and the viscerocranium. The neurocranium (or braincase) is a protective vault surrounding the brain. The viscerocranium (also splanchnocranium or facial skeleton) is formed by the bones supporting the face. Both parts have different embryological origins.

Except for the mandible, all of the bones of the skull are joined together by sutures, rigid articulations permitting very little movement.

The skull is supported on the summit of the vertebral column, and is of an oval shape, wider behind than in front. It is composed of a series of flattened or irregular bones which, with one exception (the mandible), are immovably jointed together. It is divisible into two parts: (1) the cranium, which lodges and protects the brain, consists of eight bones, and (2) the skeleton of the face, of fourteen, as follows:
 * Skull, 22 bones
 * Cranium, 8 bones
 * i. Occipital.
 * ii. Two Parietals.
 * iii. Frontal.
 * iv. Two Temporals.
 * v. Sphenoidal.
 * vi. Ethmoidal.


 * Face, 14 bones
 * i. Two Nasals.
 * ii. Two Maxillæ.
 * iii. Two Lacrimals.
 * iv. Two Zygomatics.
 * v. Two Palatines.
 * vi. Two Inferior Nasal Conchæ.
 * vii. Vomer.
 * viii.Mandible.

In the Basle nomenclature, certain bones developed in association with the nasal capsule, viz., the inferior nasal conchæ, the lacrimals, the nasals, and the vomer, are grouped as cranial and not as facial bones. The hyoid bone, situated at the root of the tongue and attached to the base of the skull by ligaments, is described in this section

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

Terminology

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