Skull

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The skull is a bony structure found in many animals which serves as the general framework for the head. Those animals having skulls are called Craniates. The skull supports the structures of the face and protects the brain against injury. The skull is made up of two parts: the cranium and the mandible, or lower jaw. A skull that is missing a mandible is only a cranium; this is the source of a very commonly made error in terminology. Protection of the brain is only one part of the function of a bony skull. For example, a fixed distance between the eyes is essential for stereoscopic vision, and a fixed position for the ears helps the brain to 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.

Human skulls

Main article: Human skull

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 form the neurocranium (braincase), a protective vault surrounding the brain and medulla oblongata. Fourteen bones form the splanchnocranium, the bones supporting the face. Encased within the temporal bones are the six ear ossicles of the middle ears. The hyoid bone, supporting the larynx, is usually not considered as part of the skull, as it does not articulate with any other bones.

The skull contains the sinus cavities, which are air-filled cavities lined with respiratory epithelium, which also lines the large airways. The exact functions of the sinuses are unclear; they may contribute to lessening the weight of the skull with a minimal reduction in strength,or they may be important in improving the resonance of the voice. In some animals, such as the elephant, the sinuses are extensive. The elephant skull needs to be very large, to form an attachment for muscles of the neck and trunk, but is also unexpectedly light; the comparatively small brain-case is surrounded by large sinuses which reduce the weight.

The meninges are the three layers, or membranes, which surround the structures of the central nervous system. They are known as the dura mater, the arachnoid mater and the pia mater. Other than being classified together, they have little in common with each other.

In humans, the anatomical position for the skull is the Frankfurt plane, where the lower margins of the orbits and the upper borders of the ear canals are all in a horizontal plane. This is the position where the subject is standing and looking directly forward. For comparison, the skulls of other species, notably primates and hominids, may sometimes be studied in the Frankfurt plane. However, this does not always equate to a natural posture in life.

Mid-facial Skeletal fracture

The mid facial skeleton is made up of a considerable number of bones which are rarely, if ever, fractured in isolation

The structure is such that it is able to withstand considerable force from below, but the bones are easily fractured by relatively trivial forces applied from other directions

Analogous to a ‘matchbox’ sitting below and in front of a hard shell containing the brain and differs quite markedly from the rigid projection of the mandible below

Le Fort I Fractures

Low-level / Guerin type fractures

Horizontal fracture of the maxilla immediately above the teeth and palate

Piriform fossa across maxilla to pterygoid fissure

May occur as a single entity or in association with le fort II and III fractures

Not infrequently present in association with a downwardly displaced fracture of the zygomatic complex

Le Fort II Fractures

Pyramidal or suprazygomatic fractures

Fracture extends from dorsum of nose, across medial walls of orbit across the maxilla below the zygomatic bone to the pterygomaxillary fissure

Le Fort III Fractures

High level or suprazygomatic fractures

The facial bones, including the zygomas are detached from the anterior cranial base

Fracture line extends from the dorsum of the nose and cribiform plate along the medial and up

the lateral wall of the orbit to the ZF suture

Animal skulls

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A hippopotamus' skull	A Tyrannosaurus skull	A cat skull, a typical skull of a carnivore	A coypu skull, a typical rodent
A bulldog skull

See also

• Bone terminology
• Anatomical terms of location
• Head and neck anatomy
• Phrenology, the pseudoscientific process of determining personality from the shape of the head.
• Skull (symbolism)

References

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

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