Please wait while we load your article...

Home > Cranium

Learn more about "Cranium"

Skull

The '''skull''' is a bone|bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury. The skull can be subdivided into two parts: the '''cranium''' and the mandible|'''mandible'''. A skull that is missing a mandible is only a cranium; this is the source of a very commonly made error in terminology. Those animals having skulls are called Craniata|craniates. 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

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 of skull|sutures, rigid articulations permitting very little movement. Eight bones form the ''neurocranium'' (braincase)—including the frontal, parietals, occipital bone, sphenoid, temporals and ethmoid—a protective cranial vault|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. The skull contains the sinus cavity|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 decreasing the weight of the skull with a minimal decrease 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 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 orbit (anatomy)|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.

Other skulls

Temporal Fenestra

The temporal fenestra are anatomical features of the amniota|amniote skull, 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 is 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 the more advanced sauropsids and synapsids did. As time progressed, sauropsids' 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:
- 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 Evolutionary, they are related as follows:
- Amniota
    - Class '''Synapsida'''
    - *Order Therapsida
    - **Class Mammalia - mammals
    - Class Sauropsida - reptiles
    - *Subclass '''Anapsida'''
    - *(unranked) Eureptilia
    - **Subclass '''Diapsida'''
    - ***(unranked) '''Euryapsida'''
    - ***Class bird|Aves - birds Image:Hippo skull dark.jpg|A hippopotamus' skull Image:Tyrannoskull.jpg|A ''Tyrannosaurus'' skull Image:Felis catus-skull-drawing.jpg|A cat skull, a typical skull of a carnivore Image:Nutriaschädel.jpg|A coypu skull, a typical rodent Image:Bulldog-skull.JPG|A bulldog skull Image:George's skull1.jpg|A gerbil skull Image:Raccoon_skull_Pengo.jpg|A raccoon's skull Image:Grizzly skull.jpg|A Grizzly bear's skull Image:Panthera leo Kruger Skull.jpg|A lion's skull Image:Cervocerus novorossiae.jpg|A Cervocerus novorossiae's skull Image:ChausinghaSkullLyd2.png|A Four-horned antelope skull drawing

References

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

External links

- Skull preparation and curation (An excellent site with information about skull and skeleton cleaning and articulation with wonderful photos)
- Animal Skull Collection (Over 300 animal skull images compiled by U.S. high-school teacher)
- Ray Bandar: A Life With Skulls Film about California animal skull collector Ray Bandar
- Skull terminology site by Texas A&M
- Anatomy of cranial cavity.
- Dept of Anth Skull Module
- Skull Anatomy Tutorial.
- GMA NEWS.TV, video, (Filipino language) Baby born without skull - 01/19/2008 Category:Skull| Category:Animal anatomy Category:Head and neck Category:Flat bones simple:Skull

Related Images

- Human skull (front)
- Human skull (side)

Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL

Welcome to Start Learning Now. Explore to your heart's content, and we hope you enjoy reading the material we have assembled for you here!

Related News

Further Resources


	home lucky pick

Related Resources