Skeleton
27609
226067163
2008-07-16T18:03:02Z
MattieTK
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Reverted edits by [[Special:Contributions/129.24.111.102|129.24.111.102]] to last version by Smith609 (using [[WP:HG|Huggle]])
{{Cleanup|date=March 2008}}
{{otheruses}}
[[Image:ElephantSkelLyd2.png|thumb|[[Endoskeleton]] of an [[Elephant]]]]
[[Image:Rana skeleton.png|thumb|[[Endoskeleton]] of a [[Frog]]]]
[[Image:Hym-myrmicinae.gif|thumb|[[Exoskeleton]] of an [[Ant]]]]
In [[biology]], the '''skeleton''' or '''skeletal system''' is a strong framework that supports the body. External rigid frameworks, such as those found in some [[invertebrate]]s (e.g. [[insect]]s), are termed [[exoskeleton]]s. Internal rigid frameworks, such as those found in most [[vertebrate]]s (e.g. [[mammals]]), are termed [[endoskeleton]]s. In some cases, Skeletons may also be referred to as '''test'''s.
The average adult [[human skeleton]] has around 206 [[bone]]s <ref> [http://www.enchantedlearning.com/subjects/anatomy/skeleton/Skelprintout.shtml Human Skeleton], ''EnchantedLearning.com'', [[2008-05-07]]. </ref> These bones meet at [[joints]], the majority of which are freely movable, making the skeleton flexible and mobile. The skeleton also contains [[cartilage]] for elasticity. [[Ligaments]] are strong strips of fibrous connective tissue that hold bones together at joints, thereby stabilizing the skeleton during movement.
==Main functions==
A skeleton provides "cages" to protect the delicate organs. For example: the ribcage helps protect the lungs, heart, etc. Moreover, it gives muscles "sides" or edges to attach to (through tendons). It also maintains balance, supports the body's shape and allows for motion. Another function of the skeleton is to make red and white blood cells. The Marrow in the bone produces these cells for when the body needs them. But all these functions are perhaps secondary to the primary function of locomotion. The bones allow the muscles of the body to move the limbs. Bones come in different sizes; the longest bone is the [[femur]], and the shortest, the [[stapes]]. The First layer of a bone is called the Periostium. It is a thin membraine coarting the compact bone. The next layer of bones are called compact bones, and the middle, called spongy or cancellous bone, which are hollow and contain bone marrow. Joints of bones are held together by ligaments, which are fine fibers attaching to each bone at joints. In order for bones to not scrape against each other, slippery cartilage and synovial fluid are covered atop the bones.
==The Skull==
The human skull shapes the head and face, protects the fragile brain, and houses and protects special sense organs for taste, smell, hearing, vision, and balance. It is constructed from 22 bones, 21 of which are locked together by immovable joints, to form a structure of great strength.
The bony framework of the head is called the [[skull]], and it is subdivided into 2 parts, namely;
====Cranial Bones====
The 8 bones of the [[cranium]] support, surround and protect the [[brain]] within the cranial cavity. They form the roof, sides, and back of the cranium, as well as the cranial floor on which the brain rests. The [[frontal bones]] and the [[parietal bones]] form the roof and sides of the cranium. Two in the temporal bone, the [[external auditory meatus]], directs sounds into the inner part of the ear that is encased within, and which contains three small, linked bones called [[ossicles]]. The occipital bones forms the posterior part of the cranium and much of the cranial floor. The [[occipital bone]] has a large opening, the [[foramen magnum]], through which the brain connects to the [[spinal cord]]. The [[occipital condyles]] articulate with the atlas (first cervical vertebra), enabling nodding movements of the head. The [[ethmoid bone]] forms part of the cranial floor, the medial walls of the orbits, and the upper parts of the nasal septum, which divides the nasal cavity vertical into left and right sides, The [[sphenoid bone]], which is shaped like a bat's wings, acts as a keystone by articulating with and holding together, all the other cranial bones.
====Facial Bones====
The 14 facial bones form the framework of the face; provide cavities for the sense organs of smell, taste, and vision; anchor the teeth; form openings for the passage of food, water, and air; and provide attachment points for the muscles that produce facial expressions. Two [[maxillae]] form the upper jaw, contain sockets for the 16 upper teeth, and link all other facial bones apart from the [[mandible]] (lower jaw). Two [[zygomatic bones]] (cheekbones), form the prominences of the cheeks and part of the lateral margins of the orbits. Two [[lacrimal bones]] form part of the medial wall of each orbit. Two [[nasal bones]] form the bridge of the nose. Two [[palatine bones]] from the posterior side walls of the nasal cavity and posterior part of the hard palate. Two inferior [[nasal conchae]] form part of the lateral wall of the [[nasal cavity]]. The [[vomer]] forms part of the [[nasal septum]]. The [[mandible]], the only skull bone that is able to move, articulates with the temporal bone allowing the mouth to open and close, and provides anchorage for the 16 lower teeth.
=== Sinuses ===
[[Sinuses]] are air-filled bubbles found in the frontal, sphenoid, ethmoid, and paired maxillae, clustered around the nasal cavity. These spaces reduce the overall weight of the skull.
=== Skull development ===
In the fetus, skull bones are formed by [[intramembranous ossification]]. A fibrous membrane ossifies to form skull bones linked by areas of as yet unossifed areas of membrane called [[fontanelles]]. At birth, these flexible areas allow the head to be slightly compressed, and permit brain growth during early infancy. These are named the anterior (Frontal) fontanelle, posterior (Occipital) fontanelle, anterolateral (Sphenoidal)fontanelle, and the posterolateral (Mastoid) fontanelle.
== Backbone and ribs ==
Together with the [[sternum]] and [[ribs]], the backbones (also known as the [[vertebral column]], [[spinal column]], or spine) forms the skeleton of the trunk. The backbone consists of a chain of irregular bones called [[vertebrae]] that meet at slightly movable joints. Each joint permits only limited movement, but collectively the joints give the backbone considerable flexibility enabling it to rotate, and to bend anteriorly, posteriorly, and laterally. The average backbone makes up about 40 percent of body height. It extends from the skull to its anchorage in the [[pelvic girdle]], where it transmits the weight of the head and trunk to the lower limbs. It also supports the skull; encloses and protects the delicate spinal cord; and provides an attachment point for the ribs, and for the muscles and ligaments that support the trunk of the body.
=== Intervertebral discs ===
[[Intervertebral discs]] are found between adjacent vertebrae from the second cervical vertebra (axis) to the [[sacrum]]. Each disc has an inner soft, pulp nucleus coverbrous covering of fibrous cartilage. Each disc forms a strong, slightly movable joint. Collectively, discs cushion vertebrae against vertical shocks, and allow various movements of the backbone.
=== Vertebral curves ===
A normal backbone has four curves that give it an S-shape. The cervical and lumbar curves are convex anteriorly, while the thoracic and sacral curves are concave anteriorly. The S-shape allows the backbone to function as a spring rather than a flexible rod, thereby absorbing shock during walking and running; enhancing the strength and flexibility of the backbone; and facilitating balance when upright by placing the trunk directly over the feet.
=== Regions of the backbone ===
An adult backbone consists of 26 vertebrae of which two, the [[sacrum]] and [[coccyx]], are composites consisting of vertebrae that fuse during childhood. The backbone has five sections. Seven small cervical vertebrae form the neck ,which is the most flexible part of the backbone. The uppermost cervical vertebra, the [[Atlas (anatomy)|atlas]] articulates with the occipital condyle of the skull to enable nodding movements of the head; articulation of the atlas with the [[Axis (anatomy)|axis]], the second [[cervical vertebra]], produces shaking movement of the head. Twelve [[thoracic vertebrae]] each articulate with a pair of ribs. Five large [[lumbar vertebrae]] form the hollow small of the back and bear most of the weight of the head and trunk. The triangular [[sacrum]], made of five fused bones, forms a strong anchorage for the [[pelvic girdle]], with which it forms the pelvis. The [[coccyx]], or tailbone, consists of four fused vertebrae.
=== Bony thorax ===
The cone-shaped bony thorax surrounds the thoracic cavity, and is formed by 12 [[thoracic vertebrae]] posteriorly, 24 ribs laterally, and the [[sternum]] and [[costal cartilages]] anteriorly. Its cage-like structure protects the thoracic and upper abdominal organs, supports the [[pectoral girdle]]s and upper limbs, and facilitates breathing.
=== Ribs ===
The [[ribs]] are curved, flat bones with a slightly twisted shaft. The 12 pairs of ribs form a ribcage that protects the heart, lungs, major blood vessels, stomach, liver, etc. At its posterior end, the head of each rib articulates with the facets on the centra of adjacent vertebrae, and with a facet on a transverse process. These vertebrocostal joints are plane joints that allow gliding movements. At their anterior ends, the upper ten pairs of ribs attach directly or indirectly to the sternum by flexible costal cartilages.. Together, vertebrocostal joints and costal cartilages give the ribcage sufficient flexibility to make movements up and down during breathing. Ribs 1-7 are called "true ribs". Ribs 8-12 are called "false ribs" of which ribs 11 and 12 are "floating" ribs that articulate with the sternum indirectly via the costal cartilage of another rib or not.
== Limbs ==
{{Expand-section|date=June 2008}}
==References==
<references/>
==See also==
{{commons|Category:Skeletons|Skeletons}}
*[[Human skeleton]]
[[Category:Skeletal system| ]]
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