Skeleton

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Exoskeleton of an Ant]

In biology, the skeleton is a strong and often a rigid framework that supports the body of an animal, holding it upright and giving it shape and strength (Also skeletal system and, in special cases, shell or test).

The skeleton may be an exoskeleton (external, such as exists in many invertebrates) or an endoskeleton (internal, as in vertebrates. It may be mineralized, or not. It may be jointed, or not. If external, it may be moulted periodically, or not.This article is mostly about the human skeleton, an endoskeleton.

The average adult human skeleton has around 206 bones [ [http://www.enchantedlearning.com/subjects/anatomy/skeleton/Skelprintout.shtml Human Skeleton] , "EnchantedLearning.com", 2008-05-07. ] 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.

Skeletal Protection and Layout

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 membrane coating 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 articulates with the occipital condyle of the skull to enable nodding movements of the head; articulation of the atlas with the 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 girdles 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

A limb (from the Old English lim) is a jointed, or prehensile (as octopus tentacles or new world monkey tails), appendage of the human or animal body.

Most animals use limbs for locomotion, such as walking, running, or climbing. Some animals can use their front limbs (or upper limbs in humans) to carry and manipulate objects. Some animals can also use hind limbs for manipulation.

In the human body, the upper and lower limbs are commonly called the arms and the legs. Human legs and feet are specialized for two-legged locomotion -- most other mammals walk and run on all four limbs. Human arms are weaker, but very mobile, allowing us to reach at a wide range of distances and angles. The arms end in specialized hands that are capable of grasping and fine manipulation of objects.

References

ee also

*Human skeleton