skeletal system

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The bodily system that consists of the bones, their associated cartilages, and the joints, and supports and protects the body, produces blood cells, and stores minerals.

The supporting tissues of animals which often serve to protect the body, or parts of it, and play an important role in the animal's physiology.

Skeletons can be divided into two main types based on the relative position of the skeletal tissues. When these tissues are located external to the soft parts, the animal is said to have an exoskeleton. If they occur deep within the body, they form an endoskeleton. All vertebrate animals possess an endoskeleton, but most also have components that are exoskeletal in origin. Invertebrate skeletons, however, show far more variation in position, morphology, and materials used to construct them.

The vertebrate endoskeleton is usually constructed of bone and cartilage; only certain fishes have skeletons that lack bone. In addition to an endoskeleton, many species possess distinct exoskeletal structures made of bone or horny materials. This dermal skeleton provides support and protection at the body surface.

Various structural components make up the human skeleton, including collagen, three different types of cartilage (hyaline, fibrocartilage, and elastic), and a variety of bone types (woven, lamellar, trabecular, and plexiform). See also Bone; Collagen; Connective tissue.

The vertebrate skeleton consists of the axial skeleton (skull, vertebral column, and associated structures) and the appendicular skeleton (limbs or appendages). The basic plan for vertebrates is similar, although large variations occur in relation to functional demands placed on the skeleton.

Axial skeleton

The axial skeleton supports and protects the organs of the head, neck, and torso, and in humans it comprises the skull, ear ossicles, hyoid bone, vertebral column, and rib cage.

Skull

The adult human skull consists of eight bones which form the cranium, or braincase, and 13 facial bones that support the eyes, nose, and jaws. There are also three small, paired ear ossicles—the malleus, incus, and stapes—within a cavity in the temporal bone. The total of 27 bones represents a large reduction in skull elements during the course of vertebrate evolution. The three components of the skull are the neurocranium, dermatocranium, and visceral cranium. See also Ear (vertebrate).

The brain and certain sense organs are protected by the neurocranium. All vertebrate neurocrania develop similarly, starting as ethmoid and basal cartilages beneath the brain, and as capsules partially enclosing the tissues that eventually form the olfactory, otic, and optic sense organs. Further development produces cartilaginous walls around the brain. Passages (foramina) through the cartilages are left open for cranial nerves and blood vessels. Endochondral ossification from four major centers follows in all vertebrates, except the cartilaginous fishes.

The visceral skeleton, the skeleton of the pharyngeal arches, is demonstrated in a general form by the elasmobranch fishes, where all the elements are cartilaginous and support the jaws and the gills. The mandibular (first) arch consists of two elements on each side of the body: the palatoquadrates dorsally, which form the upper jaw, and Meckel's cartilages, which join ventrally to form the lower jaw. The hyoid (second) arch has paired dorsal hyomandibular cartilages and lateral, gill-bearing ceratohyals. This jaw mechanism attaches to the neurocranium for support. In all jawed vertebrates except mammals, an articulation between the posterior ends of the palatoquadrate and Meckel's cartilages occurs between the upper and lower jaws. The bony fishes have elaborated on the primitive condition, where the upper jaw was fused to the skull and the lower jaw or mandible could move only in the manner of a simple hinge. Teleosts are able to protrude the upper and lower jaws. In the course of mammalian evolution, the dentary of the lower jaw enlarged and a ramus expanded upward in the temporal fossa. This eventually formed an articulation with the squamosal of the skull. With the freeing of the articular bone and the quadrate from their function in jaw articulation, they became ear ossicles in conjunction with the columella, that is, a skeletal rod that formed the first ear ossicle. The remaining visceral skeleton has evolved from jaw and gill structures in the fishes to become an attachment site for tongue muscles and to support the vocal cords in tetrapods. See also Mammalia.

Vertebral column

The vertebral column is an endoskeletal segmented rod of mesodermal origin. It provides protection to the spinal cord, sites for muscle attachment, flexibility, and support, particularly in land-based tetrapods where it has to support the weight of the body. Hard, spool-shaped bony vertebrae alternate with tough but pliable intervertebral discs. Each typical vertebral body (centrum) has a bony neural arch extending dorsally. The spinal cord runs through these arches, and spinal nerves emerge through spaces. Bony processes and spines project from the vertebrae for the attachment of muscles and ligaments. Synovial articulations between adjacent vertebrae effectively limit and define the range of vertebral motion.

Vertebral morphology differs along the length of the column. There are two recognized regions in fishes (trunk and caudal) and five in mammals (cervical, thoracic, lumbar, sacral, and caudal), reflecting regional specializations linked to function. Humans have seven cervical, twelve thoracic, five lumbar, five (fused) sacral, and four coccygeal vertebrae. Most amphibians, reptiles, and mammals have seven cervical vertebrae regardless of neck length, whereas the number is variable in birds. Specific modification to the first two cervical vertebrae in most reptiles, birds, and mammals gives the head extra mobility. The presence of large ribs in the thoracic region often limits spinal flexibility. In typical tetrapods, the sacral region is usually modified for support of the pelvic girdle, while the number of caudal vertebrae varies greatly (from 0 to 50) between and within animal groups. See also Vertebra.

Sternum and ribs

Jawed fishes have ribs that help maintain the rigidity and support of the coelomic cavity. These ribs typically follow the connective tissue septa that divide successive muscle groups. In the caudal region, they are often small paired ventral ribs, fused on the midline to form the haemal arches. Ancestral tetrapods had ribs on all vertebrae, and their lengths varied between the vertebral regions. Modern amphibia (frogs and toads) have few thoracic ribs, and these are much reduced and never meet ventrally. Reptiles have varied rib arrangements, ranging from snakes with ribs on each vertebra (important for locomotor requirements) to turtles with only eight ribs which are fused to the inside of the carapace. Flying birds and penguins have a greatly enlarged sternum that links the ribs ventrally. In humans there are twelve pairs of ribs which form a strong but movable cage encompassing the heart and lungs.

Appendicular skeleton

This section of the skeletal system comprises the pectoral and pelvic limb girdles and bones of the free appendages. The girdles provide a supporting base onto which the usually mobile limbs attach.

Pectoral girdle

The pectoral girdle has both endoskeletal and dermal components. The dermal components are derived from postopercular dermal armor of primitive fishes, and are represented by the clavicles and interclavicles in modern vertebrates, except where they are secondarily lost. Endochondral bone forms the scapula. In fishes, the main component of the girdle (the cleithrum) is anchored to the skull by other bony elements. Increased mobility of the girdle is seen in amphibia as it becomes independent of the skull. Further development and skeletal reduction have resulted in a wide range of morphologies, culminating in the paired clavicles and scapulae of mammals.

Birds have fused their paired clavicles and single interclavicle to form the wishbone or furcula. Clavicles have disappeared in certain groups of bounding mammals to allow greater movement of the scapula. Although humans, and most other mammals, have a coracoid process on the scapula, other tetrapods typically have a separate coracoid bracing the scapula against the sternum and forming part of the glenoid fossa.

Pelvic girdle

The pelvic girdle forms by endochondral ossification, that is, the conversion of cartilage into bone. In the fishes, it is a small structure embedded in the body wall musculature just anterior to the cloaca. Each half of the girdle provides an anchor and articulation point for the pelvic fins. In tetrapods, the girdle attaches to the vertebral column to increase its stability and assist in the support of body weight and locomotor forces. Humans, like all other tetrapods, have a bilaterally symmetrical pelvic girdle, each half of which is formed from three fused bones: the ischium, ilium, and pubis. A part of each of these elements forms the acetabulum, the socket-shaped component of the hip joint, that articulates with the femoral head.

All urogenital and digestive products have to pass through the pelvic outlet. This accounts for the pelvic sexual dimorphism seen in most mammals, where the pelvic opening is broader in females, because of the physical demands of pregnancy and parturition. In birds (with the exception of the ostrich and the rhea), both sexes have an open pelvic girdle, a condition also found in female megachiropteran bats (flying foxes), gophers, and mole-rats.

Paired fins and tetrapod limbs

Paired fins in fishes come in different forms, but all are involved in locomotion. In the simplest form they are fairly rigid and extend from the body, functioning as stabilizers, but they are also capable of acting like a wing to produce lift as in sharks. In many fishes, the pectoral fins have narrow bases and are highly maneuverable as steering fins for low-speed locomotion. In addition, some fishes use their pectoral and pelvic fins to walk on the river bed, while others have greatly enlarged pectoral fins that take over as the main propulsive structures.

The basic mammalian pectoral limb consists of the humerus, radius, ulna, carpals, five metacarpals, and fourteen phalanges; and the pelvic limb consists of the femur, tibia, fibula, tarsal, five metatarsals, and fourteen phalanges. A typical bird pelvic limb consists of a femur, tibiotarsus (formed by fusion of the tibia with the proximal row of tarsal bones), fibula, and tarsometatarsus (formed by fusion of metatarsals II–IV), metatarsal I, and four digits (each consisting of two to five phalanges).

All bones and cartilage of the body that collectively provide the supporting framework for the muscles and organs.

System in the body consisting of bone and cartilage which provides the basic framework through which muscles act.

The framework of the body, consisting of bones and other connective tissues, which protects and supports the body tissues and internal organs. The human skeleton contains 206 bones, six of which are the tiny bones of the middle ear (three in each ear) that function in hearing. The largest bone in the body is the thigh bone, or femur.

For other uses, see Skeleton (disambiguation).

Endoskeleton of an Elephant

The Skeletons of a Man and a Horse placed in a lifelike pose in a museum display

Endoskeleton of a Frog

Skeletons of Snakes and other reptiles

Exoskeleton of an Ant

In biology, a skeleton is a rigid framework that provides protection and structure in many types of animal, particularly those of the phylum Chordata and of the superphylum Ecdysozoa. Exoskeletons are external, as is typical of many invertebrates; they enclose the soft tissues and organs of the body. Exoskeletons may undergo periodic moulting as the animal grows. Endoskeletons are internal, as is typical of many vertebrates; they are usually surrounded by skin and musculature, though they often enclose vital organs. Endoskeletons are attachment points for musculature and act as leverage for movement, and in many animals contain marrow, which produces blood cells. Skeletons may or may not be mineralized - human skeletons are calcified, while shark skeletons are cartilaginous - and may be jointed for flexibility and motility or rigid for structural strength.

The average adult human skeleton has around 206 bones. ^[1] These bones meet at joints, the majority of which are freely movable. 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.

Contents 1 The Human Skull 1.1 Cranial bones 1.2 Facial bones 1.3 Sinuses 1.4 Skull development 2 Ribs 3 Limbs 4 Animal skeletons 5 References 6 See also

The Human Skull

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Main article: Human skull

The human skull shapes the head and face, protects the 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 eight 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 (mainly 7 on each side) 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.

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)^{[citation needed]} 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; however, 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. Femur, Humerus, Radius and Ulna, Cranium, Sternum, Clavicle, Fibulu and Tibia, Vertebrae, Scalpula, Pelvic Bone, and Coccyx.

Animal skeletons

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References

1. ^ Human Skeleton, EnchantedLearning.com, 2008-05-07.

See also

Wikimedia Commons has media related to: Skeletons

• Human skeleton
• Exoskeleton
• Endoskeleton

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