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For other uses, see Skeleton (disambiguation). Skeleton_sentence_0

This article is about skeletons in general. Skeleton_sentence_1

For the human skeleton, see Human skeleton. Skeleton_sentence_2


GreekSkeleton_header_cell_0_3_0 σκελετόςSkeleton_cell_0_3_1
MeSHSkeleton_header_cell_0_4_0 Skeleton_cell_0_4_1
TA98Skeleton_header_cell_0_5_0 Skeleton_cell_0_5_1
TA2Skeleton_header_cell_0_6_0 Skeleton_cell_0_6_1
FMASkeleton_header_cell_0_7_0 Skeleton_cell_0_7_1

The skeleton is the body part that provides support, shape and protection to the soft tissues and delicate organs of animals. Skeleton_sentence_3

There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside the body, the hydroskeleton, a flexible skeleton supported by fluid pressure, and the cytoskeleton present in the cytoplasm of all cells, including bacteria, and archaea. Skeleton_sentence_4

The term comes from Greek σκελετός (skeletós) 'dried up'. Skeleton_sentence_5

Types of skeletons Skeleton_section_0

There are two major types of skeletons: solid and fluid. Skeleton_sentence_6

Solid skeletons can be internal, called an endoskeleton, or external, called an exoskeleton, and may be further classified as pliant (elastic/movable) or rigid (hard/non-movable). Skeleton_sentence_7

Fluid skeletons are always internal. Skeleton_sentence_8

Exoskeleton Skeleton_section_1

Main article: Exoskeleton Skeleton_sentence_9

Exoskeletons are external, and are found in many invertebrates; they enclose and protect the soft tissues and organs of the body. Skeleton_sentence_10

Some kinds of exoskeletons undergo periodic moulting or ecdysis as the animal grows, as is the case in many arthropods including insects and crustaceans. Skeleton_sentence_11

The exoskeleton of insects is not only a form of protection, but also serves as a surface for muscle attachment, as a watertight protection against drying, and as a sense organ to interact with the environment. Skeleton_sentence_12

The shell of mollusks also performs all of the same functions, except that in most cases it does not contain sense organs. Skeleton_sentence_13

An external skeleton can be quite heavy in relation to the overall mass of an animal, so on land, organisms that have an exoskeleton are mostly relatively small. Skeleton_sentence_14

Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. Skeleton_sentence_15

The southern giant clam, a species of extremely large saltwater clam in the Pacific Ocean, has a shell that is massive in both size and weight. Skeleton_sentence_16

Syrinx aruanus is a species of sea snail with a very large shell. Skeleton_sentence_17

Endoskeleton Skeleton_section_2

Main article: Endoskeleton Skeleton_sentence_18

The endoskeleton is the internal support structure of an animal, composed of mineralized tissue and is typical of vertebrates. Skeleton_sentence_19

Endoskeletons vary in complexity from functioning purely for support (as in the case of sponges), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. Skeleton_sentence_20

A true endoskeleton is derived from mesodermal tissue. Skeleton_sentence_21

Such a skeleton is present in echinoderms and chordates. Skeleton_sentence_22

Pliant skeletons Skeleton_section_3

Pliant skeletons are capable of movement; thus, when stress is applied to the skeletal structure, it deforms and then reverts to its original shape. Skeleton_sentence_23

This skeletal structure is used in some invertebrates, for instance in the hinge of bivalve shells or the mesoglea of cnidarians such as jellyfish. Skeleton_sentence_24

Pliant skeletons are beneficial because only muscle contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape. Skeleton_sentence_25

Cartilage is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture of proteins, polysaccharides, and water. Skeleton_sentence_26

For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Skeleton_sentence_27

Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton. Skeleton_sentence_28

Rigid skeletons Skeleton_section_4

Rigid skeletons are not capable of movement when stressed, creating a strong support system most common in terrestrial animals. Skeleton_sentence_29

Such a skeleton type used by animals that live in water are more for protection (such as barnacle and snail shells) or for fast-moving animals that require additional support of musculature needed for swimming through water. Skeleton_sentence_30

Rigid skeletons are formed from materials including chitin (in arthropods), calcium compounds such as calcium carbonate (in stony corals and mollusks) and silicate (for diatoms and radiolarians). Skeleton_sentence_31

Cytoskeleton Skeleton_section_5

Main article: Cytoskeleton Skeleton_sentence_32

The cytoskeleton (gr. Skeleton_sentence_33

kytos = cell) is used to stabilize and preserve the form of the cells. Skeleton_sentence_34

It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion (using structures such as flagella, cilia and lamellipodia), and plays important roles in both intracellular transport (the movement of vesicles and organelles, for example) and cellular division. Skeleton_sentence_35

Fluid skeletons Skeleton_section_6

Hydrostatic skeleton (hydroskeleton) Skeleton_section_7

Main article: Hydrostatic skeleton Skeleton_sentence_36

A hydrostatic skeleton is a semi-rigid, soft tissue structure filled with liquid under pressure, surrounded by muscles. Skeleton_sentence_37

Longitudinal and circular muscles around their body sectors allow movement by alternate lengthening and contractions along their lengths. Skeleton_sentence_38

A common example of this is the earthworm. Skeleton_sentence_39

Organisms with skeletons Skeleton_section_8

Invertebrates Skeleton_section_9

The endoskeletons of echinoderms and some other soft-bodied invertebrates such as jellyfish and earthworms are also termed hydrostatic; a body cavity the coelom is filled with coelomic fluid and the pressure from this fluid acts together with the surrounding muscles to change the organism's shape and produce movement. Skeleton_sentence_40

Sponges Skeleton_section_10

The skeleton of sponges consists of microscopic calcareous or silicious spicules. Skeleton_sentence_41

The demosponges include 90% of all species of sponges. Skeleton_sentence_42

Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Skeleton_sentence_43

Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges. Skeleton_sentence_44

Echinoderms Skeleton_section_11

The skeleton of the echinoderms, which include, among other things, the starfish, is composed of and a small amount of magnesium oxide. Skeleton_sentence_45

It lies below the epidermis in the mesoderm and is within cell clusters of frame-forming cells. Skeleton_sentence_46

This structure formed is porous and therefore firm and at the same time light. Skeleton_sentence_47

It coalesces into small calcareous ossicles (bony plates), which can grow in all directions and thus can replace the loss of a body part. Skeleton_sentence_48

Connected by joints, the individual skeletal parts can be moved by the muscles. Skeleton_sentence_49

Vertebrates Skeleton_section_12

In most vertebrates, the main skeletal component is referred to as bone. Skeleton_sentence_50

These bones compose a unique skeletal system for each type of animal. Skeleton_sentence_51

Another important component is cartilage which in mammals is found mainly in the joint areas. Skeleton_sentence_52

In other animals, such as the cartilaginous fishes, which include the sharks, the skeleton is composed entirely of cartilage. Skeleton_sentence_53

The segmental pattern of the skeleton is present in all vertebrates (mammals, birds, fish, reptiles and amphibians) with basic units being repeated. Skeleton_sentence_54

This segmental pattern is particularly evident in the vertebral column and the ribcage. Skeleton_sentence_55

Bones in addition to supporting the body also serve, at the cellular level, as calcium and phosphate storage. Skeleton_sentence_56

Fish Skeleton_section_13

Main article: Fish anatomy Skeleton_sentence_57

The skeleton, which forms the support structure inside the fish is either made of cartilage as in the (Chondrichthyes), or bones as in the (Osteichthyes). Skeleton_sentence_58

The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. Skeleton_sentence_59

The ribs attach to the spine and there are no limbs or limb girdles. Skeleton_sentence_60

They are supported only by the muscles. Skeleton_sentence_61

The main external features of the fish, the fins, are composed of either bony or soft spines called rays, which with the exception of the caudal fin (tail fin), have no direct connection with the spine. Skeleton_sentence_62

They are supported by the muscles which compose the main part of the trunk. Skeleton_sentence_63

Birds Skeleton_section_14

Main article: Bird anatomy Skeleton_sentence_64

The bird skeleton is highly adapted for flight. Skeleton_sentence_65

It is extremely lightweight, yet still strong enough to withstand the stresses of taking off, flying, and landing. Skeleton_sentence_66

One key adaptation is the fusing of bones into single ossifications, such as the pygostyle. Skeleton_sentence_67

Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Skeleton_sentence_68

Birds also lack teeth or even a true jaw, instead having evolved a beak, which is far more lightweight. Skeleton_sentence_69

The beaks of many baby birds have a projection called an egg tooth, which facilitates their exit from the amniotic egg. Skeleton_sentence_70

Marine mammals Skeleton_section_15

To facilitate the movement of marine mammals in water, the hind legs were either lost altogether, as in the whales and manatees, or united in a single tail fin as in the pinnipeds (seals). Skeleton_sentence_71

In the whale, the cervical vertebrae are typically fused, an adaptation trading flexibility for stability during swimming. Skeleton_sentence_72

Humans Skeleton_section_16

Main article: Human skeleton Skeleton_sentence_73

The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. Skeleton_sentence_74

It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs, heart and spinal cord. Skeleton_sentence_75

Although the teeth do not consist of tissue commonly found in bones, the teeth are usually considered as members of the skeletal system. Skeleton_sentence_76

The biggest bone in the body is the femur in the upper leg, and the smallest is the stapes bone in the middle ear. Skeleton_sentence_77

In an adult, the skeleton comprises around 14% of the total body weight, and half of this weight is water. Skeleton_sentence_78

Fused bones include those of the pelvis and the cranium. Skeleton_sentence_79

Not all bones are interconnected directly: There are three bones in each middle ear called the ossicles that articulate only with each other. Skeleton_sentence_80

The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments. Skeleton_sentence_81

There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the hip bones on each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether the coccyx or tail bone is counted as one or four separate bones, and does not count the variable wormian bones between skull sutures. Skeleton_sentence_82

Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. Skeleton_sentence_83

There is also a variable number of small sesamoid bones, commonly found in tendons. Skeleton_sentence_84

The patella or kneecap on each side is an example of a larger sesamoid bone. Skeleton_sentence_85

The patellae are counted in the total, as they are constant. Skeleton_sentence_86

The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together. Skeleton_sentence_87

These bones are organized into a longitudinal axis, the axial skeleton, to which the appendicular skeleton is attached. Skeleton_sentence_88

The human skeleton takes 20 years before it is fully developed, and the bones contain marrow, which produces blood cells. Skeleton_sentence_89

There exist several general differences between the male and female skeletons. Skeleton_sentence_90

The male skeleton, for example, is generally larger and heavier than the female skeleton. Skeleton_sentence_91

In the female skeleton, the bones of the skull are generally less angular. Skeleton_sentence_92

The female skeleton also has wider and shorter breastbone and slimmer wrists. Skeleton_sentence_93

There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. Skeleton_sentence_94

The female pelvis is wider and shallower than the male pelvis. Skeleton_sentence_95

Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. Skeleton_sentence_96

The angle between the pubic bones is known to be sharper in males, which results in a more circular, narrower, and near heart-shaped pelvis. Skeleton_sentence_97

Parts Skeleton_section_17

Bone Skeleton_section_18

Main article: Bone Skeleton_sentence_98

Bones are rigid organs that form part of the endoskeleton of vertebrates. Skeleton_sentence_99

They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Skeleton_sentence_100

Bone tissue is a type of dense connective tissue. Skeleton_sentence_101

Bones have a variety of shapes with a complex internal and external structure they are also lightweight, yet strong and hard. Skeleton_sentence_102

One of the types of tissue that makes up bone tissue is mineralized tissue and this gives it rigidity and a honeycomb-like three-dimensional internal structure. Skeleton_sentence_103

Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. Skeleton_sentence_104

Cartilage Skeleton_section_19

Main article: Cartilage Skeleton_sentence_105

During embryonic development the precursor to bone development is cartilage that mostly becomes replaced by bone, after flesh such as muscle has formed around it. Skeleton_sentence_106

Cartilage is a stiff and inflexible connective tissue found in many areas including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs. Skeleton_sentence_107

It is not as hard and rigid as bone but is stiffer and less flexible than muscle. Skeleton_sentence_108

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of Type II collagen (except fibrocartilage which also contains type I collagen) fibers, abundant ground substance rich in proteoglycans, and elastin fibers. Skeleton_sentence_109

Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components. Skeleton_sentence_110

Unlike other connective tissues, cartilage does not contain blood vessels. Skeleton_sentence_111

The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Skeleton_sentence_112

Thus, compared to other connective tissues, cartilage grows and repairs more slowly. Skeleton_sentence_113

Ligament Skeleton_section_20

Main article: Ligament Skeleton_sentence_114

A ligament is a piece of rubbery tissue that connects bone to other bone. Skeleton_sentence_115

It is commonly confused with the tendon, a similar structure that connects muscle to bone. Skeleton_sentence_116

Tendon Skeleton_section_21

Main article: Tendon Skeleton_sentence_117

A tendon is a rubber-band like tissue that connects muscle to bone. Skeleton_sentence_118

It is not to be confused with the ligament, a similar tissue that connects bone to bone. Skeleton_sentence_119

Culture Skeleton_section_22

In Western culture, the human skeleton is oftentimes seen as a fearful symbol of death and the paranormal. Skeleton_sentence_120

It is a popular motif in the holiday Halloween, as well as Day of the Dead. Skeleton_sentence_121

Movies Skeleton_section_23

Skeletons can also be found in movies. Skeleton_sentence_122

Skeletons in movies can be often depicted coming to life, commonly in horror movies. Skeleton_sentence_123

Skeletons can also be depicted in movies wearing chainmail, helmets, and shields. Skeleton_sentence_124

Commonly holding an axe or sword. Skeleton_sentence_125

In these types of movies they are commonly getting attacked, "killed", or fighting with character(s). Skeleton_sentence_126

Skeletons can also be found in a more "welcoming" and "friendly" way in movies. Skeleton_sentence_127

Such as, playing as a decoration, a Halloween costume/face paint, ETC. Another way skeletons can be shown in movies is debatably more common than the other depictions is a sign of severe burning from things such as chemicals, fire, and acid. Skeleton_sentence_128

This can also be a case of deterioration over time. Skeleton_sentence_129

See also Skeleton_section_24

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