TheInfoList

A skeleton is a structural frame that supports an
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

body. There are several different skeletal types: the
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

, which is the stable outer shell of an organism, the
endoskeleton An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the ...
, which forms the support structure inside the body, and the
hydroskeleton A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes refer ...
, a flexible skeleton supported by fluid pressure. The term comes .

Types of skeletons

There are two major types of skeletons: solid and fluid. Solid skeletons can be internal, called an
endoskeleton An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the ...
, or external, called an
exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

, and may be further classified as pliant (elastic/movable) or rigid (hard/non-movable).. Fluid skeletons are always internal.

Exoskeleton

Exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

s are external, and are found in many
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s; they enclose and protect the
soft tissue Micrograph of a tendon. H&E stain">tendon.html" ;"title="Micrograph of a tendon">Micrograph of a tendon. H&E stain. Soft tissue is all the tissue in the body that is not hard tissue, hardened by the processes of ossification or calcification such ...
s and organs of the body. Some kinds of exoskeletons undergo periodic
moult In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

ing or
ecdysis Ecdysis is the moulting In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology ...
as the animal grows, as is the case in many
arthropod An arthropod (, (gen. ποδός)) is an invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart fr ...
s including insects and
crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda, amphipods and mantis shrimp. The ...
s. 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. The
shell Shell may refer to: Architecture and design * Shell (structure)A shell is a type of structural element which is characterized by its geometry, being a three-dimensional solid whose thickness is very small when compared with other dimensions, and ...

of mollusks also performs all of the same functions, except that in most cases it does not contain sense organs. 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. Somewhat larger aquatic animals can support an exoskeleton because weight is less of a consideration underwater. The
southern giant clam ''Tridacna derasa'', the southern giant clam or smooth giant clam, is a species of extremely large marine clam in the family Cardiidae. Description The southern giant clam is one of the largest of the "giant clams", reaching up to 60 cm in ...
, a species of extremely large saltwater clam in the Pacific Ocean, has a shell that is massive in both size and weight. ''
Syrinx aruanus ''Syrinx aruanus'', common name the Australian trumpet or false trumpet, is a species of extremely large sea snail measuring up to 91 cm long and weighing up to 18 kg. It is a marine (ocean), marine gastropod mollusk in the family Turbi ...
'' is a species of sea snail with a very large shell.

Endoskeleton

The
endoskeleton An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the ...
is the internal support structure of an animal, composed of
mineralized tissue, sea shells, conch, dentin, radiolarian, antler, bone Mineralized tissues are biological tissue (biology), tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusk ...
and is typical of
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s. Endoskeletons vary in complexity from functioning purely for support (as in the case of
sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

), to serving as an attachment site for muscles and a mechanism for transmitting muscular forces. A true endoskeleton is derived from
mesodermal In all bilaterian animals, the mesoderm is one of the three primary germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrate Vertebrates () comprise all species of ani ...

tissue. Such a skeleton is present in
echinoderms An echinoderm () is any member of the phylum Echinodermata (; ) of marine life, marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include starfish, sea urchins, sand dollars, and sea cucumbers, as we ...
and
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess 5 Apomorphy and synapomorphy , synapomorphies, or primary characteristics, at some point during their larval or adulthood stages that distinguish them from all other ta ...
s.

Pliant skeletons

Pliant skeletons are capable of movement; thus, when stress is applied to the skeletal structure, it deforms and then reverts to its original shape. This skeletal structure is used in some invertebrates, for instance in the hinge of
bivalve shell A bivalve shell is part of the body, the exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal ...

s or the
mesoglea Mesoglea refers to the tissue found in cnidaria Image:Sea nettles.jpg, Chrysaora fuscescens, Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found bot ...
of
cnidarians Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...
such as
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

. Pliant skeletons are beneficial because only
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

contractions are needed to bend the skeleton; upon muscle relaxation, the skeleton will return to its original shape.
Cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

is one material that a pliant skeleton may be composed of, but most pliant skeletons are formed from a mixture of
proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

,
polysaccharides , a beta-glucan polysaccharide Image:amylose 3Dprojection.svg, 350px, Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the o ...
, and water. For additional structure or protection, pliant skeletons may be supported by rigid skeletons. Organisms that have pliant skeletons typically live in water, which supports body structure in the absence of a rigid skeleton.

Rigid skeletons

Rigid skeletons are not capable of movement when stressed, creating a strong support system most common in
terrestrial animals Terrestrial animals are animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellul ...
. Such a skeleton type used by animals that live in water are more for protection (such as
barnacle A barnacle is a type of arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference sh ...

and
snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (co ...

shells) or for fast-moving animals that require additional support of musculature needed for swimming through water. Rigid skeletons are formed from materials including
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

(in arthropods),
calcium Calcium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

compounds such as
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

(in
stony coral Scleractinia, also called stony corals or hard corals, are marine animals in the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the gra ...
s and
mollusk Mollusca is the second-largest phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number ...
s) and
silicate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
(for
diatom Diatoms (''diá-tom-os'' 'cut in half', from ''diá'', 'through' or 'apart', and the root of ''tém-n-ō'', 'I cut') are a major group of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make ...

s and
radiolarian The Radiolaria, also called Radiozoa, are protozoa Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryotes, either free-living or parasitic, which feed on organic matter such as other microor ...

s).

Fluid skeletons

Hydrostatic skeleton (hydroskeleton)

A hydrostatic skeleton is a semi-rigid, soft tissue structure filled with liquid under pressure, surrounded by muscles. Longitudinal and circular muscles around their body sectors allow movement by alternate lengthening and contractions along their lengths. Common examples exist in both
bilateria The Bilateria or bilaterians are animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material ...
ns (some less advanced ones) and
cnidaria Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

ns, implying a common ancestor having it.

Cytoskeleton

The cytoskeleton (''cyto-'' meaning cell) is used to stabilize and preserve the form of the cells. It is a dynamic structure that maintains cell shape, protects the cell, enables cellular motion (using structures such as
flagella A flagellum (; ) is a hairlike appendage that protrudes from a wide range of microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology, an organism () is any organic, life, living system that fu ...

,
cilia The cilium (; the plural is cilia) is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

and
lamellipodia The lamellipodium (plural lamellipodia) (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. ...
), and plays important roles in both intracellular transport (the movement of
vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features of ...
and
organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
s, for example) and cellular division. Even though called a "skeleton", the word's usage is commonly more restricted to animals who have them as, for example, a plant cell has cell wall and fluid filled vacuole inside which provide a structural framework even though they aren't called together a hydroskeleton.

Organisms with skeletons

Invertebrates

The endoskeletons of echinoderms and some other soft-bodied
invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the chordata, chordate subphylum vertebrate, Vertebra ...
s such as
jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum In zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Con ...

and
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to man ...

s are also termed
hydrostatic Fluid statics or hydrostatics is the branch of fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical object ...
; a body cavity the
coelom The coelom (or celom) is the main body cavity A body cavity is any space or compartment, or potential space Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social ...
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.

Sponges

The skeleton of
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

s consists of microscopic
calcareous ''Calcareous'' is an adjective meaning "mostly or partly composed of calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecula ...
or silicious spicules. The
demosponge '' ( Poecilosclerida) File:Geodia barretti.jpg, ''Geodia barretti'' (Tetractinellida) Demosponges (Class Demospongiae) are the most diverse Class (biology), class in the phylum Porifera. They include 76.2% of all species of sponges with nearl ...
s include 90% of all species of
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the Diploblasts. They are Multicellular organism, multicellular organisms that have bodies full of pores and channels allowing water ...

s. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral
silica Silicon dioxide, also known as silica, is an oxide An oxide () is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any su ...

, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar .

Echinoderms

The skeleton of the
echinoderm An echinoderm () is any member of the phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of ...
s, which include, among other things, the
starfish Starfish or sea stars are star-shaped In mathematics, a Set (mathematics), set S in the Euclidean space \R^n is called a star domain (or star-convex set, star-shaped set or radially convex set) if there exists an s_0 \in S such that for ...

, is composed of
calcite Calcite is a carbonate mineral Carbonate minerals are those mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid E ...

and a small amount of
magnesium oxide Magnesium oxide (Mg O), or magnesia, is a white hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environm ...

. It lies below the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
in the
mesoderm The mesoderm is the middle layer of the three germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoa ...

and is within cell clusters of frame-forming cells. This structure formed is porous and therefore firm and at the same time light. It coalesces into small
calcareous ''Calcareous'' is an adjective meaning "mostly or partly composed of calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecula ...
ossicles (bony plates), which can grow in all directions and thus can replace the loss of a body part. Connected by joints, the individual skeletal parts can be moved by the muscles.

Vertebrates

In most vertebrates, the main skeletal component is referred to as
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

. These bones compose a unique skeletal system for each type of animal. Another important component is cartilage which in mammals is found mainly in the joint areas. In other animals, such as the cartilaginous fishes, which include the
shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified withi ...

s, the skeleton is composed entirely of cartilage. The segmental pattern of the skeleton is present in all vertebrates (mammals, birds, fish, reptiles and amphibians) with basic units being repeated. This segmental pattern is particularly evident in the vertebral column and the ribcage. Bones in addition to supporting the body also serve, at the cellular level, as calcium and phosphate storage.

Fish

The skeleton, which forms the support structure inside the fish is either made of cartilage as in the (
Chondrichthyes Chondrichthyes (; ) is a class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an an ...

), or bones as in the (
Osteichthyes Osteichthyes (), popularly referred to as the bony fish, is a diverse taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may al ...

). The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. They are supported only by the muscles. The main external features of the fish, the
fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people o ...
, 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. They are supported by the muscles which compose the main part of the trunk.

Birds

The bird skeleton is highly adapted for flight. It is extremely lightweight, yet still strong enough to withstand the stresses of taking off, flying, and landing. One key adaptation is the fusing of bones into single
ossification Ossification (or osteogenesis) in bone remodeling is the process of laying down new bone material by Cell (biology), cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in the formation of no ...
s, such as the
pygostyle Pygostyle describes a skeletal condition in which the final few caudal vertebra In the vertebrate spinal column, each vertebra is an irregular bone with a complex structure composed of bone and some hyaline cartilage, the proportions of wh ...
. Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true
jaw The jaw is any opposable articulated structure at the entrance of the , typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serving t ...

beak The beak, bill, and/or rostrum is an external anatomical structure found mostly in birds Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellu ...

, which is far more lightweight. The beaks of many baby birds have a projection called an
egg tooth An egg tooth is a temporary, sharp projection present on the bill Bill(s) may refer to: Common meanings * Banknote A banknote (often known as a bill (in the US and Canada), paper money, or simply a note) is a type of negotiable instrument ...
, which facilitates their exit from the amniotic egg.

Marine mammals

To facilitate the movement of
marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as Pinniped, seals, Cetacea, whales, Sirenia, manatees, sea otters and polar bears. They are an informal group, ...
s in water, the hind legs were either lost altogether, as in the whales and
manatee Manatees (family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. Ideal ...

s, or united in a single tail fin as in the
pinnipeds Pinnipeds (pronounced ), commonly known as seals, are a widely distributedDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized functions, are objects th ...
(seals). In the whale, the
cervical vertebrae In tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and therefore birds), and synapsids (including ...

Humans

The skeleton consists of both fused and individual bones supported and supplemented by
ligament A ligament is the Connective tissue#Types, fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body incl ...

s,
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant ...

s,
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

s and
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain,
lungs The lungs are the primary organs of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animal ...
,
heart The heart is a muscular MUSCULAR (DS-200B), located in the United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,Usage is mixed. The Guardian' and Telegraph' use ...

and
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which contain ...

. Although the teeth do not consist of tissue commonly found in bones, the teeth are usually considered as members of the skeletal system. The biggest bone in the body is the
femur The femur (; ), or thigh bone, is the proximal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe s ...

in the upper leg, and the smallest is the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its Annular ligament of stapes, annular ligam ...
bone in the
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

. In an adult, the skeleton comprises around 13.1% of the total body weight, and half of this weight is water. Fused bones include those of the
pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen The abdomen (colloquially called the belly, tummy, midriff or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in ...
and the
cranium The skull is a bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, store mi ...

. Not all bones are interconnected directly: There are three bones in each
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

called the
ossicles The ossicles (also called auditory ossicles) are three bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a spe ...
that articulate only with each other. The
hyoid bone The hyoid bone (lingual bone or tongue-bone) () is a horseshoe A horseshoe is a fabricated product, normally made of metal A metal (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ε ...
, which is located in the neck and serves as the point of attachment for the
tongue The tongue is a muscular organ (anatomy), organ in the mouth of a typical tetrapod. It manipulates food for mastication and swallowing as part of the digestive system, digestive process, and is the primary organ of taste. The tongue's upper surfa ...

, does not articulate with any other bones in the body, being supported by muscles and ligaments. There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the
hip bone The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrate Vertebrates () comprise all species of animal Animals (also called Meta ...

s 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. Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in the total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together. These bones are organized into a longitudinal axis, the
axial skeleton Axial may refer to: * one of the anatomical directions Standard anatomical terms of location deal unambiguously with the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of concerned with the study of the structure of s an ...
, to which the
appendicular skeleton The appendicular skeleton is the portion of the skeleton The skeleton refers to the frames of support of animal bodies. There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskele ...
is attached.. The human skeleton takes 20 years before it is fully developed, and the bones contain marrow, which produces blood cells. There exist several general differences between the male and female skeletons. The male skeleton, for example, is generally larger and heavier than the female skeleton. In the female skeleton, the bones of the skull are generally less angular. The female skeleton also has wider and shorter breastbone and slimmer wrists. There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. The female pelvis is wider and shallower than the male pelvis. Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. 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..

Parts

Bone

Bones are rigid
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
that form part of the
endoskeleton An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the ...
of
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s. They function to move, support, and protect the various organs of the body, produce
red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength Image:dominant wavelength.png, frame, Dominant/complementary wavelength example on the CIE color ...

and
white blood cell White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
s and store minerals. Bone tissue is a type of dense
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
. Bones have a variety of shapes with a complex internal and external structure they are also lightweight, yet strong and hard. One of the types of tissue that makes up bone tissue is
mineralized tissue, sea shells, conch, dentin, radiolarian, antler, bone Mineralized tissues are biological tissue (biology), tissues that incorporate minerals into soft matrices. Typically these tissues form a protective shield or structural support. Bone, mollusk ...
and this gives it rigidity and a honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow,
endosteum The endosteum (plural endostea) is a thin vascular membrane of connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biolo ...
and
periosteum The periosteum is a membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other sma ...
,
nerve A nerve is an enclosed, cable-like bundle of fibers (called axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences Despite the various English dialects Dialect The term diale ...

s,
blood vessel The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an organ system An organ system is a biological system A biological system is a comp ...
s and
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

.

Extra-skeletal bones in mammals

These bones, primarily formed separately in subcutaneous tissues, include headgears (such as bony core of horns, antlers, and ossicones), osteoderm, and os penis/ os clitoris.

Cartilage

During
embryonic development An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...
the precursor to bone development is
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

that mostly becomes replaced by bone, after flesh such as muscle has formed around it. Cartilage is a stiff and inflexible
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
found in many areas including the
joint A joint or articulation (or articular surface) is the connection made between bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the skeleton in most vertebrate animals. Bones protect the various organs of th ...

s between bones, the
rib cage The rib cage is the arrangement of rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus fa ...

, the ear, the nose, the elbow, the knee, the ankle, the
bronchial tubes A bronchus is a passage or airway in the respiratory tract, respiratory system that conducts Atmosphere of Earth, air into the lungs. The first bronchi to branch from the trachea are the right main bronchus and the left main bronchus, also known as ...
and the
intervertebral disc An intervertebral disc (or intervertebral fibrocartilage) lies between adjacent vertebrae in the vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining ...
s. It is not as hard and rigid as bone but is stiffer and less flexible than
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

. Cartilage is composed of specialized cells called
chondrocyte Chondrocytes (, from Greek (language), Greek χόνδρος, ''chondros'' = cartilage + κύτος, ''kytos'' = cell) are the only Cell (biology), cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consist ...

s that produce a large amount of
extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms ...
composed of Type II
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

(except fibrocartilage which also contains
type I collagen Type I collagen is the most abundant collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray c ...
) fibers, abundant
ground substance Ground substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix In biology Biology is the natural science that studies life and living organisms, including their anatom ...
rich in
proteoglycan Proteoglycans are protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958 ...
s, and
elastin Elastin is a key protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, f ...
fibers. Cartilage is classified in three types,
elastic cartilage Elastic cartilage or yellow cartilage is a type of cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue, rubber-like padding that covers and protects the ends of long bone A bone is a Stiffness, rigid tissue (an ...
,
hyaline cartilage Hyaline cartilage is the glass-like (hyaline) but translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering o ...

and
fibrocartilage Fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions. It owes its inflexibility and toughness to the former of these constituents, and its elasticity Elasticity often refers to: *Elastici ...
, which differ in the relative amounts of these three main components. Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.

Ligament

A ligament is a piece of rubbery tissue that connects bone to other bone. It is commonly confused with the
tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant ...

, a similar structure that connects muscle to bone.

Tendon

A tendon is a rubber-band like tissue that connects muscle to bone. It is not to be confused with the
ligament A ligament is the Connective tissue#Types, fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body incl ...

, a similar tissue that connects bone to bone.

*
Bonesetter A bonesetter is a practitioner of joint manipulation. Before the advent of chiropractors, osteopaths and physical therapists, bonesetters were the main providers of this type of treatment. Traditionally, they practiced without any formal train ...
*
Endochondral ossification Endochondral ossification is one of the two essential processes during fetal A fetus American and British English spelling differences#ae and oe, or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops fro ...
*
Intramembranous ossification Intramembranous ossification is one of the two essential processes during fetal A fetus American and British English spelling differences#ae and oe, or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops fr ...
*
Exoskeleton An exoskeleton (from Greek έξω, ''éxō'' "outer" and σκελετός, ''skeletós'' "skeleton") is the external skeleton A skeleton is a structural frame that supports an animal Animals (also called Metazoa) are multicellular ...

*
Osteoblast Osteoblasts (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...

* Osteometric points *
Skeletal system of the horse 300px, Skeleton of a horse The skeletal system of the horse The horse (''Equus ferus caballus'') is a domesticated odd-toed ungulate mammal. It belongs to the taxonomic family Equidae and is one of two Extant taxon, extant subspecies of wild ho ...

Bibliography

* * * * * * * * *

3-D Viewer
of a male
American mastodon American(s) may refer to: * American, something of, from, or related to the United States of America, commonly known as the United States The United States of America (USA), commonly known as the United States (U.S. or US), or America, is ...
skeleton, with bones labelled, at the
University of Michigan , mottoeng = "Arts, Knowledge, Truth" , former_names = Catholepistemiad, or University of Michigania (1817–1821) , budget = $8.99 billion (2018) , endowment =$17 billion (2021)As of October 25, 2021. ...
br>Mammutidae digital fossil repository

Interactive views
of various
primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...

skeletons at eSkeletons.org (associated with the
University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public university, public research university in Austin, Texas, founded in 1883. The University of Texas was included in the Association of American Universities in 1929. The i ...
{{Authority control Animal anatomy