Editing Skeleton (section)

== Vertebrate skeletons ==
[[File:Huxley - Mans Place in Nature.jpg|thumb|'''Pithecometra''': From [[Thomas Huxley]]'s 1863 ''[[Evidence as to Man's Place in Nature]]'', the compared skeletons of apes to humans.]]

Vertebrate skeletons are endoskeletons, and the main skeletal component is bone.<ref name="de Buffrénil et al. 2021" /> Bones compose a unique skeletal system for each type of animal. Another important component is cartilage which in [[mammal]]s is found mainly in the joint areas. In other animals, such as the [[Chondrichthyes|cartilaginous fishes]], which include the [[shark]]s, the skeleton is composed entirely of [[cartilage]]. The [[segmentation (biology)|segmental]] pattern of the skeleton is present in all vertebrates, with basic units being repeated, such as in the vertebral column and the ribcage.<ref>{{cite journal |last1=Billet |first1=Guillaume |last2=Bardin |first2=Jérémie |title=Segmental Series and Size: Clade-Wide Investigation of Molar Proportions Reveals a Major Evolutionary Allometry in the Dentition of Placental Mammals |journal=Systematic Biology |date=13 October 2021 |volume=70 |issue=6 |pages=1101–1109 |doi=10.1093/sysbio/syab007 |pmid=33560370 |url=https://doi.org/10.1093/sysbio/syab007}}</ref><ref>{{cite journal |last1=Buffrénil |first1=Vivian de |last2=Quilhac |first2=Alexandra |title=An Overview of the Embryonic Development of the Bony Skeleton |journal=Vertebrate Skeletal Histology and Paleohistology |date=2021 |pages=29–38 |doi=10.1201/9781351189590-2 |url=https://www.taylorfrancis.com/chapters/edit/10.1201/9781351189590-2/overview-embryonic-development-bony-skeleton-vivian-de-buffr%C3%A9nil-alexandra-quilhac |publisher=CRC Press|isbn=978-1351189590 |s2cid=236422314 }}</ref>

Bones are rigid [[organ (biology)|organs]] providing structural support for the body, assistance in movement by opposing [[muscle contraction]], and the forming of a protective wall around internal organs. Bones are primarily made of inorganic minerals, such as [[hydroxyapatite]], while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones are [[osteoblast]]s, [[osteoclast]]s, or [[osteocyte]]s.<ref>{{Cite journal |last1=Sommerfeldt |first1=D. |last2=Rubin |first2=C. |date=2001-10-01 |title=Biology of bone and how it orchestrates the form and function of the skeleton |url=https://doi.org/10.1007/s005860100283 |journal=European Spine Journal |language=en |volume=10 |issue=2 |pages=S86–S95 |doi=10.1007/s005860100283 |issn=1432-0932 |pmc=3611544 |pmid=11716022}}</ref>

Bone tissue is a type of dense [[connective tissue]], a type of [[mineralized tissue]] that gives rigidity and a [[Honeycomb (geometry)|honeycomb-like]] three-dimensional internal structure. Bones also produce [[red blood cell|red]] and [[white blood cell]]s and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include [[bone marrow]], [[endosteum]] and [[periosteum]], [[nerve]]s, [[blood vessel]]s and cartilage.

During [[embryonic development]], bones are developed individually from skeletogenic cells in the ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone.<ref>{{Citation |last1=Lefebvre |first1=Véronique |title=Chapter Eight - Vertebrate Skeletogenesis |date=2010-01-01 |journal=Current Topics in Developmental Biology |volume=90 |pages=291–317 |editor-last=Koopman |editor-first=Peter |series=Organogenesis in Development |publisher=Academic Press |language=en |last2=Bhattaram |first2=Pallavi|doi=10.1016/S0070-2153(10)90008-2 |pmid=20691853 |pmc=3077680 }}</ref> Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased in [[perichondrium]] tissue.<ref name="Gillis-2019">{{Citation |last=Gillis |first=J. Andrew |title=The Development and Evolution of Cartilage |date=2019 |url=https://www.sciencedirect.com/science/article/pii/B9780128096338907702 |work=Reference Module in Life Sciences |publisher=Elsevier |language=en |isbn=978-0-12-809633-8}}</ref> [[Ligament]]s are elastic tissues that connect bones to other bones, and [[tendon]]s are elastic tissues that connect muscles to bones.<ref>{{Cite encyclopedia |title=Tendon vs. Ligament |encyclopedia=A.D.A.M. Medical Encyclopedia |publisher=Ebix |url=https://medlineplus.gov/ency/imagepages/19089.htm |access-date=2021-08-06 |last=Vorvick |first=Linda J. |date=2020-08-13 |via=MedLinePlus}}</ref>

=== Amphibians and reptiles ===
The skeletons of turtles have evolved to develop a [[Turtle shell|shell]] from the ribcage, forming an exoskeleton.<ref>{{Cite journal |last1=Nagashima |first1=Hiroshi |last2=Kuraku |first2=Shigehiro |last3=Uchida |first3=Katsuhisa |last4=Kawashima-Ohya |first4=Yoshie |last5=Narita |first5=Yuichi |last6=Kuratani |first6=Shigeru |date=2012-03-01 |title=Body plan of turtles: an anatomical, developmental and evolutionary perspective |url=https://doi.org/10.1007/s12565-011-0121-y |journal=Anatomical Science International |language=en |volume=87 |issue=1 |pages=1–13 |doi=10.1007/s12565-011-0121-y |pmid=22131042 |s2cid=41803725 |issn=1447-073X}}</ref> The skeletons of [[snake]]s and [[caecilian]]s have significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards.<ref>{{Cite journal |last=M. Woltering |first=Joost |date=2012-06-01 |title=From Lizard to Snake; Behind the Evolution of an Extreme Body Plan |journal=Current Genomics |volume=13 |issue=4 |pages=289–299 |doi=10.2174/138920212800793302|pmid=23204918 |pmc=3394116 }}</ref>

=== Birds ===
{{Further|Bird anatomy#Skeletal system}}The skeletons of birds are adapted for [[Flying and gliding animals|flight]]. The bones in bird skeletons are hollow and lightweight to reduce the metabolic cost of flight. Several attributes of the shape and structure of the bones are optimized to endure the physical stress associated with flight, including a round and thin [[humeral shaft]] and the fusion of skeletal elements into single [[ossification]]s.<ref>{{Cite journal |last=Dumont |first=Elizabeth R. |date=2010-07-22 |title=Bone density and the lightweight skeletons of birds |journal=Proceedings of the Royal Society B: Biological Sciences |volume=277 |issue=1691 |pages=2193–2198 |doi=10.1098/rspb.2010.0117 |pmc=2880151 |pmid=20236981}}</ref> Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true [[jaw]], instead having evolved a [[beak]], which is far more lightweight. The beaks of many baby birds have a projection called an [[egg tooth]], which facilitates their exit from the amniotic egg.

=== Fish ===
{{Further|Fish anatomy#Skeleton|Fish bone}}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]]. 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 [[fish fin|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. They are supported by the muscles which compose the main part of the trunk.

Cartilaginous fish, such as sharks, rays, skates, and chimeras, have skeletons made entirely of cartilage. The lighter weight of cartilage allows these fish to expend less energy when swimming.<ref name="National Geographic-2021" />

=== Mammals ===

====Marine mammals====
[[File:Zalophus californianus 01.JPG|thumb|left|[[Californian sea lion]]]]
To facilitate the movement of [[marine mammal]]s in water, the hind legs were either lost altogether, as in the whales and [[manatee]]s, or united in a single [[fish fin|tail fin]] as in the [[pinnipeds]] (seals). In the whale, the [[cervical vertebrae]] are typically fused, an adaptation trading flexibility for stability during swimming.<ref>{{cite journal |last1=Bebej |first1=Ryan M |last2=Smith |first2=Kathlyn M |title=Lumbar mobility in archaeocetes (Mammalia: Cetacea) and the evolution of aquatic locomotion in the earliest whales |journal=Zoological Journal of the Linnean Society |date=17 March 2018 |volume=182 |issue=3 |pages=695–721 |doi=10.1093/zoolinnean/zlx058 |url=https://academic.oup.com/zoolinnean/article/182/3/695/4554339?login=true |access-date=7 March 2022 |issn=0024-4082}}</ref>

====Humans====
{{Main article|Human skeleton}}
[[File:Leonardo Skeleton 1511.jpg|thumb|''Study of Skeletons'', {{Circa|1510}}, by [[Leonardo da Vinci]]]]

The skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, [[muscle]]s and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, [[human lung|lungs]], [[human heart|heart]] and [[spinal cord]].<ref>{{Cite news |title=Skeletal System: Facts, Function & Diseases |work=Live Science |url=http://www.livescience.com/22537-skeletal-system.html |url-status=live |access-date=7 March 2017 |archive-url=https://web.archive.org/web/20170307123921/http://www.livescience.com/22537-skeletal-system.html |archive-date=7 March 2017}}</ref> The biggest bone in the body is the [[femur]] in the upper leg, and the smallest is the [[stapes]] bone in the [[middle ear]]. In an adult, the skeleton comprises around 13.1% of the total body weight,<ref>{{Harvnb|Reynolds|Karlotski|1977|page=161}}</ref> and half of this weight is water.

Fused bones include those of the [[Human pelvis|pelvis]] and the [[Human skull|cranium]]. Not all bones are interconnected directly: There are three bones in each [[middle ear]] called the [[ossicles]] that articulate only with each other. 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.

There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the [[hip bone]]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 bone]]s 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.{{citation needed|date=August 2021}} These bones are organized into a longitudinal axis, the [[axial skeleton]], to which the [[appendicular skeleton]] is attached.<ref name="tozeren6-10">{{Harvnb|Tözeren|2000|pages=6–10}}.</ref>

The human skeleton takes 20 years before it is fully developed, and the bones contain [[Bone marrow|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.<ref name="Balaban-61">{{Harvnb|Balaban|2008|page=61}}</ref><ref name="stein73">{{Harvnb|Stein|2007|page=73}}.</ref>