Editing Sacrum (section)

{{Short description|Bone of the spine}}
{{Other uses}}
{{Use dmy dates|date=July 2022}}
{{Infobox bone
| Name          = Sacrum
| Latin         = os sacrum
| Image         = Sacrum - anterior view04.png
| Caption       = Position of the sacrum in the [[pelvis]]
| Image2        = Sacrum - animation00.gif
| Caption2      = Animation of the sacrum in the [[human skeleton]]
| Articulations =
| Location      = Base of the [[vertebral column]]
| Pronunciation = ({{IPAc-en|'|s|ae|k|r|@|m}} or {{IPAc-en|'|s|ei|k|r|@|m}})
}}

The '''sacrum''' ({{plural form}}: '''sacra''' or '''sacrums'''<ref>[https://web.archive.org/web/20140126225723/http://www.oxforddictionaries.com/us/definition/american_english/sacrum Oxford Dictionaries] and [http://www.yourdictionary.com/sacrum Webster's New College Dictionary] (2010) admit the plural ''sacrums'' alongside ''sacra''; 
[https://www.ahdictionary.com/word/search.html?q=sacrum The American Heritage Dictionary], [http://www.collinsdictionary.com/dictionary/english/sacrum Collins Dictionary] and [http://machaut.uchicago.edu/?resource=Webster%27s&word=sacrum&use1913=on Webster's Revised Unabridged Dictionary] (1913) give ''sacra'' as the only plural.</ref>), in [[human body|human anatomy]], is a large, triangular [[bone]] at the base of the [[vertebral column|spine]] that forms by the fusing of the '''sacral vertebrae''' (S1{{ndash}}S5) between ages 18 and 30.<ref>
{{cite web|url=http://www.scientificspine.com/spinal-anatomy/sacrum-anatomy.html|title=Sacrum anatomy|last=Kilincer |first=Cumhur |display-authors=etal |year=2009|work=Scientific spine|publisher=Self|access-date=8 November 2015|location=Trakya Üniversitesi Rektörlüğü, Yerleşkesi, 22030 Edirne, Turkey}}</ref>

The sacrum situates at the upper, back part of the [[pelvic cavity]], between the two [[Ilium (bone)|wings of the pelvis]]. It forms joints with four other bones. The two projections at the sides of the sacrum are called the alae (wings), and articulate with the [[Ilium (bone)|ilium]] at the L-shaped [[sacroiliac joint]]s. The upper part of the sacrum connects with the last [[lumbar vertebrae|lumbar vertebra]] (L5), and its lower part with the [[coccyx]] (tailbone) via the sacral and coccygeal cornua.

The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures. Overall, it is [[wikt:concave|concave]] (curved upon itself). The [[base of the sacrum]], the broadest and uppermost part, is tilted forward as the sacral promontory internally. The central part is curved outward toward the [[Posterior (anatomy)|posterior]], allowing greater room for the [[pelvic cavity]].

In all other [[quadruped]]al [[vertebrates]], the pelvic vertebrae undergo a similar developmental process to form a sacrum in the adult, even while the bony tail (caudal) vertebrae remain unfused. The number of sacral vertebrae varies slightly. For instance, the S1{{ndash}}S5 vertebrae of a horse will fuse, the S1{{ndash}}S3 of a dog will fuse, and four pelvic vertebrae of a rat will fuse between the lumbar and the caudal vertebrae of its tail.<ref>{{cite web |url=http://biology.kenyon.edu/courses/biol10/110pdfs/8skeleton.pdf |title=Skeletal system |department=Dept. of Biology |publisher=Kenyon College |access-date=9 November 2015 |location=Gambier, Ohio}}</ref>

The ''[[Stegosaurus]]'' [[dinosaur]] had a greatly enlarged neural canal in the sacrum, characterized as a "[[Stegosaurus#"Second brain"|posterior brain case]]".<ref>{{cite book |title=The Dinosauria |url=https://archive.org/details/dinosauriandedit00weis |url-access=limited |edition=2nd |year=2004 |chapter=Stegosauria |editor1=Weishampel, D.B. |editor2=Dodson, P. |editor3=Osmólska, H. |author1=Galton, P.M. |author2=Upchurch, P. |page=[https://archive.org/details/dinosauriandedit00weis/page/n379 361] |publisher=University of California Press |isbn=0-520-24209-2}}</ref>