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Sacrum, pelvic surface
Image of a male pelvis (sacrum is in center)
Latin Os sacrum
Gray's p.106 
MeSH Sacrum 
TA A02.2.05.001 
FMA FMA:16202 
Anatomical terms of bone
In humans, the sacrum (/skrm/ or /sekrm/; plural: sacrums or sacra) is a large, triangular bone at the base ofthe spine and at the upper, back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones.Its upper part connects with the last lumbar vertebra, and its lower part with the coccyx (tailbone). Usually, it beginsas five unfused vertebrae which begin to fuse between the ages of 1618 years and have usually completely fusedinto a single bone by the age of 34 years.
It is curved upon itself and placed obliquely (tilted forward). It is concave, facing forward. The base projects forwardas the sacral promontory internally, which is the superiormost portion of the sacrum. It marks part of the border ofthe pelvic inlet, and comprises the iliopectineal line and the linea terminalis.The sacral promontory articulates withthe last lumbar vertebra to form the sacrovertebral angle, an angle of 30 degrees from the horizontal plane thatprovides a useful marker for sling operation.The central part is curved outward toward the posterior, allowing greater room for the pelvic cavity. The two lateralprojections of the sacrum are called the ala (wings), and articulate with the ilium at the L-shaped sacroiliac joints.
StructureThe sacrum consists of several parts: The pelvic surface of the sacrum is concave from above downward, and slightly so from side to side. The dorsal surface of the sacrum is convex and narrower than the pelvic. The lateral surface of the sacrum is broad above, but narrowed into a thin edge below. The base of the sacrum, which is broad and expanded, is directed upward and forward. The apex (apex oss. sacri) is directed downward, and presents an oval facet for articulation with the coccyx. The vertebral canal (canalis sacralis; sacral canal) runs throughout the greater part of the bone; above, it is
triangular in form; below, its posterior wall is incomplete, from the non-development of the laminae and spinousprocesses. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foraminathrough which these nerves emerge.
ArticulationsThe sacrum articulates with four bones: the last lumbar vertebra above the coccyx (tailbone) below the illium portion of the hip bone on either sideRotation of the sacrum superiorly and anteriorly whilst the coccyx moves posteriorly relative to the ilium issometimes called "nutation" (from the Latin term nutatio which means "nodding") and the reverse, postero-inferiormotion of the sacrum relative to the ilium whilst the coccyx moves anteriorly, "counter-nutation." In uprightvertebrates, the sacrum is capable of slight independent movement along the sagittal plane. When you bendbackward the top (base) of the sacrum moves forward relative to the ilium; when you bend forward the top movesback.
The sacrum is called so when referred to all of the parts combined. Its parts are called sacral vertebrae when referredindividually.
VariationsIn some cases the sacrum will consist of six pieces or be reduced in number to four. The bodies of the first andsecond vertebrae may fail to unite.Sometimes the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacralcanal may be open throughout a considerable part of its length, in consequence of the imperfect development of thelaminae and spinous processes.The sacrum also varies considerably with respect to its degree of curvature.
The sacrum is noticeably sexually dimorphic (differently shaped in males and females).In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper;the upper half is nearly straight, the lower half presenting the greatest amount of curvature. The bone is also directedmore obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle moreprominent.In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether larger thanin the female.
DevelopmentThe somites that give rise to the vertebral column begin to develop from head to tail along the length of thenotochord. At day 20 of embryogenesis the first four pairs of somites appear in the future occipital bone region.Developing at the rate of three or four a day, the next eight pairs form in the cervical region to develop into thecervical vertebrae; the next twelve pairs will form the thoracic vertebrae; the next five pairs the lumbar vertebrae andby about day 29 the sacral somites will appear to develop into the sacral vertebrae; finally on day 30 the last threepairs will form the coccyx.
In oncologyThe sacrum is one of the main sites for the development of the sarcomas known as chordomas (chordosarcomas) thatare derived from the remnants of the embryonic notochord.
In osteopathic medicineSacral Diagnosis is a common issue in Osteopathic Manipulative Medicine. There are many types of sacraldiagnoses, such as torsion and shear. To diagnose a sacral torsion, the axis of rotation is found with the axis namedafter its superior pole. If the opposite side of the pole is rotated anteriorly, it is rotated towards the pole, in whichcase it is called either a right-on-right (R on R) or left-on-left (L on L) torsion. The first letter in the diagnosispertains to the direction of rotation of the superior portion of the sacrum opposite the side of the superior axis pole,and the last letter pertains to the pole.
EtymologyThe 'English' name sacrum  is derived from the full Latin expression os sacrum, a translation of AncientGreek , attested in the writings of Greek physician Galen. Both os as mean bone  andsacrum as mean holy. Formerly the os sacrum was called holy bone  in English. In other languages similarexpressions, like heiliges Bein or Heiligenbein  (German) and heilig been  (Dutch) exist.There are a few probable etymological explanations for . Supposedly the sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation). Alternatively, it was believed that that the soul of the man resided in the sacrum. Others attribute the adjective to the ancient belief that this specific bone would be indestructable. Another source mentions that the os sacrum, being the largest of the vertebrae, was also called  by the Greeks, with , big and , vertebra. In certain instances was considered as a synonym of in Ancient Greek, hence the transformation from to . Latin vertebra magna, with magna, big, is a translation of
.Besides the aforementioned expressions, the Ancient Greeks, as attested in the poetry of Greek poet Antimachus,used , Latinized as clonis. to refer to the sacrum. is cognate to Latin clunis ("buttock"). The lattercan be found in the genitive plural (='"of the buttocks") in the synonymous Latin expression for the ossa sacra, ossaclunium. Due to the fact that the os sacrum is broad and thick at its upper end, the os sacrum is alternatively called oslatum, from latum, broad.
Image of a female pelvis seenanteriorly, sacrum at centre.
Lateralsurfaces ofsacrum and
Base of sacrum. Mediansagittal
section ofthe sacrum.
Left Levator ani fromwithin.
The posterior divisions of thesacral nerves.
Behind view. Cut side view.
Lumbar and sacral plexus. Deepdissection.Anterior view.
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