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The sacrum is a large, triangular bone at the base of the spine
Vertebral column

In human anatomy, the vertebral column is a column of 24 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsum aspect of the torso, separated by spinal discs....
 and at the upper and back part of the pelvic cavity
Pelvic cavity

The pelvic cavity is a body cavity that is bounded by the bones of the pelvis and which primarily contains reproductive organs, the urinary bladder, and the rectum....
, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra
Lumbar vertebrae

The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body....
, and bottom part with the coccyx
Coccyx

The coccyx , commonly referred to as the tailbone, is the final segment of the human spine . Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx....
 (tailbone). In children, it consists of usually five unfused vertebrae which begin to fuse between ages 16-18 and are usually completely fused into a single bone by age 26.

It is curved upon itself and placed obliquely (that is, tilted forward).






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Encyclopedia


The sacrum is a large, triangular bone at the base of the spine
Vertebral column

In human anatomy, the vertebral column is a column of 24 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsum aspect of the torso, separated by spinal discs....
 and at the upper and back part of the pelvic cavity
Pelvic cavity

The pelvic cavity is a body cavity that is bounded by the bones of the pelvis and which primarily contains reproductive organs, the urinary bladder, and the rectum....
, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra
Lumbar vertebrae

The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body....
, and bottom part with the coccyx
Coccyx

The coccyx , commonly referred to as the tailbone, is the final segment of the human spine . Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx....
 (tailbone). In children, it consists of usually five unfused vertebrae which begin to fuse between ages 16-18 and are usually completely fused into a single bone by age 26.

It is curved upon itself and placed obliquely (that is, tilted forward). It is kyphotic — that is, concave facing forwards. The base projects forward as the sacral promontory
Sacral promontory

The sacral promontory is the anatomical term for the superiormost portion of the sacrum. It marks part of the border of the pelvic inlet.With the iliopectineal line, it comprises the linea terminalis....
 internally, and articulates with the last lumbar vertebra to form the prominent sacrovertebral angle
Sacrovertebral angle

The sacrum is curved upon itself and placed very obliquely, its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra....
. The central part is curved outward towards the posterior, allowing greater room for the pelvic cavity
Pelvic cavity

The pelvic cavity is a body cavity that is bounded by the bones of the pelvis and which primarily contains reproductive organs, the urinary bladder, and the rectum....
.

Etymology

The name is derived from the Latin
Latin

Latin is an Italic language, historically spoken in Latium and Ancient Rome. Through the Military history of the Roman Empire, Latin spread throughout the Mediterranean and a large part of Europe....
 sacer, "sacred", a translation of the Greek
Greek language

Greek is an Indo-European languages native to the southern Balkan peninsula, the language of the Greek people. It forms an independent branch within Indo-European....
 hieron (osteon), meaning sacred or strong bone. This is supposedly because the sacrum was the part of an animal offered in sacrifice
Sacrifice

Sacrifice is commonly known as the practice of offering food, objects , or the lives of animals or people to the deity as an act of propitiation or worship....
. In Slavic languages
Slavic languages

File:Slavic europe.svgThe Slavic languages , a group of closely related languages of the Slavic peoples and a subgroup of Indo-European languages, have speakers in most of Eastern Europe, in much of the Balkans, in parts of Central Europe, and in the northern part of Asia....
 and in German
German language

German is a West Germanic languages, thus related to and classified alongside English language and Dutch language. It is one of the world's world language and the most widely spoken mother tongue in the European Union....
 this bone is called the "cross bone".

Parts

  • The pelvic surface of the sacrum
    Pelvic surface of sacrum

    The pelvic surface of sacrum is concave from above downward, and slightly so from side to side.Its middle part is crossed by four transverse ridges, the positions of which correspond with the original planes of separation between the five segments of the bone....
     is concave from above downward, and slightly so from side to side.
  • The dorsal surface of the sacrum
    Dorsal surface of sacrum

    The dorsal surface of sacrum is convex and narrower than the pelvic.In the middle line it displays a crest, the middle sacral crest, surmounted by three or four tubercles, the rudimentary spinous processes of the upper three or four sacral vertebrae....
     is convex and narrower than the pelvic.
  • The lateral surface of the sacrum
    Lateral surface of sacrum

    The lateral surface of sacrum is broad above, but narrowed into a thin edge below. The upper half presents in front an ear-shaped surface, the auricular surface, covered with cartilage in the fresh state, for articulation with the ilium....
     is broad above, but narrowed into a thin edge below.
  • The base of the sacrum
    Base of the sacrum

    The base of the sacrum, which is broad and expanded, is directed upward and forward.In the middle is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral fibrocartilage....
    , 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 spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out.


Articulations


The sacrum articulates with four bones:
  • the last lumbar vertebra above
  • the coccyx (tailbone)
    Coccyx

    The coccyx , commonly referred to as the tailbone, is the final segment of the human spine . Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx....
     below
  • the ileum
    Ilium (bone)

    The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium....
     portion of the hip bone
    Hip bone

    The hip bone, innominate bone or coxal bone is a large, flattened, irregularly shaped bone, constricted in the center and expanded above and below....
     on either side


Rotation of the sacrum forward a few degrees vis-à-vis the ilia
Ilium (bone)

The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium....
 is sometimes called "nutation
Nutation

Nutation is a slight irregular motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope or a planet.Nutation is also the name of one of the Euler_angles#Euler_rotations, the Euler rotation that measures the change in angle due to the "nodding" mentioned above....
", and the reverse motion "counter-nutation."

It is called the sacrum when referred to all of the parts combined, but sacral vertebrae when referred individually.

Sexual dimorphism


The sacrum is noticeably sexually dimorphic
Sexual dimorphism

Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. Examples include color , size, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks....
 (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 directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent.

In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female.

Variations


The sacrum, in some cases, consists of six pieces ; occasionally the number is reduced to four . The bodies of the first and second 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 sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminae and spinous processes.

The sacrum also varies considerably with respect to its degree of curvature.

Additional images


See also

  • Bone terminology
    Bone

    Bones are rigid organ that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red blood cell and white blood cells and store minerals....
  • Terms for anatomical location
  • Pelvimetry
    Pelvimetry

    Pelvimetry is the assessment of the female pelvis in relation to the birth of a baby. Traditional obstetrical services relied heavily on pelvimetry in the conduct of delivery in order to decide if natural or operative childbirth was possible or if and when to use a cesarean section....
  • Rump (croup)
  • Coccyx (tailbone)
    Coccyx

    The coccyx , commonly referred to as the tailbone, is the final segment of the human spine . Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between the sacrum and the coccyx....
  • Ileum bone
    Ilium (bone)

    The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium....


External links