Sacrum
Overview
In vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 the sacrum (plural: sacrums or sacra) is a large, triangular bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 at the base of the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

 and at the upper and back part of the pelvic cavity
Pelvic cavity
-External links:* * *...

, where it is inserted
Insertion (anatomy)
Insertion is the point at which a muscle attaches to the skin, a bone, or another muscle. The insertion attaches to the structure that will be moved by the contraction of the muscle. Insertions are usually connections of muscle via tendon to bone. The opposite end of the muscle is called the origin....

 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 vertebral column. 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...

 (tailbone). It consists of usually five initially 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).
Encyclopedia
In vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

 anatomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 the sacrum (plural: sacrums or sacra) is a large, triangular bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 at the base of the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

 and at the upper and back part of the pelvic cavity
Pelvic cavity
-External links:* * *...

, where it is inserted
Insertion (anatomy)
Insertion is the point at which a muscle attaches to the skin, a bone, or another muscle. The insertion attaches to the structure that will be moved by the contraction of the muscle. Insertions are usually connections of muscle via tendon to bone. The opposite end of the muscle is called the origin....

 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 vertebral column. 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...

 (tailbone). It consists of usually five initially 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
Kyphosis (disambiguation)
The medical term kyphosis has several meanings.* A deformity, where the back is bowed * A term describing the normally convex segments of the spine, also called primary curvatures....

 — 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. The rectosigmoid junction is at the level of the sacral promontory....

 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.It is also known as the "lumbosacral angle"....

. The central part is curved outward towards the posterior, allowing greater room for the pelvic cavity
Pelvic cavity
-External links:* * *...

.

Etymology

The name is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 (os) sacrum, (sacer, sacra, sacrum, "sacred"), a translation of the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 hieron (osteon), meaning sacred or strong bone. Since the sacrum is the seat of the organs of procreation, animal sacrums were offered in sacrifices. In Slavic languages
Slavic languages
The 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.-Branches:Scholars traditionally divide Slavic...

 and in German
German language
German is a West Germanic language, related to and classified alongside English and Dutch. With an estimated 90 – 98 million native speakers, German is one of the world's major languages and is the most widely-spoken first language 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.The portions of bone intervening...

     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...

    , 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 vertebral column. 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...

     below
  • the illium
    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.The name comes from the Latin ,...

     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.The name comes from the Latin ,...

 is sometimes called "nutation
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...

" (L. "nodding"), 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 a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

 (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 more massive than in the female.

Variations

In some cases the sacrum will consist of six pieces http://www.webcitation.org/query?url=http://www.geocities.com/akramjfr/sacralization.html&date=2009-10-25+12:10:24 or be reduced in number to four http://www.webcitation.org/query?url=http://www.geocities.com/akramjfr/lumbarization.html&date=2009-10-25+12:10:08. 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.

See also

  • Bone terminology
  • 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 vaginal delivery was possible or if and when to use a cesarean...

  • Rump (croup)
  • Coccyx (tailbone)
    Coccyx
    The coccyx , commonly referred to as the tailbone, is the final segment of the vertebral column. 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...

  • 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.The name comes from the Latin ,...

  • Lumbarization
    Lumbarization
    Lumbarization is a term that refers to an anatomic anomaly in the human spine. It is defined by the nonfusion of the first and second segments of the sacrum. The lumbar spine subsequently appears to have six vertebrae or segments, not five. Conversely the sacrum appears to have only four segments...


External links

- "The Female Pelvis: Articulated bones of pelvis" - "The Female Pelvis: Bones"
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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