Sacrum

The sacrum (/ˈsækrəm/ or /ˈseɪkrəm/; plural: sacra or sacrums), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of sacral vertebrae S1–S5 between 18 and 30 years of age.Image of a female pelvis seen anteriorly, sacrum at centre.Lateral surfaces of sacrum and coccyx.Base of sacrum.Median sagittal section of the sacrum.Left levator ani from within.The posterior divisions of the sacral nerves.Sacrum. Pelvic surface.Sacrum. Dorsal surface. The sacrum (/ˈsækrəm/ or /ˈseɪkrəm/; plural: sacra or sacrums), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of sacral vertebrae S1–S5 between 18 and 30 years of age. The sacrum is situated at the upper, back part of the pelvic cavity, between the two 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 at the L-shaped sacroiliac joints. The upper part of the sacrum connects with the last 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 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, allowing greater room for the pelvic cavity. In all other quadrupedal 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–S5 vertebrae of a horse will fuse, the S1–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. The Stegosaurus dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a 'posterior brain case”. English sacrum was introduced as a technical term in anatomy in the mid-18th century, as a shortening of the Late Latin name os sacrum 'sacred bone', itself a translation of Greek ἱερόν ὀστέον, the term found in the writings of Galen. Prior to the adoption of sacrum, the bone was also called holy bone in English, paralleling German heiliges Bein or Heiligenbein (alongside Kreuzbein) and Dutch heiligbeen. The origin of Galen's term is unclear. Supposedly the sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation). Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible. As the Greek adjective ἱερός may also mean 'strong', it has also been suggested that os sacrum is a mistranslation of a term intended to mean 'the strong bone'. This is supported by the alternative Greek name μέγας σπόνδυλος by the Greeks, translating to 'large vertebra', translated into Latin as vertebra magna. In Classical Greek the bone was known as κλόνις (Latinized clonis); this term is cognate to Latin clunis 'buttock', Sanskrit śróṇis 'haunch' and Lithuanian šlaunis 'hip, thigh'. The Latin word is found in the alternative Latin name of the sacrum, ossa clunium, as it were 'bones of the buttocks'. Due to the fact that the os sacrum is broad and thick at its upper end, the sacrum is alternatively called os latum, 'broad bone'. The sacrum is a complex structure providing support for the spine and accommodation for the spinal nerves. It also articulates with the hip bones. The sacrum has a base, an apex, and three surfaces – a pelvic, dorsal and a lateral surface. The base of the sacrum, which is broad and expanded, is directed upward and forward. On either side of the base is a large projection known as an ala of sacrum and these alae (wings) articulate with the sacroiliac joints. The alae support the psoas major muscles and the lumbosacral trunk which connects the lumbar plexus with the sacral plexus. In the articulated pelvis the alae are continuous with the iliac fossa. Each ala is slightly concave from side to side, and convex from the back and gives attachment to a few of the fibers of the iliacus muscle. The posterior quarter of the ala represents the transverse process, and its anterior three-quarters the costal process of the first sacral segment. Each ala also serves as part of the border of the pelvic brim. The alae also form the base of the lumbosacral triangle. The iliolumbar ligament and lumbosacral ligaments are attached to the ala. In the middle of the base 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. Behind this is the large triangular orifice of the sacral canal, which is completed by the lamina and spinous process of the first sacral vertebra. The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra. They are attached to the body of the first sacral vertebra and to the each ala, by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebrae.