The sacral promontory marks part of the border of the pelvic inlet, and comprises the iliopectineal line and the linea terminalis. A sacral angle of lesser degree suggests funneling of the pelvis. The sacral angle is the angle formed by the true conjugate with the two pieces of sacrum. The sacral canal as a continuation of the vertebral canal runs throughout the greater part of the sacrum. The apex is directed downward and presents an oval facet for articulation with the coccyx. They are attached to the body of the first sacral vertebra and to 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. 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. 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. 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. The iliolumbar ligament and lumbosacral ligaments are attached to the ala. The alae also form the base of the lumbosacral triangle. Each ala also serves as part of the border of the pelvic brim. 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 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. In the articulated pelvis the alae are continuous with the iliac fossa. The alae support the psoas major muscles and the lumbosacral trunk which connects the lumbar plexus with the sacral plexus. 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 base of the sacrum, which is broad and expanded, is directed upward and forward. The sacrum has a base, an apex, and three surfaces – a pelvic, dorsal and a lateral surface. The sacrum is a complex structure providing support for the spine and accommodation for the spinal nerves. The Stegosaurus dinosaur had a greatly enlarged neural canal in the sacrum, characterized as a " posterior brain case". 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 number of sacral vertebrae varies slightly. 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 central part is curved outward toward the posterior, allowing greater room for the pelvic cavity. The base of the sacrum, the broadest and uppermost part, is tilted forward as the sacral promontory internally.
Overall it is concave (curved upon itself). The sacrum has three different surfaces which are shaped to accommodate surrounding pelvic structures.
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 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 sacrum situates at the upper, back part of the pelvic cavity, between the two wings of the pelvis. The sacrum (plural: sacra or sacrums ), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1–S5) between ages 18 and 30.
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