Anatomy Guide › Pelvis & perineum
Sacrum
The sacrum is the broad, shield-shaped bone at the base of your spine, wedged between the two hip bones like the keystone of an arch. It begins life as five separate vertebrae; between roughly ages 18 and 30 they fuse into a single solid bone. Faint horizontal lines on its front surface still mark the boundaries where the individual vertebrae once met.
The keystone comparison is not decorative — it is exactly how the sacrum works. The entire weight of your head, arms, and torso funnels down the spine into the sacrum, which passes it sideways through the sacroiliac joints into the hip bones and down the legs. When you stand, the sacrum is under continuous load, and its wedge shape lets it lock more tightly into the pelvis the harder it is pressed. Strong ligaments on all sides make the sacroiliac joints among the most stable in the body, though they can still become inflamed and are a well-known source of low back pain.
The bone is far from solid plate. A central canal continues the vertebral canal, carrying the final nerve roots of the spinal cord, and four pairs of openings on both front and back surfaces let those nerves exit to supply the pelvis and legs. The sacrum also shows some of the clearest differences between male and female skeletons: it tends to be shorter, wider, and more curved in females, part of the pelvic architecture that accommodates childbirth.
In Anamo, the sacrum is a central landmark of the pelvic region. It borders the coccyx below and sits near the iliac vessels and pelvic muscles, so it is a strong first guess whenever the heat map points low and midline.
- Formed by five vertebrae that fuse in early adulthood
- Transmits all upper-body weight into the pelvis via the sacroiliac joints
- Pierced by four pairs of openings for spinal nerve roots
- Noticeably wider and more curved in the female skeleton
Think you could find the sacrum on a 3D model? Play Anamo and test yourself — every guess is color-coded by how close it lands to the mystery structure.