Sechenov School
Home › Human Anatomy › Sacrum and Coccyx: Anatomy, Surfaces and Features

Sacrum and Coccyx

Os sacrum et os coccygis

For medical students2 min readUpdated 2026-10-10

The sacrum and coccyx are the inferior segments of the vertebral column, consisting of fused vertebrae into single monolithic structures. They form the posterior wall of the pelvis, protect the sacral canal, and serve as a robust anchoring base for pelvic floor ligaments and muscles.

Bone fusionThe sacrum consists of 5 fused vertebrae, while the coccyx consists of 2–4 fused rudimentary vertebrae.
Anatomical landmarkThe sacral promontorium is a crucial landmark for assessing pelvic obstetric dimensions.
SurfacesThe sacrum features a smooth, concave pelvic surface and a rough, convex dorsal surface.
Neural pathwaysAnterior and posterior rami of the sacral spinal nerves exit through four pairs of foramina.

General Structure of the Sacrum: Base and Apex

In adults, the sacrum is a single, massive bone formed by the natural fusion of five sacral vertebrae (vertebrae sacrales S₁–S₅). This bone performs a vital weight-bearing function as it directly contributes to forming the posterior pelvic wall.

Anatomically, it is divided into two terminal parts:

Surface Relief of the Sacrum

The sacrum features two distinct surfaces with contrasting relief reflecting their functions:

  1. Pelvic surface (facies pelvica). Concave and relatively smooth. It clearly displays transverse lines (lineae transversae), which mark the fusion sites of the sacral vertebral bodies (typically four lines). Four pairs of anterior sacral foramina (foramina sacralia anteriora) open here, allowing the anterior rami of the sacral spinal nerves to pass into the pelvic cavity.
  2. Dorsal surface (facies dorsalis). Convex and extremely rough. It features prominent longitudinal bony crests:
  3. Median sacral crest (crista sacralis mediana) — formed by the completely fused spinous processes.
  4. Intermediate sacral crests (cristae sacrales intermediae) — located lateral to the median crest, resulting from the fusion of articular processes.
  5. Lateral sacral crests (cristae sacrales laterales) — positioned even more laterally, originating from rudimentary transverse processes.

Four pairs of posterior sacral foramina (foramina sacralia posteriora) open on this surface for the passage of posterior nerve rami.

Lateral Parts and the Sacral Canal

On the lateral masses of the sacrum lies the articular auricular surface (facies auricularis), which articulates with the corresponding surface of the hip bone (ilium). Directly posterior to it is the sacral tuberosity (tuberositas ossis sacri), a massive, rough area for the attachment of strong supporting ligaments.

Running vertically through the bone from top to bottom is the sacral canal (canalis sacralis), a direct anatomical continuation of the vertebral canal. Inferiorly on the dorsal surface, this canal terminates at the sacral hiatus (hiatus sacralis). Flanking this hiatus inferiorly are the paired sacral cornua (cornua sacralia).

Anatomy of the Coccyx

The coccyx (os coccygis) is an evolutionary vestige (rudiment) of the caudal spine. In humans, it forms via the fusion of two to four small coccygeal vertebrae (vertebrae coccygeae Co₁–Co₄).

The first coccygeal vertebra is the largest and bears rudimentary articular processes on its superior surface called coccygeal cornua (cornua coccygea). These project superiorly to articulate with the sacral cornua.

Although the coccyx plays a minor role in body weight support, it has significant biomechanical value as a reliable anchor point for ligaments and parts of the pelvic floor musculature.

Mnemonic

To easily remember the origin of the crests on the dorsal surface of the sacrum, match them to the structure of a standard vertebra from center to edges: Median — Spinous, Intermediate — Articular, Lateral — Transverse (MS - IA - LT).

Frequently asked questions

What passes through the anterior and posterior sacral foramina?

Branches of the sacral spinal nerves pass through the anterior and posterior sacral foramina.

  • Anterior sacral foramina (foramina sacralia anteriora) — serve as exit sites for the anterior rami of the sacral spinal nerves.
  • Posterior sacral foramina (foramina sacralia posteriora) — serve as exit sites for the posterior rami of the sacral nerves.
What are the anatomical contents of the sacral canal?

The anatomical contents of the sacral canal include the cauda equina, venous plexuses, and adipose tissue.

  • Cauda equina (cauda equina) — lies directly within the canal cavity.
  • Internal vertebral venous plexuses (plexus venosi vertebrales interni) — run within the epidural space to the inferior end of the sacral canal.

The sacral epidural space also contains adipose tissue (fat).

Which muscle originates on the pelvic surface of the sacrum?

Only one muscle mentioned in standard sources originates on the anterior (pelvic) surface of the sacrum — the piriformis.

  • Piriformis muscle (m. piriformis) — originates on the anterior surface of the sacrum, exits the pelvic cavity through the greater sciatic foramen (foramen ischiadicum majus), and inserts onto the apex of the greater trochanter.
What anatomically bounds the sacral hiatus?

The sacral hiatus is anatomically bounded by the dorsal surface of the sacral apex and the sacral cornua.

  • Apex of sacrum (apex ossis sacri) — the dorsal surface of the apex forms the inferior boundary of the canal opening.
  • Sacral cornua (cornua sacralia) — located on either side of the sacral hiatus, directed inferiorly to bound it.

Externally, this hiatus is closed by the superficial dorsal sacrococcygeal ligament (sacrococcygeal membrane).

What is the sacral promontory and why is it clinically important?

The promontory (promontorium) is the anteriorly projecting point on the anterior margin of the first sacral vertebra body. It serves as a vital obstetric landmark for assessing the true conjugate and overall pelvic dimensions.

Where do the transverse lines on the pelvic surface of the sacrum come from?

The transverse lines (lineae transversae) are bony ridges representing the fusion lines of originally separate sacral vertebral bodies into a single bone. There are typically four lines.

What structures pass through the sacral foramina?

The anterior rami of the sacral spinal nerves exit through the anterior sacral foramina, whereas their posterior rami exit through the posterior sacral foramina.

What is the function of the sacral and coccygeal cornua?

These are rudimentary articular processes of the respective vertebrae. The sacral cornua flank the sacral hiatus and articulate with the coccygeal cornua, forming the sacrococcygeal joint.

Go deeper

More topics in Human Anatomy

PharynxTelencephalon (Cerebrum): Anatomy and StructureSphenoid BoneEndocrine Part of the PancreasInternal Carotid ArteryShoulder JointThoracic Fascia: Anatomy, Layers and Clinical SignificanceTrachea and Main BronchiOphthalmic NerveParasympathetic Nervous SystemSternum: Anatomy, Parts and ArticulationsCerebral Arteries: Anatomy, Branches, and Circle of WillisHuman Anatomy →