Anatomy and Physiology 2e · The Appendicular Skeleton
The Pelvic Girdle and Pelvis
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The Pelvic girdle The two hip (coxal) bones that connect the lower limbs to the trunk Full entry → is the ring of two hip (coxal) bones that connects the lower limbs to the trunk. The Pelvis The girdle plus the sacrum and coccyx Full entry → is the larger bony basin formed by the girdle plus the sacrum and coccyx behind. Where the pectoral girdle is built for mobility, the pelvic girdle is built for the opposite: it is a massive, nearly closed ring designed to transfer the body's weight from the vertebral column to the legs, protect the organs of the lower abdomen and pelvis, and — in people who give birth — form the bony birth canal.
Each hip bone is really three bones that fuse together at the Acetabulum The deep cup on the hip bone that receives the femoral head Full entry →, the deep socket that receives the head of the femur: the Ilium The large, wing-like superior part of the hip bone Full entry → (superior), the Ischium The posteroinferior part of the hip bone Full entry → (posteroinferior), and the Pubis The anteroinferior part of the hip bone Full entry → (anteroinferior). The two hip bones meet in front at the Pubic symphysis The cartilaginous joint between the two pubic bones Full entry → and articulate behind with the sacrum at the sacroiliac joints, closing the ring. Stand on one leg and feel your hip: most of the force of your body weight passes through the ilium into the acetabulum and down the femur, which is why the pelvis is so thick and heavily reinforced with ligaments.
The pelvis is commonly divided into a False (greater) pelvis The space above the pelvic brim, part of the abdominal cavity Full entry → above and a True (lesser) pelvis The bony canal below the pelvic brim Full entry → below the Pelvic brim The oblique ridge (sacral promontory to pubic symphysis) dividing the pelvis Full entry → — a division that matters for obstetrics and for understanding where abdominal versus pelvic organs sit.
Why this matters
The pelvis is the load-bearing hub of the body, and its anatomy shows up in several high-stakes situations:
- Weight transfer and posture: the pelvis redirects the weight of the head, trunk, and upper limbs from the vertebral column out to the hip joints. Its strength is why humans can stand, carry loads, and run on two legs.
- Pelvic fractures are high-energy injuries (motor vehicle crashes, falls from height). The pelvis surrounds a dense network of blood vessels, so a disrupted pelvic ring can cause major internal bleeding, and the bladder and urethra lie close enough to be injured as well. Anyone who works in emergency care needs to recognize how serious a "broken pelvis" can be.
- Childbirth: the dimensions of the true pelvis — its inlet, cavity, and outlet — define the birth canal. Understanding pelvic shape is foundational in obstetrics and midwifery education.
- Palpable landmarks like the iliac crest (the "hip bone" you feel at your waist) and the ischial tuberosities (the bones you sit on) are used every day in physical assessment, injections, and body-measurement techniques.
The college version
Core Concepts
The ilium: the great wing
The ilium is the largest and most superior part of the hip bone. Its upper margin, the iliac crest, ends in the anterior superior iliac spine (ASIS) in front and the posterior superior iliac spine (PSIS) behind — the ASIS is the sharp point you can feel at the front of your hip, and the PSIS is marked by the dimples above the buttocks. The inner surface is the smooth iliac fossa; the outer surface gives origin to the gluteal muscles. The ilium's posterior border is notched by the greater sciatic notch, through which the sciatic nerve and other structures leave the pelvis.
The ischium and pubis: the seat and the front
The ischium forms the posteroinferior part. Its thick, rough ischial tuberosity is the "sit bone" — the weight-bearing point when you sit — and the sharp ischial spine projects medially above it, separated by the lesser sciatic notch. The pubis forms the anterior part; the two pubic bones meet at the pubic symphysis (a cartilaginous joint), and their inferior rami form the pubic arch. Together the ischium and pubis enclose the large obturator foramen, which is closed by a membrane and is a common site of "obturator" nerve and muscle references.
The acetabulum: where all three meet
The acetabulum is the deep, cup-shaped socket on the lateral surface of the hip bone where the ilium, ischium, and pubis fuse. It receives the head of the femur to form the hip joint. Compare it with the glenoid cavity of the scapula (Topic 1): the acetabulum is deep and surrounds most of the femoral head, which is why the hip dislocates far less often than the shoulder — the stability-over-mobility trade-off again, now favoring stability.
The pelvis as a whole: ring, brim, and canal
The bony pelvis includes the two hip bones, the sacrum, and the coccyx. The pelvic brim is the oblique line running from the sacral promontory (front edge of the sacrum) along the arcuate lines of the ilia and the superior border of the pubic symphysis. It divides the pelvis into:
- the false (greater) pelvis above the brim — actually part of the abdominal cavity, holding the lower abdominal organs (parts of the intestines, for example); and
- the true (lesser) pelvis below the brim — the bony canal containing the bladder, rectum, and internal reproductive organs, bounded by the pelvic inlet (superior opening) and pelvic outlet (inferior opening).
The sacroiliac joints and the pubic symphysis complete the ring, reinforced by powerful ligaments (including the sacrotuberous and sacrospinous ligaments).
Sex differences in the pelvis
Human pelves vary widely between individuals, but the commonly taught general trends are: the female pelvis tends to be wider and shallower, with a larger, more rounded pelvic inlet, a wider subpubic angle (commonly cited as greater than about 90 degrees versus a narrower angle in males), less prominent ischial spines, and a shorter, less curved sacrum — features that enlarge the birth canal. These are statistical tendencies, not reliable "gender tests": there is broad overlap between individuals, and sex is not binary in all people. What matters educationally is that the female pelvis is adapted for childbirth, and that variation is normal.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Pelvic girdle | Pelvis | Girdle = the two hip bones only; pelvis = hip bones + sacrum + coccyx |
| Ilium | Ischium or pubis | Ilium is the superior wing (iliac crest, ASIS); ischium is posteroinferior (sit bone); pubis is anterior (pubic symphysis) |
| False (greater) pelvis | True (lesser) pelvis | False pelvis is above the pelvic brim and holds abdominal organs; true pelvis is below and forms the birth canal |
| Acetabulum | Glenoid cavity | Acetabulum is deep (stable hip); glenoid cavity is shallow (mobile shoulder) |
| Ischial tuberosity | Ischial spine | Tuberosity = the blunt "sit bone"; spine = the sharper projection above the lesser sciatic notch |
| Greater sciatic notch | Lesser sciatic notch | Greater notch lies between the PSIS and ischial spine (sciatic nerve exits here); lesser notch is below the ischial spine |
| Pelvic inlet | Pelvic outlet | Inlet = superior opening bounded by the pelvic brim; outlet = inferior opening bounded by the pubic arch, ischial spines/tuberosities, and coccyx |
| "Male vs. female pelvis" as an absolute | A statistical tendency | Pelvic shape varies widely; differences are trends with broad overlap, not reliable individual identifiers |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your pelvis is like a sturdy mixing bowl made of bone at the bottom of your body. Three pieces of bone fuse into one on each side — a big wing (ilium), a seat bone (ischium), and a front bone (pubis) — and they lock together in a ring with your tailbone. The ring holds your intestines, bladder, and other organs like a bowl, and it passes your body's weight down into your legs. In people who give birth, the bottom opening of the bowl is the passage the baby travels through.
Worked example
A construction worker falls from a ladder and lands hard on his side. In the emergency department, the team's concern is not just "is the bone broken?" — it is how much force the pelvis absorbed. Walking through the anatomy: the pelvis is a ring, and a ring broken in one place often breaks (or at least strains) in a second place, like a pretzel snapping at two points. The worker's pain is over the ilium, but the team must consider the whole ring — the sacroiliac joints, the pubic symphysis, and the sacrum. The pelvic cavity wraps around a dense venous plexus and the iliac vessels, so a displaced pelvic fracture can bleed extensively into the retroperitoneum; the bladder and urethra sit just inside the pubic arch and are at risk too. The care team will stabilize the ring, image it, and monitor for hemorrhage — and the bedside clinician's job is rapid assessment, monitoring, and reporting, with diagnosis and treatment decided by the care team. The educational point: every structure near the pelvic ring — vessels, bladder, urethra, nerves — is on the list of "what else did this injury reach?", which is why the anatomy of the pelvis is genuinely lifesaving knowledge.
Key takeaways
- Pelvic girdle = two hip (coxal) bones; pelvis = girdle + sacrum + coccyx. Don't use the words interchangeably.
- Each hip bone = ilium + ischium + pubis, fused at the acetabulum (deep socket for the femoral head).
- Landmarks: iliac crest and ASIS (front hip point), ischial tuberosity (sit bone), ischial spine, pubic symphysis, obturator foramen, greater and lesser sciatic notches.
- The pelvic brim separates the false (greater) pelvis above (abdominal cavity) from the true (lesser) pelvis below (birth canal; bladder, rectum, reproductive organs).
- The pelvis transfers body weight: vertebral column → sacrum → ilia → acetabula → femurs.
- Acetabulum is deep → the hip is stable and dislocates far less often than the shoulder (compare glenoid cavity).
- Commonly taught sex differences: female pelvis wider/shallower, larger inlet, wider subpubic angle, less prominent ischial spines; these are trends with wide individual variation.
- Pelvic ring fractures are high-energy injuries with risk of major hemorrhage and bladder/urethral injury — an emergency, not a routine fracture.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What bones make up the pelvic girdle, and what bones make up the full pelvis?
Show answer
The pelvic girdle is the two hip (coxal) bones; the pelvis adds the sacrum and coccyx.
Name the three parts of each hip bone and the socket where they fuse.
Show answer
Ilium (superior), ischium (posteroinferior), and pubis (anteroinferior), fusing at the acetabulum.
What structure divides the false pelvis from the true pelvis, and what does each contain?
Show answer
The pelvic brim (sacral promontory → arcuate lines of the ilia → superior pubic symphysis). Above it lies the false/greater pelvis (abdominal cavity); below it lies the true/lesser pelvis (birth canal with bladder, rectum, and reproductive organs).
Why is the hip joint more stable than the shoulder joint, anatomically?
Show answer
The acetabulum is a deep cup that surrounds most of the femoral head, whereas the glenoid cavity is shallow — depth is the main stability difference.
List four palpable landmarks of the pelvis.
Show answer
Iliac crest, anterior superior iliac spine (ASIS), ischial tuberosity, pubic symphysis (also: PSIS, ischial spine).
Why is a pelvic ring fracture considered a high-risk injury beyond the bone break itself?
Show answer
The pelvis is a ring surrounding major blood vessels, the bladder, and the urethra; a disrupted ring can cause severe hemorrhage and injury to those organs, so it is managed as an emergency.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Pelvic girdle
- The two hip (coxal) bones that connect the lower limbs to the trunk
- Pelvis
- The girdle plus the sacrum and coccyx
- Hip (coxal) bone
- The fused ilium + ischium + pubis of one side
- Ilium
- The large, wing-like superior part of the hip bone
- Ischium
- The posteroinferior part of the hip bone
- Pubis
- The anteroinferior part of the hip bone
- Acetabulum
- The deep cup on the hip bone that receives the femoral head
- Pubic symphysis
- The cartilaginous joint between the two pubic bones
- Pelvic brim
- The oblique ridge (sacral promontory to pubic symphysis) dividing the pelvis
- True (lesser) pelvis
- The bony canal below the pelvic brim
- False (greater) pelvis
- The space above the pelvic brim, part of the abdominal cavity
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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