Bony Pelvis – Logic-Based Note

1. Components & Overall Concept

• The bony pelvis is formed by three bones:
• Hip bones (ilium, ischium, pubis)
• Sacrum
• Coccyx
• These bones are described individually, but function as one unit when articulated.
• When articulated, they enclose a cavity → the pelvic cavity.

2. Pelvic Cavity & Iliac Fossae

• From the pelvic brim, the ala of each ilium:
• Projects upwards
• Forms the iliac fossa
• The iliac fossa contributes to the posterior abdominal wall.
• From the pelvic brim, the pelvic cavity:
• Projects backwards
• Extends toward the buttocks

3. Pelvic Brim – Exact Anatomical Boundaries

The pelvic brim is a continuous ring formed by (anterior → posterior):

1. Pubic crest
2. Pectineal line of the pubis
3. Arcuate line of the ilium
4. Ala of the sacrum
5. Sacral promontory

👉 These structures together form the inlet to the pelvic cavity.

4. Plane of the Pelvic Brim

• The plane of the pelvic brim:
• Lies at 60° to the horizontal
• Is oblique, not horizontal
• Clinical correlation:
• The vagina lies in the same plane as the pelvic brim

5. Pelvic Joints & Ligaments

• The pelvic joints and ligaments are:
• Not detailed here
• Described separately elsewhere (pages 323 onwards)

6. Sex Differences – Fundamental Logic

Sex differences arise due to two main reasons:

A. Functional Reason

• The female pelvis is broader
• Purpose: facilitate passage of the fetal head

B. Structural Reason

• Female bones are more slender
• Includes slender femoral head
• Male bones are thicker and more robust

7. Subpubic Angle – Visual & Logical Difference

Male Pelvis

• Subpubic angle is acute
• Bones are sturdy
• Shape resembles a Gothic arch (pointed)

Female Pelvis

• Subpubic angle is wide
• Bones are slender
• Shape resembles a Roman arch (rounded)

8. Pelvic Brim Shape – Male vs Female

Male Pelvic Brim

• Sacral promontory indents the outline
• Brim is widest posteriorly
• Overall shape: Heart-shaped

Female Pelvic Brim

• Less indentation by sacral promontory
• Brim is widest more anteriorly
• Overall shape: Transversely oval

9. Position of the Pelvis in the Erect Individual

Vertical Plane Alignment

• The following lie in the same vertical plane:
• Anterior superior iliac spines (ASIS)
• Upper margin of the symphysis pubis

Horizontal Plane Alignment (Extremely High-Yield)

The following structures lie in the same horizontal plane:

• Upper border of symphysis pubis
• Ischial spine
• Tip of coccyx
• Head of femur
• Apex of greater trochanter

10. Clinical Importance of This Horizontal Plane

• This plane:
• Passes through the pelvic cavity
• Corresponds to the level reached by the tip of the examiner’s finger
• During rectal examination
• During vaginal examination

Structures at This Level

• Female: Lower poles of the ovaries
• Male: Seminal vesicles

One-Line Exam Lock (Compression)

The bony pelvis is formed by the hip bones, sacrum, and coccyx enclosing a cavity whose brim—formed by the pubic crest, pectineal line, arcuate line, and sacral promontory—lies obliquely at 60°, shows sex-specific shape differences, and is positioned so that key pelvic landmarks and reproductive organs align in predictable vertical and horizontal planes of major clinical importance.

Pelvic Walls – Logic-Based Note (Zero Omission)

1. Meaning & Overall Shape of the Pelvis

• The word pelvis comes from Latin, meaning a basin.
• When the bony pelvis is:
• Tilted forwards
• Placed in the anatomical position
• It resembles a pudding basin:
• Except that much of the anterior wall is missing.

2. Completion of the Anterior Pelvic Wall

• The deficiency of the front wall is compensated by:
• The lower part of the anterior abdominal wall
• Here:
• The aponeuroses of all three anterolateral abdominal muscles
• Lie in front of rectus abdominis
• This arrangement completes the functional anterior boundary.

3. Pelvic Brim & Functional Division

• The pelvic brim divides the pelvis into two regions:

A. False Pelvis

• Lies above the pelvic brim
• Is part of the general abdominal cavity

B. True Pelvis (Pelvic Cavity)

• Lies below the pelvic brim
• Forms the pelvic cavity proper

4. Muscles of the True Pelvis

The true pelvis contains four key muscles:

Lower-limb muscles (also forming pelvic walls):

1. Obturator internus
2. Piriformis

Pelvic floor muscles:

1. Levator ani
2. Coccygeus

• Levator ani + coccygeus, together with their contralateral counterparts, form:
• The pelvic floor
• Also called the pelvic diaphragm

5. Formation of Pelvic Walls

Side Wall of the Pelvis

• Formed by:
• Hip bone
• Covered (clad) by obturator internus
• And its fascia

Posterior Wall of the Pelvis

• Formed by:
• Sacrum
• Piriformis:
• Passes laterally
• Leaves the pelvis through the greater sciatic foramen

6. Piriformis – Pelvic Component

Origin

• Arises from:
• The anterior surface of the middle three sacral segments
• On its own half of the sacrum
• Specifically:
• From the lateral mass
• Extending medially between the anterior sacral foramina

Relation to Nerves

• Because of this origin:
• Sacral nerves and sacral plexus
• Lie on the anterior surface of piriformis

Course

• Runs transversely
• Exits the pelvis via the greater sciatic foramen

Fascial Covering

• The pelvic surface of piriformis and the sacral plexus are covered by:
• Pelvic fascia
• This fascia:
• Is attached to the sacral periosteum
• Along the margin of the muscle

Additional Details

• Its:
• Course in the gluteal region
• Nerve supply
• Action
• Are described elsewhere.

7. Obturator Internus – Pelvic Component

Obturator Foramen & Membrane

• In life, the obturator foramen is filled by:
• A felted mass of fibrous tissue
• Called the obturator membrane
• There is a gap superiorly:
• Converts the obturator notch into the obturator canal
• Transmits:
• Obturator nerve
• Obturator vessels

Origin of Obturator Internus

The muscle arises from:

• The entire obturator membrane
• The bony margins of the obturator foramen

Extent of Origin

• Extends:
• Posteriorly up to the pelvic brim
• Across the flat pelvic surface of the ischium
• To the margin of the greater sciatic notch
• On the ischial tuberosity:
• The origin extends down to the falciform ridge

Muscle Fibre Direction

• From this wide origin:
• Muscle fibres converge fan-wise
• Towards the lesser sciatic notch

Tendon Formation & Bony Relations

• As the muscle bears on the lesser sciatic notch:
• Tendinous fibres develop on its surface
• The bone often shows:
• Low ridges and grooves
• Where the tendon makes a right-angled turn
• To pass into the buttock
• At this site:
• Bone is lined by hyaline cartilage
• Tendon is separated from bone by a bursa

8. Obturator Fascia & Pelvic Floor Landmark

• Obturator internus is covered by:
• A strong obturator fascia

Attachments of Obturator Fascia

• Attached:
• To bone at the margins of the muscle
• Inferiorly:
• Fuses with the falciform process
• Of the sacrotuberous ligament
• At the ischial tuberosity

Tendinous Arch of Levator Ani

• The tendinous arch:
• Is formed on the obturator internus fascia
• Slopes obliquely across it
• Functional division:
• Above the arch → pelvic cavity
• Below the arch → ischioanal fossa

One-Line Exam Lock

The pelvic walls are formed by bone and muscle, with the pelvic brim dividing the false from the true pelvis, the side wall lined by obturator internus and fascia, the posterior wall by sacrum and piriformis, and the obturator fascia giving rise to the levator ani arch that separates the pelvic cavity from the ischioanal fossa.

Pelvic Floor – Logic-Based Note (Zero Omission)

1. What the pelvic floor is (shape + what it “slings”)

• The pelvic floor consists of a gutter-shaped sheet of muscle called the pelvic diaphragm.
• This pelvic diaphragm is slung around the midline viscera:
• Urethra
• Anal canal
• And in the female, the vagina

2. Which muscles form it + where they arise and insert

• The muscles of the pelvic floor are:
• Levator ani
• Coccygeus
• They arise in continuity from:
• The body of the pubis
• The tendinous arch over the obturator fascia
• The spine of the ischium
• They are inserted into:
• The coccyx
• The postanal plate (details below)
• Fibre direction and floor shape:
• From their origin, fibres slope downwards and backwards to the midline
• The pelvic floor produced is a gutter
• It slopes downwards and faces forwards

Levator Ani (full details)

3. Main parts + common origin line

• Levator ani has two main parts:
• Pubococcygeus
• Iliococcygeus
• Their fibres arise in continuity from:
• The body of the pubis
• To the ischial spine
• Across the obturator fascia
• Along a condensation of that fascia called the tendinous arch
• Developmental/anatomical migration fact:
• Levator ani originally arose from the pelvic brim (this is its present origin in most mammals)
• In humans it has migrated down the side wall of the pelvis
• It carried the tendinous arch with it
• Residual fibres fact:
• Residual aponeurotic fibres of levator ani contribute to the strength of the obturator fascia above the tendinous arch

4. Pubococcygeus: exact origin + functional fibre sets

Origin of pubococcygeus (as a part)

• Pubococcygeus is the part of levator ani arising from:
• The anterior half of the tendinous arch
• The posterior surface of the body of the pubis

Functional fibre sets of pubococcygeus (named subdivisions)

A. Pubococcygeus muscle proper

• The bulk of the posterior fibres:
• Sweep backwards in a flat sheet
• On the pelvic surface of iliococcygeus
• Form a tendinous plate in the midline
• This plate is attached posteriorly to the front of the coccyx
• These posterior fibres constitute the pubococcygeus muscle proper

B. Puborectalis

• Fibres arising more anteriorly from the body of the pubis:
• Swing more medially and more inferiorly
• Pass around the anorectal junction
• Join with fibres of the opposite side
• Join with the top of the external anal sphincter
• This part is called puborectalis
• Puborectalis forms a U-shaped sling
• It holds the anorectal junction angled forwards

C. Puboanalis

• Some fibres blend with:
• The longitudinal muscle of the rectum
• The conjoint longitudinal coat of the anal canal
• These fibres are termed puboanalis

D. Puboprostaticus / Pubourethralis (male)

• The most medial fibres of pubococcygeus:
• Pass backwards alongside the prostate
• Pass alongside the sphincter urethrae
• Decussate across the midline behind the urethra
• They are referred to as:
• Puboprostaticus or Pubourethralis

E. Pubovaginalis (female)

• In the female, the corresponding fibres:
• Sling around the posterior wall of the vagina
• They are referred to as pubovaginalis

F. Perineal body attachment (both sexes)

• In both sexes, fibres also attach to the perineal body

5. Iliococcygeus: origin, insertion, overlap, name logic

Origin

• Iliococcygeus arises from:
• The posterior half of the tendinous arch
• The pelvic surface of the ischial spine

Relations and insertion

• Its fibres:
• Overlap the pelvic surface of coccygeus
• Insert into:
• The side of the coccyx
• The anococcygeal raphe
• The anococcygeal raphe:
• Extends from the tip of the coccyx
• To the junction of rectum and anal canal

Name explanation

• Although iliococcygeus does not arise from the ilium, its name derives from its former origin on the iliac bone at the pelvic brim

6. Postanal plate (anococcygeal ligament): what it is + layers (top → bottom)

• The postanal plate, also called the anococcygeal ligament, is:
• A layered musculotendinous structure
• Located between the anal canal and the caudal part of the vertebral column
• The terminal rectum sits on it
• From above downwards, it consists of:
1. The superior fascia of the pelvic diaphragm
2. The tendinous plate of pubococcygeus
3. The muscular raphe of iliococcygeus
4. The posterior parts of puborectalis
5. The external anal sphincter

Coccygeus (full details)

7. Coccygeus = ischiococcygeus, attachments, relations, ligament fact

• Coccygeus is best thought of as ischiococcygeus
• Origin:
• From the tip of the ischial spine
• Fibres fan out to insert into:
• The side of the coccyx
• The lowest piece of the sacrum
• Relations:
• Lies edge to edge with the lower border of piriformis
• Is overlapped anteriorly by iliococcygeus
• Surface/ligament identity:
• Its gluteal surface is fibrous tissue
• This fibrous tissue is indeed the sacrospinous ligament

Nerve Supply (exact)

8. Levator ani nerve supply

• Levator ani is mainly supplied from the sacral plexus by branches of:
• S3
• S4
• These branches enter the upper (pelvic) surface of the muscle
• Some of these somatic fibres may travel:
• In, or very close to, the pelvic splanchnic nerves

9. Puborectalis / Pubourethralis / Pubovaginalis supply (from below)

• Puborectalis, pubourethralis, and pubovaginalis are supplied from below by:
• The perineal branch of S4
• The inferior rectal branch of the pudendal nerve
• They share this pattern in common with the external anal sphincter

10. Fibre-type fact

• Levator ani, like the external anal sphincter and urethral sphincter muscles, has:
• A high proportion of slow twitch fibres

11. Coccygeus nerve supply

• Coccygeus is supplied by branches of:
• S3
• S4

Actions (exact physiology + defecation + micturition + parturition)

12. General actions of pelvic floor

• The pelvic floor helps to:
• Support the pelvic viscera
• Retain them in their normal positions
• The floor contracts to counteract increased intra-abdominal pressure, which may be:
• Momentary (coughing, sneezing)
• Prolonged (muscular efforts like lifting)
• If an expulsive effort is required, the floor relaxes

13. Defecation mechanics (step-by-step)

• During defecation:
• The abdominal wall and diaphragm contract
• Puborectalis relaxes
• This straightens the anorectal junction
• The pelvic floor descends
• The floor becomes more funnel-shaped
• The floor then rises again as the process ends

14. Female micturition support

• The pubovaginalis fibres of levator ani may be important for:
• Assisting the urethral sphincter
• At the end of micturition in the female

15. Parturition (labour) role + damage risk

• In parturition:
• The pelvic floor initially directs the fetal head to the pelvic outlet
• The degree of stretching of the muscular and fibrous parts of the floor:
• May make it liable to damage by tearing

Pelvic Fascia (parietal + piriformis fascia + rectosacral fascia + pelvic diaphragm fasciae)

16. Parietal pelvic fascia over obturator internus

• The parietal pelvic fascia on the pelvic surface of obturator internus is:
• A strong membrane
• Fuses with the periosteum at the upper margin of the muscle
• Below the tendinous arch (origin of levator ani):
• This fascia becomes thin
• Where it covers obturator internus on the lateral wall of the ischioanal fossa

17. Fascia over piriformis + what lies behind/in front

• Fascia on the pelvic surface of piriformis fuses with periosteum at:
• The medial margins of the anterior sacral foramina
• Therefore:
• The sacral anterior primary rami emerging from these foramina lie behind this fascia
• The internal iliac vessels lie:
• In front of the fascia over piriformis
• The large (presacral) lateral sacral veins:
• Lie initially behind this fascia
• As they emerge from the anterior sacral foramina

18. Rectosacral fascia + fusion point + vein relation

• From the front of the lower sacrum there is a condensation of connective tissue called the rectosacral fascia
• Varies in thickness
• It:
• Passes downwards and forwards
• Fuses with the mesorectal fascia
• At 3–5 cm proximal to the anorectal junction
• The large (presacral) lateral sacral veins lie:
• Behind this fascia
• On the front of the sacrum

19. Superior fascia of pelvic diaphragm (levator ani + coccygeus surface fascia)

• The fascia on the pelvic surface of levator ani and coccygeus is:
• The superior fascia of the pelvic diaphragm
• Attachments:
• In front: to the posterior surface of the body of the pubis
• At the back: to the ischial spine
• Between these attachments:
• It blends with the obturator fascia
• A thickening of these two fused fasciae forms the tendinous arch of origin of levator ani

20. Inferior fascia of pelvic diaphragm

• The inferior fascia of the pelvic diaphragm is:
• The thin fascia covering the undersurface of levator ani
• On the sloping medial wall of the ischioanal fossa
• It blends:
• Laterally with the obturator fascia
• Medially with the fascia on the external anal sphincter and external urethral sphincter

One-sentence exam lock (compressed but complete)

• The pelvic floor is a gutter-shaped pelvic diaphragm formed by levator ani and coccygeus arising from pubis, tendinous arch on obturator fascia, and ischial spine and inserting into coccyx/postanal plate, with levator ani subdivided into pubococcygeus and iliococcygeus (plus named fibre slings), innervated mainly by S3–S4 (with key branches from below), acting to support viscera and regulate continence/expulsive functions, and invested by pelvic fascia layers including superior/inferior pelvic diaphragm fascia and rectosacral fascia with specific vessel–nerve relationships.

🦴 Bony Pelvis, Pelvic Walls, Pelvic Floor & Pelvic Fascia — MASTER TABLE (Zero Omission)

TABLE 1 — BONY PELVIS: COMPONENTS, BRIM, PLANES & SEX DIFFERENCES

TABLE 2 — PELVIC WALLS: STRUCTURE, MUSCLES & FASCIA

TABLE 3 — PELVIC FLOOR (PELVIC DIAPHRAGM): MUSCLES & ARCHITECTURE

TABLE 4 — LEVATOR ANI: PARTS & NAMED FIBRES

TABLE 5 — ILIOCOCCYGEUS & COCCYGEUS

TABLE 6 — POSTANAL PLATE (TOP → BOTTOM)

Anococcygeal Raphe vs Postanal Plate — Complete Anatomy Table

Exam Lock (2 lines to remember forever)

• Anococcygeal raphe = midline fibrous fusion line of levator ani from anus to coccyx
• Postanal plate = fibromuscular posterior buttress of anal canal formed by levator ani + deep EAS + raphe

TABLE 7 — NERVE SUPPLY

TABLE 8 — ACTIONS (FUNCTIONAL PHYSIOLOGY)

TABLE 9 — PELVIC FASCIA (ZERO-OMISSION)

🧠 Ultimate One-Line Exam Reflex

The pelvis is a basin-shaped bony and muscular unit whose oblique brim defines false and true pelvis, whose walls are lined by obturator internus and piriformis, whose floor is a gutter-shaped pelvic diaphragm formed by levator ani and coccygeus with precise slings for continence and childbirth, and whose layered pelvic fasciae determine critical nerve and venous relationships.