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The abdomen is the largest cavity in the body, located between the thorax above and the pelvis below.
It houses most of the digestive, urinary, and reproductive organs.
Superiorly: Diaphragm (separates it from thoracic cavity).
Inferiorly: Continuous with the pelvic cavity at the pelvic brim.
Anteriorly and laterally: Muscles of the abdominal wall.
Posteriorly: Lumbar vertebrae, psoas major, quadratus lumborum, and fascia.
Abdominal cavity proper – upper part, extending from diaphragm to pelvic brim.
Pelvic cavity – below the pelvic brim, contains pelvic viscera.
Peritoneal cavity – a potential space lined by peritoneum.
The abdomen is divided into nine regions by two vertical and two horizontal planes:
Vertical (midclavicular) planes: Pass from midpoint of clavicle to midinguinal point.
Horizontal planes:
Subcostal plane (below 10th costal cartilage).
Transtubercular plane (through iliac tubercles).
Nine regions (R→L, top→bottom):
Right hypochondriac | Epigastric | Left hypochondriac
Right lumbar | Umbilical | Left lumbar
Right iliac | Hypogastric | Left iliac
Xiphoid process: Marks lower end of sternum, level of T9 vertebra.
Costal margin: Inferior border of rib cage; overlies diaphragm and liver.
Umbilicus: Level of intervertebral disc between L3–L4 in adults.
Pubic symphysis: Marks lower midline of anterior abdominal wall.
Number: 5 (L1–L5).
Distinctive features:
Body: Large, kidney-shaped, wider transversely.
Pedicles: Strong, projecting backward.
Laminae: Short and broad, forming posterior wall of vertebral canal.
Spinous process: Thick, quadrilateral, directed horizontally backward.
Transverse processes: Long, slender (accessory processes present at base).
Articular processes:
Superior facets → directed medially and backward.
Inferior facets → directed laterally and forward.
Mammillary processes: Project from posterior surface of superior articular processes — give attachment to multifidus.
Vertebral foramen: Triangular and smaller than cervical but larger than thoracic.
Large bodies with nearly horizontal spinous processes.
Accessory and mammillary processes characteristic.
Body: Deep anteriorly → forms lumbosacral angle.
Transverse processes: Thick and short.
Inferior articular facets: Face forward; articulate with sacral promontory.
Primary centers:
1 for body (centrum).
1 for each neural arch (two total).
Appear during 8th week of intrauterine life.
Secondary centers (after puberty):
1 each for the superior and inferior surfaces of the body (ring epiphyses).
1 for the tip of each transverse process.
1 for the tip of spinous process.
These fuse by 25 years of age.
Performed in lower lumbar region (L3–L4 or L4–L5), below termination of spinal cord.
Used to obtain CSF or administer spinal anesthesia.
The line joining iliac crests (Tuffier’s line) passes through the L4 spine — a key surface landmark.
Forward curvature of lumbar spine; physiological in normal adults.
Excessive lordosis: Common in pregnancy or obesity due to anterior shift of weight.
Flattened lordosis: Occurs in muscle weakness or disc pathology.
Forward slipping of one vertebra over another, usually L5 over S1.
Due to congenital defect in pars interarticularis or trauma.
Produces back pain and nerve compression symptoms.
Protrusion of nucleus pulposus through annulus fibrosus compresses spinal nerves (usually L4–L5 or L5–S1).
Causes sciatica — pain radiating down the back of thigh and leg.
Common at L1 or L2 from falls on buttocks or blows to head (compression type).
May damage cauda equina or spinal roots, causing paralysis or sensory loss.
Formed between long axis of lumbar spine and sacrum.
Increased angle = more stress on intervertebral discs → low back pain.
Tuberculosis of lumbar vertebrae may track along psoas sheath, presenting as a fluctuant swelling in groin or thigh.
The sacrum is a large, triangular bone formed by fusion of five sacral vertebrae.
Forms the posterosuperior part of the pelvis, articulating with hip bones at the sacroiliac joints.
Upper part — broad and strong to support body weight; lower part — tapered and light.
Surfaces:
Pelvic surface: Smooth and concave.
Dorsal surface: Irregular and convex.
Lateral surfaces: Irregular; partly articular (auricular surface) for hip bones.
Pelvic surface faces downwards and forwards.
Upper surface of S1 body slopes forward at 30° (sacral promontory).
Upper end of sacral canal directed upwards and slightly backward.
Formed by upper surface of first sacral vertebra.
Features:
Body articulates with L5 at the lumbosacral joint; anterior projection forms sacral promontory.
Vertebral foramen → leads into sacral canal, triangular.
Superior articular processes → facets directed backward and medially.
Small, oval surface articulating with base of coccyx.
Four transverse ridges mark fusion lines of vertebral bodies.
Anterior sacral foramina (4 pairs) transmit ventral rami of S1–S4 and lateral sacral arteries.
Median sacral crest — fused spines of vertebrae.
Intermediate crest — fused articular processes.
Lateral crest — fused transverse processes.
Posterior sacral foramina transmit dorsal rami of S1–S4.
Continuation of vertebral canal, triangular in cross-section.
Opens inferiorly as sacral hiatus (due to failure of laminae of S5 and S4 to meet).
Transmits cauda equina, filum terminale, and spinal meninges.
At sacral hiatus: exits S5 nerves, coccygeal nerves, and filum terminale.
Pelvic surface related to:
Median sacral vessels, sympathetic trunks, peritoneum, and rectum.
Lateral surface related to:
Sympathetic chain, lumbosacral trunk, iliolumbar artery, and obturator nerve (forming triangle of Marcille).
Dorsal surface: attachment to erector spinae and multifidus muscles.
Anterior (pelvic) surface:
Piriformis arises from anterior S2–S4.
Posterior surface:
Gluteus maximus and multifidus attached.
Lateral border:
Attachment for sacroiliac, sacrotuberous, and sacrospinous ligaments.
Ala (wing): gives origin to iliacus.
Sacral Index = (Breadth across base × 100) / (Length from promontory to apex).
Male ≈ 105 (long, narrow sacrum).
Female ≈ 115 (short, wide sacrum).
Body of S1: broader than alae in males; equal in females.
Ventral concavity: shallower and uniform in males; irregular and deeper in females.
Pelvic cavity: larger in females for childbirth.
Develops from five separate vertebrae.
Primary centers (21 total):
5 for bodies, 10 for arches, 6 for costal bars (upper three vertebrae).
Secondary centers (14 total):
10 for body epiphyses, 2 for auricular surfaces, 2 for margins below auricular areas.
Timeline:
Primary centers appear in 2nd–8th week of fetal life.
Fuse between 2nd–8th years.
Secondary centers appear at puberty and fuse by 25 years.
Small triangular bone formed by fusion of four rudimentary coccygeal vertebrae.
Curves downward and forward, continuous with sacral curve.
First coccygeal piece: largest, may remain separate.
Has body, cornua (horns), and rudimentary transverse processes.
Cornua articulate with sacral cornua by intercornual ligaments.
Remaining segments: small nodules decreasing in size inferiorly.
Four primary centers — one for each segment.
Appear between 1st and 20th years; fuse by 20–30 years.
Fuses with sacrum late in life.
Slight movement persists at sacrococcygeal joint until old age
Definition:
The pelvis (Latin = basin) is a ring of bones forming the lower part of the trunk between the vertebral column and the lower limbs.
Constituents:
Two hip bones (ossa coxae) — one on each side.
Sacrum — posteriorly.
Coccyx — posterior and inferior tip.
Joints forming the pelvis:
Sacroiliac joints (between sacrum and ilium).
Symphysis pubis (between two pubic bones).
Lumbosacral joint (between L5 and sacrum).
Sacrococcygeal joint (between sacrum and coccyx).
Function:
Supports the trunk and transmits body weight to lower limbs.
Protects pelvic viscera (urinary bladder, rectum, reproductive organs).
Provides attachment for pelvic and lower limb muscles.
Greater (false) pelvis
Above the pelvic brim.
Formed by the iliac fossae and lumbar vertebrae.
Supports abdominal contents.
Lesser (true) pelvis
Below the pelvic brim.
Formed by sacrum, coccyx, and lower ilium, ischium, and pubis.
Contains pelvic viscera.
Formed by:
Posteriorly: Sacral promontory and alae.
Laterally: Arcuate line of ilium and pectineal line of pubis.
Anteriorly: Pubic crest and symphysis pubis.
Pelvic cavity: Funnel-shaped space between inlet and outlet.
Pelvic outlet: Bounded by coccyx, ischial tuberosities, and pubic arch.
| Feature | Male Pelvis | Female Pelvis |
|---|---|---|
| General | Thick, heavy, narrow | Light, broad, shallow |
| Pelvic inlet | Heart-shaped | Oval or round |
| Pelvic outlet | Smaller | Larger |
| Subpubic angle | Acute (~70°) | Wide (~90–100°) |
| Ischial spines | Project medially | Everted laterally |
| Sacrum | Long, narrow, concave | Short, wide, less curved |
| Acetabulum | Large and faces laterally | Smaller, faces more anteriorly |
| Greater sciatic notch | Narrow and deep | Wide and shallow |
| Obturator foramen | Oval | Triangular |
| Pelvic cavity | Conical | Cylindrical |
| Pubic arch | Narrow | Wide, rounded |
Functional note:
The female pelvis is adapted for childbirth, with a larger inlet and outlet and wider pubic arch.
When the pelvis is in its anatomical position:
ASIS (anterior superior iliac spine) and upper border of pubic symphysis lie in the same vertical plane.
Tip of coccyx and upper border of pubic symphysis are in the same horizontal plane.
Pelvic inlet faces forwards and upwards; outlet faces downwards and backwards.
Sacral promontory forms the posterior boundary of pelvic brim.
Type: Secondary cartilaginous joints (amphiarthroses).
Components: Between adjacent vertebral bodies.
Structure:
Intervertebral disc between the bodies.
Thin layer of hyaline cartilage covering each end-plate.
Movements:
Permit slight movement (flexion, extension, lateral bending, rotation).
Range of motion greatest in the cervical and lumbar regions.
Reinforcing ligaments:
Anterior longitudinal ligament (strong, limits hyperextension).
Posterior longitudinal ligament (narrow, limits flexion).
Type: Fibrocartilaginous pad between two vertebral bodies.
Composition:
Annulus fibrosus:
Outer concentric lamellae of fibrocartilage.
Resists torsion and provides strength.
Nucleus pulposus:
Central gelatinous mass derived from the notochord.
Acts as a shock absorber and allows flexibility.
Function:
Allows cushioning and slight motion between vertebrae.
Absorbs vertical shock and maintains spinal curvature.
Clinical relevance:
Prolapsed intervertebral disc (slipped disc):
Nucleus pulposus herniates through annulus fibrosus, compressing spinal roots (commonly L4–L5 or L5–S1).
Degenerative disc disease: Leads to loss of disc height and back pain with aging.
Mnemonic: “U R R”
U – Urinary bladder
R – Rectum
R – Reproductive organs (uterus in females, prostate and seminal vesicles in males)
Mnemonic: “PSA APP”
P – Promontory of sacrum
S – Sacral ala
A – Arcuate line of ilium
A – Pectineal line of pubis
P – Pubic crest and symphysis
Mnemonic: “WIDER FEMALE”
W – Wider subpubic angle
I – Inlet more circular
D – Deeper pelvic cavity (male: deeper; female: shallower)
E – Everted ischial spines
R – Round obturator foramen (female; male = oval)
F – Flatter sacrum
E – Enlarged outlet
M – More space between iliac crests
A – Arch broader
L – Lighter bones
E – Everything designed for childbirth
Mnemonic: “5 Cute Men”
5 – Five pairs of sacral spinal nerves (S1–S5)
C – Cauda equina
M – Meninges (dura and arachnoid mater)
Mnemonic: “A Noble Cushion”
A – Annulus fibrosus
N – Nucleus pulposus
C – Cartilaginous end plates
Mnemonic: “SASS”
S – Sacroiliac joints
A – Sacrococcygeal joint
S – Symphysis pubis
S – Lumbosacral joint
The pelvis is the link between the vertebral column and lower limbs.
The true pelvis lies below the pelvic brim and contains pelvic viscera.
The false pelvis is part of the abdominal cavity.
The pelvic inlet faces forward and upward, while the pelvic outlet faces backward and downward.
The female pelvis is shorter, broader, and lighter, adapted for childbirth.
The pelvic index (breadth/length × 100) is higher in females (~115) than in males (~105).
The subpubic angle is acute in males (70°) and obtuse in females (90–100°).
The intervertebral discs make up about one-fourth of the total length of the vertebral column.
The nucleus pulposus is a remnant of the notochord.
Spondylolisthesis results from a defect in the pars interarticularis (usually L5).
The sacrum is concave anteriorly, forming the posterior wall of the pelvis.
The lumbosacral angle normally measures about 140°; increased angle → lordosis.
The pelvic cavity in females is cylindrical, while in males it is funnel-shaped.
The pelvic outlet in females is larger due to everted ischial spines and a wider pubic arch.
The intervertebral joints are secondary cartilaginous joints, permitting limited movement but providing stability.
Case:
A patient complained of chronic dull low backache. One day, during sudden bending, he developed radiating pain in the calf.
Questions:
What is the reason for the low backache?
What triggered the radiating pain in the calf?
Answer:
The low backache is likely due to a slipped intervertebral disc in the lumbosacral region.
Initially, the slip is mild, producing dull pain.
During sudden bending and straightening, the disc herniated posterolaterally, narrowing the intervertebral foramen between L5 and S1.
This compressed one of the roots of the sciatic nerve, causing shooting pain along the area of its cutaneous supply — i.e., the posterior thigh and calf
The intervertebral disc is a fibrocartilaginous joint between vertebral bodies, composed of:
Annulus fibrosus: Outer concentric lamellae of fibrocartilage.
Nucleus pulposus: Central gelatinous core derived from the notochord.
Functions: Acts as a shock absorber, maintains spinal curvature, and allows limited movement.
Prolapsed disc (slipped disc): Posterolateral herniation of the nucleus pulposus compresses adjacent spinal roots (commonly L4–L5 or L5–S1), causing pain and neurological deficits (sciatica).
Paired processes:
Superior articular processes
Inferior articular processes
Transverse processes
Unpaired process:
Spinous process
Body (centrum) (though not a projection, it is the central unpaired part in morphology).
Male pelvis:
Heavy and narrow.
Heart-shaped inlet.
Acute subpubic angle (50–60°).
Sacrum long and narrow.
Ischial spines point medially.
Female pelvis:
Light and shallow.
Oval or round inlet.
Wide subpubic angle (80–100°).
Sacrum short and wide.
Ischial spines everted laterally.
Pelvic surface is smooth and concave; related to:
Median sacral vessels
Sympathetic trunks
Pelvic splanchnic nerves
Rectum
Peritoneum
Contents:
Cauda equina
Filum terminale
Dural sac and meninges (ending at S2)
Ventral and dorsal roots of sacral and coccygeal nerves
Internal vertebral venous plexus
Contents: Stomach, intestines, liver, gallbladder, pancreas, spleen, kidneys, ureters, and major vessels (aorta, IVC), plus part of the urinary bladder and reproductive organs.
In adults, the spinal cord ends at the level of the lower border of L1 vertebra (sometimes between L1–L2).
In newborns, it extends to L3.
Lumbar vertebra:
Large kidney-shaped body.
Absence of costal facets.
Triangular vertebral foramen.
Quadrangular spinous process.
Mammillary and accessory processes present.
Thoracic vertebra:
Heart-shaped body.
Costal facets for ribs.
Circular vertebral foramen.
Long, downward-pointing spinous process.
Definition: Fusion of L5 vertebra with the sacrum either partially or completely.
Clinical importance: Decreases lumbar mobility; may cause backache or nerve compression symptoms.
Definition: Forward slipping of L5 over the sacrum due to defect or fracture of the pars interarticularis.
Clinical features: Low back pain, restricted movement, and nerve compression (sciatica).
The true pelvis is located:
A. Above pelvic brim B. Below pelvic brim C. Between ilium and sacrum D. At level of pubic arch
✅ Answer: B
The pelvic inlet is formed posteriorly by:
A. Sacral ala B. Sacral promontory C. Arcuate line D. Pubic crest
✅ Answer: B
The pelvic outlet is bounded posteriorly by:
A. Coccyx B. Sacrum C. Ischial tuberosity D. Pubic arch
✅ Answer: A
The subpubic angle in males is approximately:
A. 30° B. 50°–60° C. 70°–80° D. 90°–100°
✅ Answer: B
The subpubic angle in females is approximately:
A. 50° B. 70° C. 80°–100° D. 40°
✅ Answer: C
The true conjugate (anteroposterior diameter of inlet) extends from:
A. Sacral promontory to upper border of symphysis pubis B. Sacral promontory to lower border of symphysis pubis
C. S1 to pubic crest D. Ischial spine to pubic tubercle
✅ Answer: A
The pelvic cavity in females is generally:
A. Deep and conical B. Shallow and cylindrical C. Flat and narrow D. Triangular
✅ Answer: B
The lumbosacral angle in normal adults is about:
A. 120° B. 130° C. 140° D. 150°
✅ Answer: C
The intervertebral joints are classified as:
A. Primary cartilaginous B. Secondary cartilaginous C. Plane synovial D. Fibrous
✅ Answer: B
The nucleus pulposus is derived from:
A. Mesoderm B. Notochord C. Ectoderm D. Sclerotome
✅ Answer: B
The annulus fibrosus is composed of:
A. Elastic fibres B. Fibrocartilage C. Hyaline cartilage D. Ligamentous fibres
✅ Answer: B
Prolapse of lumbar disc most commonly occurs at:
A. L2–L3 B. L3–L4 C. L4–L5 or L5–S1 D. S1–S2
✅ Answer: C
The sacral canal contains:
A. Spinal cord B. Cauda equina and filum terminale C. Lumbosacral trunk D. Femoral nerve
✅ Answer: B
The pelvic index (breadth × 100 / length) is higher in:
A. Males B. Females C. Children D. Elderly
✅ Answer: B
Which type of pelvis is best suited for childbirth?
A. Android B. Anthropoid C. Gynaecoid D. Platypelloid
✅ Answer: C
Spondylolisthesis most commonly occurs at:
A. L4–L5 B. L5–S1 C. T12–L1 D. S1–S2
✅ Answer: B
The joint between bodies of vertebrae is strengthened anteriorly by:
A. Anterior longitudinal ligament B. Posterior longitudinal ligament C. Ligamentum flavum D. Supraspinous ligament
✅ Answer: A
Which of the following is a secondary cartilaginous joint?
A. Sacroiliac joint B. Symphysis pubis C. Zygapophyseal joint D. Costotransverse joint
✅ Answer: B
The vertebral column curvature that develops first in the embryo is:
A. Cervical B. Thoracic C. Lumbar D. Sacral
✅ Answer: B
The ligament connecting the tips of spinous processes is:
A. Supraspinous ligament B. Interspinous ligament C. Ligamentum flavum D. Posterior longitudinal ligament
✅ Answer: A
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