Anatomy of the Pelvis

The Pelvis

• The pelvis is the region of the trunk that lies below the abdomen and has three main functions:
  • Transmits the weight of the body from the vertebral column to the femur
  • Contains, supports, and protects the pelvic viscera
  • Provides attachment for trunk and lower limb muscles
• The bony pelvis is composed of four bones:
  • Two hip bones (form the lateral and anterior walls)
  • Sacrum (part of the vertebral column, forms the posterior wall)
  • Coccyx (part of the vertebral column, forms the posterior wall)

False Pelvis and True Pelvis

• The pelvis is divided into two parts by the pelvic brim:
  • False pelvis (above the brim): part of the abdominal cavity, supports the abdominal contents
  • True pelvis (below the brim): has an inlet, outlet, and cavity
• The pelvic inlet is formed by:
  • Posterior: sacral promontory (anterior and upper margins of the first sacral vertebra)
  • Anterior: upper surface of symphysis pubis
  • Lateral: ileopectineal line (line that runs downward and forward around the inner surface of the ilium)
• The pelvic outlet is bounded by:
  • Anterior: pubic arch
  • Lateral: ischial tuberosities
  • Posterior: coccyx

Pelvic Cavity

• The pelvic cavity is a short, curved canal with a shallow anterior wall and a deep posterior wall
• The pelvic cavity has four walls:
  • Anterior wall: formed by the posterior surfaces of the bodies of the pubic bones, pubic rami, and symphysis pubis
  • Posterior wall: formed by the sacrum, coccyx, and piriformis muscle with their covering of parietal pelvic fascia
  • Lateral wall: formed by the hip bone, obturator foramen, and sacrotuberous and sacrospinous ligaments
  • Inferior wall: formed by the pelvic floor (supports the pelvic viscera)

Sacrum

• The sacrum is a single, wedge-shaped bone formed by the fusion of five rudimentary vertebrae
• Characteristics of the sacrum:
  • Superior border articulates with the fifth lumbar vertebra
  • Inferior border articulates with the coccyx
  • Laterally, the sacrum articulates with the two iliac bones to form the sacroiliac joint
  • The anterior and upper margins of the first sacral vertebra bulge forward as the sacral promontory
  • The vertebral foramina together form the sacral canal
  • The anterior and posterior surfaces of the sacrum have four foramina for passage of the anterior and posterior rami of the upper four sacral nerves

Coccyx

• The coccyx is a small, triangular bone formed by the fusion of four vertebrae
• The coccyx articulates with the lower end of the sacrum

Lateral Wall of the Pelvis

• The lateral wall of the pelvis is formed by:
  • Part of the hip bone below the pelvic inlet
  • Obturator foramen and membrane
  • Sacrotuberous and sacrospinous ligaments
  • Obturator internus muscle and its covering fascia

Hip Bone

• The hip bone is a large, irregular bone composed of three bones fused together:
  • Ilium (superior)
  • Ischium (posterior and inferior)
  • Pubis (anterior and inferior)
• Characteristics of the hip bone:
  • The outer surface of the hip bone has a deep depression, the acetabulum, which articulates with the head of the femur
  • Behind the acetabulum is a large notch, the greater sciatic notch, which is separated from the lesser sciatic notch by the spine of the ischium
  • The sciatic notches are converted into greater and lesser sciatic foramina by the sacrotuberous and sacrospinous ligaments

Sex Differences of the Pelvis

• Differences in the pelvis between males and females:
  • False pelvis is shallow in females and deep in males
  • The pelvic inlet is oval in females and heart-shaped in males
  • The pelvic outlet is larger in females than in males
  • The pelvic cavity is roomier in females than in males
  • The sacrum is shorter, wider, and flatter in females than in males
  • The pubic arch is more rounded and wider in females than in males

Pelvic Floor

• The pelvic floor is formed by the pelvic diaphragm and supports the pelvic viscera
• The pelvic floor is divided into the main pelvic cavity above and the perineum below

Pelvic Diaphragm

• The pelvic diaphragm is formed by the levator ani muscles and small coccygeus muscles and their coverings of fascia
• The pelvic diaphragm is incomplete anteriorly to allow passage of the urethra in males and the urethra and vagina in females

Perineum

• The perineum is the region between the pelvic floor and the anus
• The perineum is divided into two parts:
  • Urogenital triangle (anterior): contains the urethra and vagina in females
  • Anal triangle (posterior): contains the anus

Pelvic Fascia

• The pelvic fascia is divided into two layers:
  • Parietal fascia: lies on the walls of the pelvis and is named according to the muscle it overlies
  • Visceral fascia: covers and supports the pelvic viscera

Peritoneum in the Pelvic Cavity

• The peritoneum in the pelvic cavity is different in males and females
• In males:
  • The peritoneum passes from the anterior abdominal wall to the superior surface of the bladder
  • Then it runs posterior to the superior surface of the vas deferens and reaches the upper end of the seminal vesicles
  • It sweeps backward to reach the anterior surface of the rectum, forming the rectovesical pouch
  • Then it covers the upper 2/3 of the rectum and becomes continuous with the peritoneum on the posterior abdominal wall
• In females:
  • The peritoneum passes from the anterior abdominal wall to the superior surface of the bladder
  • Then it passes to the anterior surface of the uterus, forming the uterovesical pouch
  • It covers the anterior surface of the body and fundus of the uterus and the posterior surface of the body, fundus, and cervix of the uterus
  • Then it covers the posterior wall of the vagina and passes to the front of the rectum to form the rectouterine pouch

Contents of the Pelvic Cavity

• The pelvic cavity contains:
  • Sigmoid colon
  • Rectum
  • Bladder
  • Uterus and vagina (in females)
  • Prostate (in males)

Sigmoid Colon

• The sigmoid colon is a curved tube that begins as a continuation of the descending colon in front of the pelvic brim
• Characteristics of the sigmoid colon:
  • 25-38 cm long
  • Attached to the posterior pelvic wall by a fan-shaped sigmoid mesocolon
  • Curves to the right of the midline before joining the rectum
  • Relations: anterior to the urinary bladder (in males) and uterus and upper part of vagina (in females); posterior to the rectum, terminal part of ilium, and sacrum
  • Arterial supply: sigmoidal branches of the inferior mesenteric artery
  • Venous drainage: inferior mesenteric vein
  • Nerve supply: inferior hypogaséric plexus

Rectum

• The rectum is a curved tube that begins as a continuation of the sigmoid colon in front of the third sacral vertebra
• Characteristics of the rectum:
  • 13 cm long
  • Passes downward to end 2.5 cm in front of the tip of the coccyx
  • The lower part of the rectum is dilated to form the rectal ampulla
  • The peritoneum covers the anterior and lateral surfaces of the upper third, anterior surface of the middle third, and leaves the lower third devoid of peritoneum
  • Relations: posterior to the sacrum, coccyx, piriformis, and coccygeus muscles; anterior to the sigmoid colon and coils of the ileum (in males) and posterior surface of the vagina (in females)
  • Blood supply: superior rectal artery, middle rectal artery, and inferior rectal artery
  • Lymph drainage: pararectal lymph nodes
  • Nerve supply: sympathetic supply from the inferior mesenteric plexus, parasympathetic supply from the pelvis splanchnic nerve, and pain fibers that accompany both sympathetic and parasympathetic nerves

This quiz covers the anatomy of the pelvis, including its functions and composition of bones.