Anatomy of the Pelvis, Posterior Abdominal Wall, and Perineum

The Pelvis, Posterior Abdominal Wall, and Perineum

Introduction

The pelvis, posterior abdominal wall, and perineum are key anatomical regions that support vital functions such as locomotion, protection of organs, and reproduction. This guide covers their boundaries, musculoskeletal structures, innervation, blood supply, and major anatomical differences between sexes.

Learning Objectives

• Identify the three anatomical regions: Posterior abdominal wall, Pelvis, and Perineum, including their borders.

• Describe the musculoskeletal structures found in each region.

• Explain the innervation, blood supply, and other anatomical features (e.g., erectile tissues, joints) of these regions.

• Recognize major anatomical differences between males and females in these regions.

Posterior Abdominal Wall

Bones

The posterior abdominal wall provides structural support and serves as an attachment site for muscles.

• Ribs 11 and 12: The lowest ribs, which articulate with the vertebral column.

• 5 Lumbar vertebrae (L1-L5): Support the trunk and protect the spinal cord. Intervertebral discs separate these vertebrae and allow flexibility.

• Sacrum: A triangular bone at the base of the spine, forming the posterior part of the pelvis.

• Pelvic bones: Includes the iliac crest and fossa, which are important for muscle attachment.

Muscles

Several muscles form the posterior abdominal wall, contributing to posture, movement, and protection of internal organs.

1. Diaphragm (superiorly): Main muscle of respiration, separates thoracic and abdominal cavities.

2. Iliacus muscle (inferiorly): Flexes the thigh at the hip joint.

3. Psoas major muscle (medially): Flexes the hip and stabilizes the lumbar spine.

4. Psoas minor muscle (medially): Assists in flexion of the lumbar spine (not always present).

5. Quadratus lumborum muscle (laterally): Stabilizes the pelvis and lumbar spine, aids in lateral flexion of the trunk.

Fascial Linings

Fasciae in the posterior abdominal wall compartmentalize muscles and provide pathways for nerves and vessels.

• Thoracolumbar fascia: Encloses the deep muscles of the back and trunk.

• Transversalis fascia: Lies deep to the abdominal muscles, separating them from the peritoneum.

Arteries and Veins

The posterior abdominal wall is supplied by major vessels that also serve the pelvis and lower limbs.

• Abdominal aorta: Main artery, bifurcates into common iliac arteries.

• Common iliac arteries: Divide into internal and external iliac arteries.

• Veins: Common iliac veins drain into the inferior vena cava.

Nerves

Innervation is provided by the lumbar plexus and associated nerves.

• Lumbar plexus (L1-L4): Includes femoral, obturator, and iliohypogastric nerves.

• T12: Subcostal nerve.

Pelvis

Definitions and Boundaries

The pelvis is a bony basin formed by the sacrum and two hip bones (os coxae). It supports the weight of the upper body and transfers it to the lower limbs.

• True pelvis: The lower, smaller cavity containing pelvic organs.

• False pelvis: The upper, larger part, supporting abdominal organs.

Bony Pelvis - Functions

• Supports and transfers weight from the axial skeleton to the lower limbs.

• Protects pelvic organs (bladder, reproductive organs, rectum).

• Provides attachment sites for muscles of locomotion, posture, and the pelvic floor.

Pelvic Inlet and Outlet

• Pelvic inlet (brim): Superior opening into the true pelvis, bounded by the sacral promontory, arcuate line, pectineal line, and pubic symphysis.

• Pelvic outlet (floor): Inferior opening, bounded by the pubic arch, ischial tuberosities, and coccyx.

Sexual Dimorphism

There are notable differences between male and female pelves.

• Female pelvis: Wider, shallower, larger pelvic inlet and outlet, adapted for childbirth.

• Male pelvis: Narrower, deeper, more robust, smaller pelvic inlet and outlet.

Bony Pelvis - Osteology

• Sacrum: Forms the posterior wall.

• Os coxae (hip bones): Each consists of ilium, ischium, and pubis.

Joints and Ligaments

• Pubic symphysis: Cartilaginous joint uniting the left and right pubic bones.

• Sacroiliac joints: Synovial joints between sacrum and ilium, stabilized by strong ligaments.

• Ligaments: Sacrotuberous and sacrospinous ligaments stabilize the pelvis and form foramina for neurovascular structures.

Pelvic Floor (Pelvic Diaphragm)

Muscles

The pelvic floor supports pelvic organs and controls openings of the urethra and anus.

• Levator ani: Main muscle group, includes puborectalis, pubococcygeus, and iliococcygeus.

• Coccygeus: Supports the coccyx and pelvic organs.

• Obturator internus: Lateral rotator of the hip.

• Piriformis: Lateral rotator of the hip.

Function

• Supports pelvic viscera.

• Maintains continence (urinary and fecal).

• Facilitates childbirth in females.

Puborectalis Muscle

• Forms a sling around the rectum, attaching it to the pubic symphysis.

• Must relax to allow defecation.

Pelvic Foramina and Outlets

Sciatic Notches and Foramina

• Greater sciatic foramen: Passage for sciatic nerve, gluteal arteries, and piriformis muscle.

• Lesser sciatic foramen: Passage for internal pudendal artery and nerve, tendon of obturator internus.

• Obturator foramen: Passage for obturator nerve, artery, and vein.

Main Structures Passing Through Pelvic Outlets

• Pelvic outlet: Rectum, urethra, vagina (in females), and passage for childbirth.

• Inguinal ligament: Femoral nerve, artery, vein, round ligament, spermatic cord.

Blood Supply of the Pelvis

Internal Iliac Artery

The internal iliac artery is the main blood supply to pelvic organs, gluteal region, and perineum.

Branch	Region Supplied
Anterior trunk	Inferior gluteal, middle rectal, internal pudendal, vaginal (in females), inferior vesical
Posterior trunk	Iliolumbar, superior gluteal, lateral sacral

Nerves of the Pelvis

Spinal Nerves and Plexuses

• Lumbar plexus: Femoral, obturator, iliohypogastric nerves.

• Lumbosacral trunk: Connects lumbar and sacral plexuses.

• Sacral plexus (L4-S4): Sciatic, pudendal, superior and inferior gluteal nerves.

Autonomic Nerves

• Sympathetic: Controls blood flow, contraction of smooth muscle.

• Parasympathetic: Controls bladder contraction, sexual function.

The Perineum

Boundaries and Divisions

The perineum is a diamond-shaped area between the pubic symphysis, coccyx, and ischial tuberosities. It contains openings for the urethra and anus, and is divided into two triangles:

• Urogenital triangle: Contains external genitalia and urethral opening.

• Anal triangle: Contains the anus.

Muscles and Fascia

• Superficial perineal muscles: Ischiocavernosus, bulbospongiosus, superficial transverse perineal.

• Deep perineal muscles: Deep transverse perineal, external urethral sphincter.

• Perineal membrane: Deep fascia supporting the urogenital diaphragm.

Blood Supply and Innervation

• Internal pudendal artery: Main blood supply to the perineum.

• Pudendal nerve: Main sensory and motor nerve of the perineum.

Example: Childbirth

The female pelvis is adapted for childbirth, with a wider pelvic inlet and outlet, and a more flexible pubic symphysis.

Additional info: The pelvic floor muscles play a crucial role in continence and support of pelvic organs. Dysfunction can lead to conditions such as prolapse or incontinence.