Chapter 33 - Muscles of the Abdominal and Pelvic Walls

Bony Landmarks

–Rib cage

• Sternum - inferior tip is xiphoid process

• 12 pairs of ribs, with costal cartilages

• 12 thoracic vertebrae

–Pelvis - two hipbones and the sacrum

–Hipbone

• Ilium - iliac crest, iliac fossa, anterior superior iliac spine

• Ischium - ischial spine, ischial tuberosity

• Pubis - pubic symphysis, pubic tubercle

• Ischiopubic ramus

  → Subpubic angle

–Pelvic brim

• Borders - superior margin of pubis, inferior margin of iliac fossa, and superior margin of sacrum

• Separates greater and lesser pelvic regions

–Greater (false) pelvis (greater pelvic cavity)

• Area above pelvic brim

• Contents: small intestine, sigmoid colon

–Lesser (true) pelvis (lesser pelvic cavity)

• Narrow area below pelvic brim

• Contents: pelvic organs (bladder, rectum, uterus, vagina, prostate gland)

–Inferior pelvic outlet

• Borders - pubic symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligaments, coccyx

• Pelvic outlet of female is wider and rounder compared to male outlet

Abdominopelvic Cavity

–The abdominal cavity and pelvic cavity form a continuous body cavity

• Pelvis is tilted anteriorly at 45° angle

–Borders:

• Superior: rib cage & thoracic diaphragm

• Inferior: pelvis & pelvic diaphragm

• Anterior/Lateral: abdominal wall muscles

• Posterior: vertebral column, m. psoas major, m. quadratus lumborum

Muscles of Abdominal Wall

Abdominal wall - consists of 4 muscles:

–Three layers of muscles that begin laterally and extend to anterior mid-line

• M. External abdominal oblique

• M. Internal abdominal oblique

• M. Transverse abdominis

–One muscle of anterior abdominal wall

• M. Rectus abdominis

–Three lateral muscles extend anteriorly - become aponeurotic (transition to broad, flat tendon)

• Each lateral muscle inserts into its paired muscle from opposite side (form raphe)

–Linea alba - anterior, mid-line raphe formed by all three lateral muscles

• Serves as insertion for all lateral muscles

• Extends down anterior mid-line of abdominal wall, from xiphoid process of sternum to pubic symphysis of pelvis

M. Rectus abdominis

• Elongated, anterior abdominal wall muscle that runs vertically (from pelvis to rib cage), parallel to anterior mid-line

• Surrounded by rectus sheath - dense connective tissue covering formed by aponeurosis from all lateral abdominal wall muscles

M. External abdominal oblique

–Origin: lower margin of rib cage

–Insertion: linea alba, iliac crest, anterior superior iliac spine, pubic tubercle

• Most superficial of lateral abdominal wall muscles

• Muscle fibers run in down and medial direction

• Inferior free margin (spanning between anterior superior iliac spine and pubic tubercle of pelvis) forms the inguinal ligament

M. Internal abdominal oblique

–Origin: iliac crest, thoracolumbar aponeurosis

• Thoracolumbar aponeurosis - broad tendinous structure arising from spinous processes of lower thoracic and all lumbar vertebrae

–Insertion: lower rib cage and linea alba

• Middle layer of lateral abdominal wall muscles

• Muscle fibers run in up and medial direction

M. Transverse abdominis

–Origin: lower rib cage, thoracolumbar aponeurosis, iliac crest

–Insertion: linea alba

• Deepest layer of lateral abdominal wall muscles

• Muscle fibers run in transverse (horizontal) direction

M. Rectus abdominis

–Origin: pubic portion of pelvis

–Insertion: xiphoid process of sternum, lower rib cage

• Muscle runs vertically, parallel to anterior mid-line of abdomen

• Muscle divided into segments by tendinous intersections (prevent “bowstringing” of muscle)

–Muscle is covered on both anterior (superficial) and posterior (deep) sides by rectus sheath

• Sheath formed by fused aponeurosis from all three lateral muscles of abdominal wall

• Fused aponeurosis then passes both superficial and deep to rectus abdominis muscle

• Both layers of rectus sheath come together at anterior mid-line (medial to rectus abdominis) to insert into linea alba

M. Rectus abdominis

• Anteriorly flexes trunk of body

Actions of abdominal wall muscles

All four muscles serve to contain the abdominal organs and compress abdominal wall during respiration, defecation, urination, vomiting, or childbirth

Actions of External and internal abdominal oblique muscles

• Contraction of both muscles on one side only - lateral flexion of body trunk toward that side

• Contraction of both muscles on both right & left sides simultaneously - anterior flexion of body trunk

• Primary muscles for rotation of body trunk

• Contraction of external abdominal oblique on one side plus contraction of internal abdominal oblique on opposite side  produces rotation of body trunk toward internal abdominal oblique side

• Example: contraction of right external abdominal oblique plus contraction of left internal abdominal oblique rotates body trunk toward left side