Mednotes.

Muscles of the Abdominal Wall

Summary

  • Muscles of the anterolateral abdominal wall
  • Muscles of the posterior abdominal wall

Muscles of the anterolateral abdominal wall

  • These lie deep to the superficial fascia (aka Camper’s and Scarpa’s fascia)
  • 4 muscles:
    • 3 pairs of flat, aponeurotic, muscles
    • 1 strap muscle (rectus abdominis)

  • Actions
    • Flexion, lateral flexion, and rotation of the spine
    • Also act as accessory muscles of respiration
  • Innervation – comes from two main sources
    1. Intercostal nerves (T7-12)
    2. Iliohyogastric, ilioinguinal nerves (L1)

 

External oblique

 

  • Most superficial of all four muscles
    • Inferomedial fibres (‘Hands in pockets’)
  • T7-T12 innervation by intercostal nerves
  • Attachments:
    • Ribs 5-12 => Iliac crest, linea alba (aponeurosis), inguinal ligament
Structure of External Oblique.
Internal oblique muscles

 

  • Middle layer
    • Superomedial fibres (perpendicular to EO fibres)
  • T7-L1 innervation
  • Attachments
    • Iliac crest, lateral inguinal ligament, thoracolumbar fascia => costal margin of ribs 8-12, linea alba (via aponeurosis)
Structure of Internal Oblique.

 

Transversus abdominus

 

  • Deepest layer of the three aponeurotic flat muscles
  • T7-12 innervation
  • Extra actions:
    • Compresses abdomen
  • Attachments
    • Iliac crest, lateral IL, TL fascia, Costal margins of 6-12 => linea alba
Structure of Transversus Abdominus.

 

Rectus abdominus

 

  • T7-L1 innervation
  • Attachments:
    • Pubic crest + pubic symphysis => costal cartilages 5-7 + xiphoid process
  • Extra actions:
    • Flexion of spine and compression of abdomen

Pyrimidalis
  • This is a small triangular muscle present in 80%

  • Function – tenses the linea alba
  • Located within the rectus sheath
    • Attachment – Pubic symphysis + pubic crest
    • Insertion – linea alba midpoint

 

Rectus Sheath
Anterior view of the Rectus Sheath.
  • This is a structure formed by the aponeuroses of the three flat muscles coming together
    • Accommodates the rectus and pyramidalis muscles
  • Lies deep to Camper’s fascia and Scarpa’s fascia (see layers of the abdominal wall)
  • Different function/structure depending on location relative to arcuate line (located halfway between umbilicus and pubic crest)
    • Above the arcuate line, it encloses the whole muscle (anterior and posterior) for superior 75% 
    • Below (i.e. inferior 25%) the arcuate line it encloses the anterior only
      • Transversalis fascia still lies deep/posterior to it
      • Clinically relevant:
        1. Increased risk of hernia
        2. Haemorrhage is less obvious

Muscles of the posterior abdominal wall

  • 5 muscles contribute to the main posterior abdominal wall
    1. Diaphragm
    2. Psoas major
    3. Psoas minor
    4. Quadratus lumborum
    5. Iliacus
      • Iliopsoas
    • The superficial muscles of the back can also be considered to be part of the posterior abdominal wall

Muscles of the posterior abdominal wall (made using Complete Anatomy by 3D4Medical
  • Most are supplied by anterior rami of lumbar nerves
    • L1-L4 are involved
    • (except iliacus which is supplied by the femoral nerve)

  1. Diaphragm
    • Extends down to L3 as well as the 12th rib

  1. Psoas major
    • Attachment/insertion – Transverse processes of T12-L5 => lesser trochanter of the femur (as iliopsoas tendon)
    • Nerve – L1-L3 ventral rami

  1. Psoas minor (in 60%)

  1. Quadratus lumborum –
    • Lateral to psoas
    • Attaches from iliac crest to 12th ribs and inserts to L1-L5
    • Like psoas, this bends the trunk

  1. Iliacus – placed most inferiorly
    • Iliac crest/Iliac fossa/Sacral ala => lesser tubercle (as iliopsoas)
    • Supplied by the femoral nerve

  • Iliopsoas (this is a combination of the iliacus + psoas major which merge inferiorly)
    • Develops inferior to inguinal ligament close to insertion onto the lesser tubercle of the femur