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Module 2 - Topic 3: Muscles of the Thorax

1. Intrinsic Thoracic Muscles

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2. Extrinsic (Axioappendicular) Muscles

Key muscles:

• Large fan-shaped muscle covering the anterior chest
• Adducts and medially rotates the arm

• Lies deep to pectoralis major
• Originates: ribs 3-5
• Inserts: coracoid process of scapula
• Also acts as a secondary respiratory muscle - assists elevation of ribs 3-5
• Clinical: Poland syndrome = congenital absence of pectoralis muscles, associated with brachydactyly (short fingers) and sometimes dextrocardia (heart on right side)

• Originates: ribs 1-8/9 (lateral surface)
• Inserts: medial border of scapula (anterior/costal surface)
• Action: protracts and rotates the scapula upward (essential for raising arm above 90°)
• Clinical: Long thoracic nerve (C5-C7) innervates it. Damage causes Winged Scapula - the medial border of the scapula sticks out like a wing because it can no longer be held flat against the thorax.

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3. Scalene Muscles

• Primarily: flex and rotate the neck
• Secondary respiratory role: elevate ribs 1-2 during forced inspiration

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4. The Diaphragm

• Ligamentum arcuatum medianum - bridges the two crura over the aorta
• Ligamentum arcuatum medialis - over the psoas major muscle
• Ligamentum arcuatum lateralis - over the quadratus lumborum muscle
• Also attaches to the xiphoid process and lower 6 costal cartilages

• Failure of the diaphragm to fully form during development
• Most common: Bochdalek hernia (posterolateral defect, usually left side)
• Abdominal organs herniate into the thorax, compressing the developing lungs - causes pulmonary hypoplasia
• Medical emergency in newborns

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5. Respiratory Muscles Summary

• Diaphragm - the main workhorse
• Intercostal muscles (external, internal, subcostal)

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Module 2 - Topic 4: Muscles of the Abdomen

1. Layers of the Anterior Abdominal Wall

1. Skin and subcutaneous tissue
2. Superficial fascia - two layers:
   • Camper's fascia (fatty layer) - continuous with superficial fat everywhere
   • Scarpa's fascia (membranous layer) - fuses with fascia lata of the thigh below inguinal ligament
3. External oblique muscle
4. Internal oblique muscle
5. Transversus abdominis muscle
6. Transversalis fascia (deep fascia)
7. Preperitoneal adipose tissue
8. Parietal peritoneum

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2. Congenital Defects of the Anterior Wall

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3. The Four Anterior Abdominal Muscles

Rectus Abdominis

• Runs vertically, flanking the midline (linea alba)
• Origin: pubic crest, pubic tubercle, pubic symphysis
• Insertion: cartilage of 5th, 6th, 7th ribs and xiphoid process
• Action: flexes trunk, compresses abdomen, increases intra-abdominal pressure (coughing, Valsalva maneuver)
• The "6-pack" muscle - divided by tendinous intersections

Pyramidalis

• Small triangular muscle anterior to the lower rectus abdominis
• Origin: pubis
• Insertion: linea alba
• Action: tenses the linea alba
• Often absent (absent in ~20% of people)

External Oblique

• Most superficial of the lateral muscles
• Origin: 5th-12th ribs (outer surfaces)
• Insertion: lateral fibers - iliac crest; medial fibers form the external oblique aponeurosis
• Action: ipsilateral lateral flexion, contralateral rotation of trunk
• Think: "hands in pockets" orientation of fibers (diagonal, downward-forward)
• Its lower free border folds on itself to form the inguinal ligament

Internal Oblique

• Middle lateral muscle (between external oblique and transversus abdominis)
• Origin: lateral 2/3 of inguinal ligament, iliac crest, thoracolumbar fascia
• Insertion: lower costal cartilages; forms internal oblique aponeurosis contributing to rectus sheath; medial fibers fuse with transversus abdominis to form the conjoint tendon
• Action: ipsilateral lateral flexion AND ipsilateral rotation (opposite to external oblique for rotation)

Transversus Abdominis

• Deepest of the lateral muscles; fibers run horizontally
• Origin: lateral 1/3 of inguinal ligament, iliac crest, thoracolumbar fascia, lower 6 costal cartilages
• Insertion: upper fibers form aponeurosis contributing to rectus sheath; lower fibers join internal oblique to form the conjoint tendon
• Action: maintains abdominal wall tone and increases intra-abdominal pressure (no rotation)

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4. Rectus Sheath

• Anterior layer = external oblique aponeurosis + anterior lamina of internal oblique
• Posterior layer = transversus abdominis aponeurosis + posterior lamina of internal oblique

• ALL three aponeuroses pass anterior to the rectus abdominis
• There is no posterior layer below the arcuate line
• The rectus is in direct contact with the transversalis fascia and peritoneum

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5. Hernias of the Abdominal Wall

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6. Inguinal Region

Inguinal Ligament

• Fibrous band from ASIS (anterosuperior iliac spine) to the pubic tubercle
• Formed by the folded lower free border of the external oblique aponeurosis
• The internal oblique and transversus abdominis originate from its lateral end
• Conjoint tendon = fusion of medial internal oblique + lower transversus abdominis fibers; reinforces the posterior wall of the inguinal canal medially

Inguinal Canal

• Males: spermatic cord
• Females: round ligament of the uterus

• Deep (internal) ring: opening in the transversalis fascia, bounded superolaterally by transversalis fascia, inferomedially by inferior epigastric vessels
• Superficial (external) ring: triangular opening in the external oblique aponeurosis, at the medial aspect

Hesselbach's (Inguinal) Triangle

• Medially: rectus abdominis
• Laterally: inferior epigastric vessels
• Inferiorly: inguinal ligament

Inguinal Hernias

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7. Umbilical Folds and Fossae (Internal View)

• Supravesical fossa - between median and medial folds (size changes with bladder filling)
• Medial inguinal fossa (Hesselbach's triangle) - medial to lateral fold - site of direct inguinal hernia
• Lateral inguinal fossa - lateral to lateral fold - site of indirect inguinal hernia

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8. Posterior Abdominal Muscles

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Quick Review - Key Clinical Pearls

1. Winged scapula = serratus anterior palsy (long thoracic nerve damage)
2. Poland syndrome = absent pectoralis + brachydactyly
3. Congenital diaphragmatic hernia = organs in thorax, pulmonary hypoplasia
4. Gastroschisis = no sac, para-umbilical; Omphalocele = has sac, at umbilicus
5. Arcuate line = where posterior rectus sheath ends; below it, no posterior sheath
6. Direct hernia = through Hesselbach's triangle, medial to epigastric vessels
7. Indirect hernia = through deep inguinal ring, lateral to epigastric vessels
8. Median fold = urachus; Medial folds = obliterated umbilical arteries; Lateral folds = inferior epigastric vessels (active!)

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📘 Module 2 - Topic 3: Muscles of the Thorax (Detailed)

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PART 1: Intrinsic Thoracic Wall Muscles

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1. Intercostal Muscles

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A. External Intercostal Muscles (most superficial)

• Attachment: From the inferior margin (lateral edge of costal groove) of the rib above → superior margin of the rib below
• Fiber direction: Obliquely anteroinferiorly (think: hands in front pockets - the direction fibers slope)
• Extent: From the tubercles of the ribs posteriorly all the way to the costal cartilages anteriorly. Beyond the cartilages, the muscle is replaced by a thin sheet of connective tissue = external intercostal membrane
• Function: Most active during inspiration - they elevate the ribs
• Innervation: Intercostal nerves T1-T11

B. Internal Intercostal Muscles (middle layer)

• Attachment: From the lateral edge of the costal groove of the rib above → superior margin of the rib below (deeper than external attachment)
• Fiber direction: Obliquely posteroinferiorly - opposite to external intercostals (the two sets cross each other like an "X")
• Extent: From the parasternal region (between costal cartilages) to the angle of the ribs posteriorly. Medial to the angle, replaced by the internal intercostal membrane
• Function: Most active during forced expiration - they depress the ribs
• Innervation: Intercostal nerves T1-T11

C. Innermost Intercostal Muscles (deepest layer)

• Fiber direction: Same as internal intercostals (posteroinferiorly)
• Extent: Most evident in the lateral thoracic wall; from the medial edge of the costal groove to the deep surface of the rib below
• Key anatomical point: The neurovascular bundle (intercostal vein, artery, nerve - VAN from top to bottom) travels in the costal groove in the plane BETWEEN the internal and innermost intercostals
• Function: Acts with internal intercostals (expiration)
• Innervation: Intercostal nerves T1-T11

Clinical pearl - Pleural tap (thoracentesis): Always insert the needle just above the upper border of the lower rib to avoid the neurovascular bundle that runs in the costal groove (inferior margin of each rib).

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D. Subcostal Muscles

• Same plane as innermost intercostals
• More numerous in the lower posterior thoracic wall
• They span 2-3 ribs (from one rib to the 2nd or 3rd rib below) - unlike intercostals which only span one space
• Function: May depress ribs

E. Transversus Thoracis Muscles

• Found on the deep surface of the anterior thoracic wall (same plane as innermost intercostals)
• Origin: Posterior surface of xiphoid process, inferior sternum body, and adjacent costal cartilages of lower ribs
• Insertion: Lower borders and internal surfaces of costal cartilages of ribs 2-6 (passing superolaterally)
• Function: Depresses costal cartilages (expiration)

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2. The Intercostal Neurovascular Bundle

• Posterior intercostal arteries (spaces 3-11): Arise from the thoracic aorta (the right-sided ones are longer because the aorta lies on the left)
• Anterior intercostal arteries (spaces 1-6): Branch from the internal thoracic artery; spaces 7-9 from the musculophrenic artery
• The internal thoracic artery arises from the subclavian artery, descends ~1 cm lateral to the sternum posterior to the costal cartilages, and terminates at the 6th intercostal space as the superior epigastric artery (entering the abdominal wall) and musculophrenic artery

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PART 2: Extrinsic (Axioappendicular) Thoracic Muscles

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1. Pectoralis Major

• Large, fan-shaped, covers the anterior chest
• Two heads: clavicular (upper) and sternocostal (lower)
• Action: adduction, medial rotation, and flexion of the arm
• As an accessory respiratory muscle: when the arms are fixed (hands on knees), it can help elevate the thorax during forced inspiration - this is why people with respiratory distress instinctively brace their arms

2. Pectoralis Minor

• Lies deep to pectoralis major
• Origin: Anterior surfaces of ribs 3, 4, and 5 (near their costal cartilages)
• Insertion: Medial border and superior surface of the coracoid process of the scapula
• Action: Depresses and protracts the shoulder; tilts the scapula anteriorly
• Accessory respiratory role: With the shoulder fixed, it elevates ribs 3-5 during forced inspiration

Clinical: Poland Syndrome - Congenital unilateral absence of the pectoralis major (and sometimes minor). Associated with ipsilateral brachydactyly (abnormally short fingers), skin/soft tissue defects of the chest wall, and occasionally dextrocardia (rightward displacement of the heart). Rare (1 in 20,000 births).

3. Serratus Anterior

• Origin: Lateral surfaces of ribs 1-8 (or 9), appearing as serrated "teeth" of muscle
• Insertion: Costal surface (anterior aspect) of the medial border of the scapula
• Action: Protracts the scapula (pulls it forward around the chest wall); rotates the glenoid cavity upward (essential for raising the arm above 90°); holds the scapula flat against the thorax
• Innervation: Long thoracic nerve (C5, C6, C7) - runs superficially down the lateral chest wall

Clinical: Winged Scapula - Damage to the long thoracic nerve (e.g., from a stab wound to the axilla, after radical mastectomy, or in some viral illnesses) paralyzes serratus anterior. The medial border of the scapula can no longer be held flat against the posterior thorax and lifts away like a bird's wing - especially visible when the patient pushes against a wall. Classic exam finding.

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PART 3: Scalene Muscles

• Origin: Transverse processes of cervical vertebrae (C2-C7)
• Insertion: First and second ribs
• Primary action: Lateral flexion and rotation of the cervical spine
• Respiratory role: Elevate ribs 1 and 2 during forced inspiration (accessory)
• The brachial plexus and subclavian artery pass between the anterior and middle scalenes (the scalene triangle) - relevant in thoracic outlet syndrome

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PART 4: The Diaphragm - In Full Detail

Structure

• Median arcuate ligament (Lig. arcuatum medianum): A fibrous arch connecting the two crura over the aorta at the midline - forms the roof of the aortic hiatus
• Medial arcuate ligament (Lig. arcuatum medialis): An arch of fascia over the psoas major muscle, connecting the body of L1/L2 to the transverse process of L1
• Lateral arcuate ligament (Lig. arcuatum lateralis): An arch of fascia over the quadratus lumborum muscle, from the transverse process of L1 to the 12th rib

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The Three Major Openings

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Diaphragm and Respiration

1. Phrenic nerve fires → diaphragm contracts
2. The dome descends (flattens) ~1.5 cm in quiet breathing, up to 10 cm in forced breathing
3. The volume of the thoracic cavity increases both inferiorly (vertical) and laterally (bucket-handle movement of ribs)
4. Intrapleural pressure drops → lungs expand → air flows in (Boyle's Law)

• Passive: Diaphragm simply relaxes; elastic recoil of lungs and chest wall drives air out
• No muscular effort needed

• Abdominal muscles (especially transversus abdominis and internal oblique) contract → push abdominal viscera upward against the diaphragm → accelerates exhalation

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Congenital Diaphragmatic Hernia (CDH)

• Posterolateral defect, left side in 85% of cases (because the right side closes earlier)
• Abdominal contents (small bowel, stomach, spleen, liver) herniate into the thorax
• Compresses the developing lungs → bilateral pulmonary hypoplasia (even the contralateral lung is hypoplastic)
• Presents at birth with: respiratory distress, cyanosis, scaphoid (flat/sunken) abdomen, bowel sounds in the thorax, shifted trachea/mediastinum
• Management: Surgical emergency - mechanical ventilation first to stabilize, then surgical repair

• Anterior (parasternal) defect - through the gap between the sternal and costal portions
• Usually contains omentum or colon
• Often asymptomatic, found incidentally

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PART 5: Respiratory Muscles - Complete Overview

Primary Muscles (active in every breath)

1. Diaphragm - the workhorse
2. External intercostals - elevate ribs (bucket-handle and pump-handle movements)

Accessory/Secondary Muscles (recruited during increased ventilatory demand - exercise, disease)

Muscles of Forced Expiration

• Rectus abdominis, internal oblique, external oblique, transversus abdominis - push the viscera up against the diaphragm
• Internal intercostals - depress ribs

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📘 Module 2 - Topic 4: Muscles of the Abdomen (Detailed)

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PART 1: Layers of the Anterior Abdominal Wall

1. Skin
2. Subcutaneous connective tissue - two layers:
   • Camper's fascia (superficial/fatty layer): a continuous fatty sheet that extends over the entire body as subcutaneous fat. Below the inguinal ligament it continues into the thigh.
   • Scarpa's fascia (deep/membranous layer): a firm, membrane-like sheet. It fuses with the fascia lata of the thigh just below the inguinal ligament. Importantly, it continues as Colles' fascia in the perineum - meaning urine from a ruptured urethra can track upward into the anterior abdominal wall under Scarpa's fascia.
3. External oblique muscle
4. Internal oblique muscle
5. Transversus abdominis muscle
6. Transversalis fascia (deep investing fascia of the abdominal wall) - fuses with the fascia lata inferiorly
7. Preperitoneal (extraperitoneal) fat
8. Parietal peritoneum

• Skin and muscles: anterior rami of T7-T12 (intercostal and subcostal nerves) sweep around the costal margin into the abdominal wall
• T6 supplies skin over the xiphoid
• T10 dermatome = the umbilicus (a key clinical landmark)
• L1 (via ilioinguinal and iliohypogastric nerves) supplies the inguinal and suprapubic region

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PART 2: Congenital Anterior Wall Defects

Gastroschisis

• Paraumbilical herniation of abdominal contents (usually intestine) through a full-thickness defect in the abdominal wall, almost always on the right side of the umbilicus
• The intestines are not covered by a sac (herniated organs are exposed directly to amniotic fluid → thickened, matted, foreshortened bowel)
• Usually an isolated anomaly (not associated with chromosomal defects)
• Associated with young maternal age, gastroschisis is not related to umbilical cord abnormalities
• Treatment: Immediate surgical repair or staged reduction using a silastic silo

Omphalocele (Exomphalos)

• Herniation of abdominal organs through the umbilicus (midline)
• Contents (can include liver, bowel, spleen) are covered by a translucent sac composed of amnion and peritoneum
• Strongly associated with chromosomal anomalies (trisomy 13, 18, 21) and other syndromes (Beckwith-Wiedemann)
• Larger and more clinically complex than gastroschisis

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PART 3: The Four Anterior Abdominal Muscles - In Full Detail

1. Rectus Abdominis

• Origin: Pubic crest, pubic tubercle, pubic symphysis
• Insertion: Cartilage of 5th, 6th, 7th ribs; xiphoid process
• Action:
  • Flexes the trunk (lumbar flexion)
  • Compresses the abdomen
  • Increases intra-abdominal pressure (coughing, sneezing, Valsalva maneuver, forced expiration, defecation, childbirth)
• Unique feature: Interrupted by 3-4 tendinous intersections (horizontal fibrous bands adherent to the anterior layer of the rectus sheath) - these create the visible "abs" segments
• Innervation: Anterior rami T7-T12
• Enclosed within the rectus sheath (see below)

2. Pyramidalis

• Origin: Anterior surface of the pubis
• Insertion: Linea alba
• Action: Tenses the linea alba
• Present in only ~80% of people; absent in ~20%
• Clinical relevance: minimal, but surgeons note its presence during midline incisions

3. External Oblique

• Origin: Outer surfaces and inferior borders of ribs 5-12 (lower 8 ribs)
• Fiber direction: Obliquely anteroinferiorly - same direction as the external intercostals - like "hands in front pockets" (fibers run downward and toward the midline)
• Insertion:
  • Lateral fibers → outer lip of the iliac crest
  • Medial/anterior fibers → become a broad flat aponeurosis that contributes to the anterior layer of the rectus sheath and inserts into the linea alba
  • The lower free border of this aponeurosis folds inward on itself to form the inguinal ligament (from ASIS to pubic tubercle)
• Actions:
  • Ipsilateral lateral flexion of trunk
  • Contralateral rotation of trunk (e.g., right external oblique turns you to the LEFT)
  • Compresses abdomen; increases intra-abdominal pressure
• Innervation: Anterior rami T5-T12, L1

4. Internal Oblique

• Origin: Lateral two-thirds of the inguinal ligament; iliac crest; thoracolumbar fascia
• Fiber direction: Obliquely superiorly and medially - perpendicular to external oblique fibers (fan-shaped, running "upward toward the midline")
• Insertion:
  • Uppermost fibers → lower 3-4 costal cartilages (ribs 9-12)
  • Middle and lower fibers → form the internal oblique aponeurosis, split to wrap around rectus abdominis as part of both layers of the rectus sheath, and insert into the linea alba
  • Medial-most fibers → fuse with the lowest fibers of transversus abdominis to form the conjoint tendon (inguinal falx), inserting into the pubic crest and pectineal line
• Actions:
  • Ipsilateral lateral flexion of trunk
  • Ipsilateral rotation of trunk (opposite to external oblique for rotation - right internal oblique turns you to the RIGHT)
  • Compresses abdomen
• Innervation: Anterior rami T7-T12, L1
• Also gives rise to the cremaster muscle (from its lower fibers), which covers the spermatic cord and testis

5. Transversus Abdominis

• Origin: Lateral one-third of the inguinal ligament; inner lip of the iliac crest; thoracolumbar fascia; inner surfaces of lower 6 costal cartilages (ribs 7-12)
• Fiber direction: Horizontal/transverse (hence the name)
• Insertion:
  • Upper fibers → form the transversus abdominis aponeurosis → contributes to the rectus sheath → inserts into the linea alba
  • Lowermost fibers → fuse with the aponeurosis of the internal oblique to form the conjoint tendon
• Actions:
  • Compresses and supports the abdominal contents (the main "corset" muscle)
  • Increases intra-abdominal pressure
  • No rotational or lateral flexion action (horizontal fibers cannot rotate or flex)
• Innervation: Anterior rami T7-T12, L1

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Summary: Rotation Trick

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PART 4: The Rectus Sheath - Detailed

• Located about halfway between the umbilicus and pubic symphysis (roughly at the level of the ASIS)
• Marks where the arrangement of aponeuroses changes

Above the Arcuate Line (upper 2/3):

• Anterior layer: External oblique aponeurosis + anterior lamina of internal oblique aponeurosis
• Posterior layer: Transversus abdominis aponeurosis + posterior lamina of internal oblique aponeurosis (the internal oblique splits and wraps around both sides)

Below the Arcuate Line (lower 1/3):

• At the arcuate line, the transversus abdominis aponeurosis AND the posterior lamina of the internal oblique aponeurosis swing anteriorly to join the anterior layer
• Anterior layer: ALL THREE aponeuroses (external oblique + internal oblique + transversus abdominis)
• Posterior layer: ABSENT
• The posterior surface of the lower rectus abdominis rests directly on the transversalis fascia (and therefore the peritoneum is very close)

• The posterior sheath ends abruptly at the arcuate line - seen as a distinct edge during surgery
• Below the arcuate line, the inferior epigastric vessels (which normally run in the posterior sheath) are now anterior to the peritoneum
• Hematomas of the rectus sheath track differently above vs. below the arcuate line

Key midline structures formed by the rectus sheath:

• Linea alba: The midline fusion of all three aponeuroses from both sides, extending from the xiphoid to the pubic symphysis. This is the bloodless midline incision site in abdominal surgery.
• Linea semilunaris: The lateral edge of the rectus sheath (visible as a curved groove in muscular individuals)
• Tendinous intersections: Horizontal bands within the rectus abdominis creating the "6-pack" appearance

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PART 5: Hernias of the Abdominal Wall

Midline Hernias

Lumbar Hernias (Posterior)

• Bounded by: 12th rib (superiorly), erector spinae muscle (medially), internal oblique muscle (laterally)
• Protrusion through here = Grynfeltt hernia

• Bounded by: Iliac crest (inferiorly), latissimus dorsi (medially), external oblique (laterally)
• Protrusion through here = Petit hernia

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PART 6: The Inguinal Region - Full Detail

The Inguinal Ligament

• A fibrous band, approximately 12-15 cm long
• Runs from the anterior superior iliac spine (ASIS) laterally to the pubic tubercle medially
• Formation: The free lower border of the external oblique aponeurosis folds posteriorly and inferiorly on itself → creating a trough/gutter (the "shelving edge")
• The internal oblique and transversus abdominis muscles originate from its lateral two-thirds
• Medially, it has a specialized extension: the lacunar ligament (Gimbernat's ligament) - a triangular expansion of fibers that fans out to attach to the pectineal line. The medial edge of the lacunar ligament forms the sharp medial wall of the femoral ring.

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Development of the Inguinal Canal

1. The gonads form on the posterior abdominal wall (near the kidneys)
2. A fibromuscular cord, the gubernaculum, forms and connects the testis to the developing scrotum
3. A peritoneal diverticulum called the processus vaginalis pushes through the anterior abdominal wall layers along the gubernaculum's path, dragging the muscular and fascial layers with it
4. The testis descends along this path into the scrotum (usually by 7-8 months gestation)
5. The processus vaginalis normally closes after birth; if it doesn't → congenital indirect inguinal hernia or hydrocele

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Inguinal Canal - Anatomy

• Male: Spermatic cord + ilioinguinal nerve + genital branch of genitofemoral nerve
• Female: Round ligament of uterus + ilioinguinal nerve + genital branch of genitofemoral nerve

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The Two Inguinal Rings

• The entrance to the canal
• Located midway between the ASIS and pubic symphysis, just above the inguinal ligament
• Formed by an outpouching of the transversalis fascia (it is NOT actually a defect - it is the beginning of the fascial sleeve around the spermatic cord = internal spermatic fascia)
• Lies lateral to the inferior epigastric vessels
• The inferior epigastric vessels are the key landmark separating direct from indirect hernias

• The exit from the canal
• A triangular opening in the external oblique aponeurosis, superior to the pubic tubercle
• The apex points superolaterally; the base is formed by the pubic crest
• Two crura (medial and lateral) define the sides; held together at the apex by intercrural fibers (preventing the ring from widening)
• The external oblique continues as the external spermatic fascia over the spermatic cord

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PART 7: Hesselbach's Triangle and Inguinal Hernias

Hesselbach's (Inguinal) Triangle

• Medially: Lateral edge of the rectus abdominis muscle
• Laterally: Inferior epigastric vessels (key landmark)
• Inferiorly: Inguinal ligament

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Inguinal Hernias - Direct vs. Indirect

Mnemonic: "DIrect" = goes DIrect through the abdominal wall, is Medial (think M before N). "INdirect" = goes IN the deep ring, is lateral (lateral comes before medial alphabetically... or remember: indirect has more letters than direct, it takes a longer path).

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PART 8: Umbilical Folds and Fossae

The 5 Umbilical Folds (Peritoneal Folds)

• Patent urachus = urine drains from the umbilicus
• Urachal cyst = a fluid-filled cyst along the ligament
• Urachal sinus = opens only at the umbilicus

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The Fossae (Depressions Between the Folds)

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PART 9: Posterior Abdominal Muscles

Psoas Major

• Origin: Vertebral bodies and intervertebral discs of T12-L4 + transverse processes of lumbar vertebrae
• Insertion: Lesser trochanter of the femur (joins iliacus = iliopsoas)
• Action: Flexes the thigh at the hip (bringing the knee toward the chest); also flexes the lumbar spine laterally when acting unilaterally
• Innervation: Anterior rami L1-L3 directly
• Clinical: A psoas abscess (e.g., from vertebral TB/Pott's disease or Crohn's disease) tracks along the psoas sheath and can present as a groin lump or cause a painful, fixed-flexion deformity of the hip. The "psoas sign" (pain on passive hip extension) suggests peritoneal irritation (e.g., appendicitis) near the psoas.

Iliacus

• Origin: Iliac fossa (inner surface of the ilium)
• Insertion: Lesser trochanter (with psoas = iliopsoas)
• Action: Flexes the thigh
• Innervation: Femoral nerve (L2, L3)

Iliopsoas

Psoas Minor

• Present in only ~50% of people
• Origin: T12-L1 vertebrae
• Insertion: Pecten pubis and iliopubic ramus (iliopectineal eminence)
• Action: Weak flexion of the lumbar spine; assists in tilting the pelvis
• Clinically insignificant

Quadratus Lumborum

• A thick, roughly quadrilateral muscle of the posterior abdominal wall
• Origin: Iliac crest and iliolumbar ligament
• Insertion: 12th rib; transverse processes of L1-L4
• Action:
  • Lateral flexion of the trunk (unilateral)
  • Fixes the 12th rib during inspiration (prevents it from being pulled upward by the diaphragm's contraction - stabilizing the diaphragm's origin)
  • Assists in expiration by pulling the 12th rib inferiorly
• Innervation: T12, L1-L4
• Clinical: A common source of low back pain - often called "the joker of low back pain" because it can mimic many conditions. Trigger points in QL refer pain to the sacroiliac joint and hip.

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Quick-Reference Clinical Summary