Anterior abdominal wall
Anterior abdominal wall: layers, rectus sheath, vessels, nerves and dermatomes
Anterior abdominal wall for FCPS Part 1: layers, rectus sheath above and below the arcuate line, epigastric vessels, nerves and dermatomes. Explains stab wounds, haematomas, incisions, venous flow patterns, lymph drainage and umbilical anomalies.
Layers from outside in
| Layer | Note |
|---|---|
| Skin | Dermatomes T7–L1 |
| Camper's fascia (fatty superficial fascia) | Continuous with superficial fat elsewhere |
| Scarpa's fascia (membranous layer) | Continues into the perineum as Colles fascia; fuses with the fascia lata just below the inguinal ligament |
| External oblique | Aponeurosis in front of rectus |
| Internal oblique | Splits around rectus above the arcuate line |
| Transversus abdominis | Behind rectus above the arcuate line |
| Transversalis fascia | The layer met immediately deep to transversus abdominis |
| Extraperitoneal fat | Between transversalis fascia and peritoneum |
| Parietal peritoneum | Innermost |
- Urine from a ruptured bulbar urethra (below the perineal membrane) spreads deep to Colles fascia into the scrotum, penis and lower abdominal wall, but not into the thighs because Scarpa's fascia is fused to the fascia lata.
- At laparotomy, between transversus abdominis and the peritoneum the surgeon meets transversalis fascia and then extraperitoneal fat.
Muscles
| Muscle | Fibres | Notes |
|---|---|---|
| External oblique | Downwards and medially ("hands in pockets") | Interdigitates with serratus anterior and latissimus dorsi; its lower free border, from the ASIS to the pubic tubercle, forms the inguinal ligament |
| Internal oblique | Upwards and medially | Lowest fibres form the cremaster; its aponeurosis joins that of transversus as the conjoint tendon and splits to enclose rectus |
| Transversus abdominis | Horizontal | Joins the conjoint tendon |
| Rectus abdominis | Vertical, with three tendinous intersections fused to the anterior sheath | Pubic crest and symphysis to the xiphoid and 5th–7th costal cartilages |
| Pyramidalis | Small, in front of lower rectus | Often absent |
- Nerve supply: all are supplied by the ventral rami of T7–T12 (the lower intercostal and subcostal nerves) plus L1 (iliohypogastric and ilioinguinal) for the lowest fibres of internal oblique and transversus. Rectus abdominis is supplied by T7–T12.
Rectus sheath
| Level | Anterior wall | Posterior wall |
|---|---|---|
| Above the costal margin | External oblique aponeurosis | None (rectus lies on the costal cartilages) |
| Costal margin to arcuate line | External oblique + anterior lamina of internal oblique | Posterior lamina of internal oblique + transversus aponeurosis |
| Below the arcuate line (about one-third of the way from umbilicus to pubis) | All three aponeuroses | Transversalis fascia only |
- Contents: rectus, pyramidalis, superior and inferior epigastric vessels, terminal parts of the lower six thoracic nerves (T7–T12).
- Rectus sheath haematoma: rupture of the inferior epigastric artery (anticoagulants, coughing, trauma), commonest below the arcuate line where no posterior aponeurosis contains it; a tender mass that does not cross the midline.
- Linea alba: midline fusion of the aponeuroses; relatively avascular, used for midline incisions.
- Diastasis (divarication) recti: after several pregnancies the linea alba stretches, so a midline ridge appears between the xiphoid and umbilicus when the head is lifted. There is no fascial defect and no risk of strangulation.
- Linea semilunaris: the lateral border of rectus. Spigelian hernia occurs along it, usually near the arcuate line.
Arteries and veins
- Superior epigastric (terminal branch of the internal thoracic) anastomoses inside the sheath with the inferior epigastric (external iliac). If the internal thoracic artery is ligated, rectus is perfused by increased flow through the inferior epigastric; in aortic coarctation flow runs the other way to the legs.
- The inferior epigastric artery runs up behind rectus from the deep ring, lying a few centimetres lateral to the midline. A stab wound just lateral to the linea alba below the umbilicus, followed by delayed hypotension, has cut it; it is also at risk in paramedian incisions, laparoscopic port sites and lateral extension of a Pfannenstiel (suprapubic transverse) incision.
- Superficial epigastric, superficial circumflex iliac and superficial external pudendal arteries are branches of the femoral artery.
- Pedicled rectus (TRAM) flaps survive on the superior epigastric; lower abdominal free flaps take the deep inferior epigastric.
| Venous pattern | Direction of flow below the umbilicus | Cause |
|---|---|---|
| Caput medusae (around the umbilicus) | Away from the umbilicus | Portal hypertension; paraumbilical veins along the ligamentum teres open up |
| Dilated veins up the flanks with leg oedema | Upwards, towards the chest | IVC obstruction (for example a large liver tumour or hepatoma compressing it); blood bypasses through superficial epigastric to thoracoepigastric veins |
A liver tumour suggests portal hypertension, but upward flow with bilateral leg oedema means the IVC is compressed.
Nerves and dermatomes
- T7–T11 intercostal and T12 subcostal nerves run forwards between internal oblique and transversus abdominis (the transversus abdominis plane, target of the TAP block), then pierce the rectus sheath. L1 iliohypogastric and ilioinguinal nerves supply the lowest part.
- Dermatomes: T4 nipple (in men, over the 4th intercostal space), T7 xiphoid, T10 umbilicus, L1 inguinal region and pubis.
- Referred pain: early appendicitis is felt around the umbilicus because the midgut's visceral afferents enter the cord at T10. Shingles or root irritation of lower thoracic nerves mimics abdominal disease.
- Cutting segmental nerves (long vertical incisions lateral to rectus) denervates muscle and predisposes to hernia.
Lymph drainage of the skin
Skin lymph follows a watershed at the umbilicus: above it to the axillary nodes (and parasternal), below it to the superficial inguinal nodes of the same side. A skin cancer 4 cm below and left of the umbilicus spreads first to the left superficial inguinal nodes; deep structures drain to iliac and para-aortic nodes.
Surface landmarks
- The transpyloric plane (L1) passes through the tips of the 9th costal cartilages. Where the right linea semilunaris meets the costal margin (tip of the 9th cartilage) lies the fundus of the gallbladder; the pylorus lies on the same plane but nearer the midline.
- The umbilicus lies at about L3/L4. The liver is attached to the anterior abdominal wall by the falciform ligament, whose free edge contains the ligamentum teres.
Incisions
| Incision | Note |
|---|---|
| Midline (linea alba) | Quick, good access, bloodless; higher hernia risk |
| Paramedian | Rectus displaced laterally; nerves preserved; inferior epigastric at risk |
| Kocher (right subcostal) | Gallbladder, liver; avoid cutting the T9 nerve |
| Gridiron / Lanz | Appendicectomy; muscles split along their fibres |
| Pfannenstiel | Transverse suprapubic; caesarean section; inferior epigastric vessels laterally |
- An incisional hernia is a bulge with a cough impulse at or near a previous scar (for example after several caesarean sections), caused by failure of the fascial closure; obesity, infection and repeated surgery increase the risk.
Umbilical anomalies
| Anomaly | Failure | Presentation |
|---|---|---|
| Patent urachus | Allantois/urachus fails to obliterate | Urine leaking from the umbilicus |
| Vitellointestinal (omphalomesenteric) fistula | Vitelline duct persists | Faecal discharge from the umbilicus |
| Omphalocele (exomphalos) | Midgut fails to return to the abdomen | Bowel through the umbilical ring in a membranous sac with the cord attached; look for other anomalies (cardiac, chromosomal such as trisomy 18 and 13) |
| Gastroschisis | Defect beside the cord, usually right | Bowel uncovered, no sac; few associated anomalies |
Remember Below the arcuate line only transversalis fascia lies behind rectus.
Exam trap Upward flow in abdominal wall veins below the umbilicus means IVC obstruction, not portal hypertension.
How FCPS asks about this
- Rectus sheath below the arcuate line: all aponeuroses in front, only transversalis fascia behind.
- Layer immediately deep to transversus abdominis: transversalis fascia.
- Stab just lateral to the linea alba with delayed shock: inferior epigastric artery.
- Hepatoma with leg oedema and upward-flowing abdominal veins: IVC obstruction.
- Urine from the umbilicus: patent urachus (allantois). Bowel in a sac at the cord: omphalocele; look for other anomalies.
- Skin lesion below the umbilicus: superficial inguinal nodes of the same side.
- Point where the right rectus edge meets the costal margin: fundus of the gallbladder; umbilicus is T10, male nipple T4.
Test yourself on this topic
5 of the 74 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A skin cancer lies 4 cm below and left of the umbilicus. To which lymph nodes will it spread first?
Answer: Left superficial inguinal nodes
Superficial lymphatics of the anterior abdominal wall divide at the umbilicus (around the T10 level). Skin above the umbilicus drains to the axillary nodes, while skin below it drains to the superficial inguinal nodes of the same side. The deeper tissues drain along the deep vessels to iliac and para-aortic nodes, but a skin cancer first reaches the superficial group.
Abdominal wall lymph
- Watershed at the umbilicus
- Above: axillary nodes
- Below: superficial inguinal nodes
- Deep drainage: iliac and para-aortic
Remember Above the navel go up; below go down.
Exam trap Skin lymph does not go to iliac nodes first; that route is for deep tissues.
Why the other options are wrong
- Left anterior axillary nodes Receive skin lymph from above the umbilicus.
- Left external iliac nodes Receive deep, not skin, drainage.
- Para-aortic (lumbar) nodes Drain deep structures and gonads.
- Left internal iliac nodes Drain pelvic organs.
A 68-year-old man has painful grouped vesicles over the right inguinal ligament, root of penis and upper scrotum. Which dermatome is affected?
Answer: L1
This is herpes zoster in the L1 dermatome. L1 supplies the skin over the inguinal ligament, the groin and, through the ilioinguinal nerve, the root of the penis and front of the scrotum. Useful landmarks are T4 at the nipple, T7 at the xiphoid, T10 at the umbilicus and L1 at the inguinal ligament. L3 covers the front of the lower thigh and knee, and S2 the back of the thigh.
Trunk dermatomes
- T4 nipple, T7 xiphoid
- T10 umbilicus
- L1 inguinal ligament and groin
- Zoster follows one dermatome
Remember Groin and upper scrotum: L1.
Exam trap T10 is the umbilicus, not the groin.
Why the other options are wrong
- T10 Supplies the umbilicus.
- T8 Supplies the skin between xiphoid and umbilicus.
- L3 Supplies the lower anterior thigh and knee.
- S2 Supplies the back of the thigh.
After falling astride a beam, a 22-year-old man has blood at the urethral meatus and swelling of the scrotum, penis and lower anterior abdominal wall. The swelling does not extend into the thighs. Why are the thighs spared?
Answer: Scarpa fascia fuses with the fascia lata
Rupture of the bulbous urethra lets blood and urine escape into the superficial perineal pouch, deep to the membranous (Colles) fascia. This layer is continuous with the dartos of the scrotum and penis and with the membranous (Scarpa) fascia of the abdominal wall. Scarpa fascia fuses with the fascia lata just below the inguinal ligament, and Colles fascia is attached to the ischiopubic rami and perineal membrane, so fluid cannot enter the thigh or the anal triangle.
Urine extravasation
- Bulbar urethra torn below perineal membrane
- Fluid deep to Colles fascia
- Tracks into scrotum, penis, abdominal wall
- Stopped at thigh: Scarpa fuses with fascia lata
Remember The membranous layer decides where urine goes.
Exam trap Fluid does not reach the thigh because of the fascial attachment, not the inguinal ligament.
Why the other options are wrong
- The inguinal ligament blocks the fluid It is not the barrier; fluid passes above it into the wall.
- Camper fascia is attached to the pubis The fatty layer has no firm attachments.
- Transversalis fascia is still intact Lies deep to the muscles; not involved.
- The perineal body seals the thigh Lies in the midline perineum; does not separate the thigh.
In an upper midline laparotomy, one tough, almost bloodless fibrous layer lies between the subcutaneous fat and the peritoneum. What is this layer?
Answer: Linea alba
The linea alba is the midline fibrous raphe formed by the interlacing aponeuroses of the three flat muscles of both sides, running from the xiphoid to the pubic symphysis. A midline incision passes through it with little bleeding and no nerve damage, which is why it gives quick, extensible access in emergencies. Its poor blood supply is one reason midline wounds are prone to incisional hernia if closure is poor.
Linea alba
- Midline fusion of aponeuroses
- Xiphoid to pubic symphysis
- Avascular, no nerves
- Fast, extensible access
Remember White line: quick, bloodless way in.
Exam trap Being avascular, the linea alba heals slowly; poor closure leads to incisional hernia.
Why the other options are wrong
- Linea semilunaris Lateral border of rectus, not midline.
- Arcuate line A curved line on the posterior sheath below the umbilicus.
- Posterior rectus sheath Lies lateral to the midline, behind rectus.
- Conjoint tendon Lies at the medial end of the inguinal canal.
In a man, the nipple usually lies over which intercostal space and dermatome?
Answer: 4th intercostal space; T4
In males and young women the nipple overlies the 4th intercostal space at the T4 dermatome; T6 is at the xiphoid process and T10 at the umbilicus. In adult women the position varies with breast size and age, so it is not a reliable landmark there.
Trunk dermatomes
- Nipple: T4
- Xiphoid: T6
- Umbilicus: T10
- Inguinal ligament: L1
Remember T4 teat, T6 xiphoid, T10 belly button.
Exam trap In adult women the nipple is not a fixed landmark.
Why the other options are wrong
- 2nd intercostal space; T2 Too high.
- 6th intercostal space; T6 T6 is at the xiphoid.
- 5th intercostal space; T7 T7 lies below the nipple line.
- 3rd intercostal space; T10 T10 is the umbilicus.
Where this sits in the CPSP syllabus
- Paper 1: Abdomen, Anterior abdominal wall, rectus sheath and dermatomes
- Surgery & Allied, Paper 2: Abdomen, Anterior abdominal wall, rectus sheath and dermatomes