Anterior abdominal wall
Anterior abdominal wall: layers, rectus sheath, vessels, nerves and incisions
Anterior abdominal wall for FCPS Part 1: layers, rectus sheath, epigastric vessels, segmental nerves, umbilical folds and incisions. Covers caesarean and laparoscopic entry, stab wounds, hernias, venous flow and pregnancy skin changes.
Layers from skin to peritoneum
- Skin; superficial fascia in two layers: fatty Camper's and membranous Scarpa's fascia (strong enough to hold sutures in obese wounds; continuous with Colles' fascia in the perineum).
- External oblique, internal oblique, transversus abdominis (or their aponeuroses).
- Transversalis fascia, extraperitoneal fat, then parietal peritoneum. So the layer met immediately deep to transversus abdominis is the transversalis fascia, and between transversus and peritoneum lie transversalis fascia and extraperitoneal fat.
- Linea alba: midline fusion of the aponeuroses from xiphoid to symphysis; relatively avascular, with few nerves, so an upper midline incision bleeds little. It is broad above the umbilicus but a narrow line below it.
- Pyramidalis: a small triangular muscle in front of the lower rectus (subcostal nerve, T12).
- An incisional hernia sac pushes out through the scar in the musculoaponeurotic layer; the sac itself is peritoneum (with extraperitoneal fat), the first layer met deep to skin and subcutaneous fat.
The three flat muscles and what they form
| Muscle | Origin and fibres | Forms |
|---|---|---|
| External oblique | Lower 8 ribs, interdigitating with serratus anterior and latissimus dorsi; fibres run down and medially ("hands in pockets") | Inguinal ligament (ASIS to pubic tubercle), lacunar ligament, superficial inguinal ring |
| Internal oblique | Thoracolumbar fascia, iliac crest, lateral inguinal ligament; fibres up and medially | With transversus, the conjoint tendon (falx inguinalis); cremaster muscle |
| Transversus abdominis | Lower costal cartilages, thoracolumbar fascia, iliac crest; fibres horizontal | Posterior rectus sheath above the arcuate line; conjoint tendon |
- In women the inguinal canal carries the round ligament of the uterus and the ilioinguinal nerve.
Rectus sheath
| Level | Anterior wall | Posterior wall |
|---|---|---|
| Above costal margin | External oblique only | None (rectus on costal cartilages) |
| Costal margin to arcuate line | External oblique + anterior lamina of internal oblique | Posterior lamina of internal oblique + transversus |
| Below arcuate line | All three aponeuroses | Transversalis fascia only |
- The arcuate line lies about midway between umbilicus and pubis; here the inferior epigastric artery enters the sheath.
- A Pfannenstiel incision is below the arcuate line: the surgeon cuts one anterior sheath and finds only transversalis fascia behind the recti.
- Usually three tendinous intersections are fused to the anterior sheath, so a collection in front of rectus stays box-shaped.
- Contents: rectus, pyramidalis, superior and inferior epigastric vessels, ends of T7–T12 nerves.
Blood vessels
- Superior epigastric (from internal thoracic) anastomoses with inferior epigastric (from external iliac, just above the inguinal ligament) inside the sheath. If the internal thoracic artery is ligated (for example used for coronary grafting), rectus is perfused by increased flow through the inferior epigastric. The anastomosis is a collateral route in coarctation and aortic occlusion; the TRAM flap survives on the superior epigastric.
- Inferior epigastric runs up behind rectus in the lateral umbilical fold. It lies medial to the deep inguinal ring: indirect hernias pass lateral to it, direct hernias medial.
- A stab wound just lateral to the linea alba below the umbilicus, or a paramedian incision, can cut the inferior epigastric artery; bleeding into the sheath can cause shock. Lateral laparoscopic ports and ascitic taps go lateral to the rectus edge. Coughing on anticoagulants can tear it: rectus sheath haematoma.
- Superficial epigastric (from femoral artery) runs up in the subcutaneous fat and is met when deepening a Pfannenstiel or suprapubic incision.
- Veins: above the umbilicus to the axillary vein, below to the femoral vein; the paraumbilical veins form a portosystemic anastomosis.
| Pattern of dilated wall veins | Cause |
|---|---|
| Radiating from the umbilicus, flow away from it (caput medusae) | Portal hypertension |
| Flow upwards from below the umbilicus, with leg oedema | IVC obstruction (for example a large liver tumour compressing it); blood reaches the SVC through thoracoepigastric veins |
- Lymph: watershed at the umbilicus; skin above drains to axillary nodes, below to superficial inguinal nodes.
Nerves
- Ventral rami T7–T11 (thoracoabdominal), T12 (subcostal) and L1 (iliohypogastric, ilioinguinal) run in the plane between internal oblique and transversus: the TAP block plane.
- Rectus abdominis: T7–T12. Transversus and internal oblique: T7–T12 plus L1 (iliohypogastric and ilioinguinal). External oblique: T7–T12.
- The nerves enter the rectus sheath from its lateral edge, so a long vertical lateral (paramedian or lateral rectus) incision denervates rectus, leaving a weak bulging muscle.
- Dermatomes: T4 nipple, T7 xiphoid, T10 umbilicus, L1 groin and suprapubic skin. L4 supplies the medial leg and knee, not the abdomen.
- Visceral pain from midgut organs (early appendicitis) is referred to the umbilicus (T10) through sympathetic afferents; when the parietal peritoneum is involved, pain shifts to the right iliac fossa.
- TAP block relieves wound pain but not uterine visceral pain. Ilioinguinal–iliohypogastric block: about 2 cm medial to the ASIS.
- Caesarean section under spinal needs a block to T4, because peritoneal handling and traction reach higher than the skin incision.
Inner surface, umbilicus and cord
| Fold | Contains |
|---|---|
| Median umbilical | Urachus (allantois remnant) |
| Medial umbilical | Obliterated umbilical artery (patent part gives superior vesical) |
| Lateral umbilical | Inferior epigastric vessels |
- Above the umbilicus the falciform ligament, with the ligamentum teres (obliterated left umbilical vein) in its free edge, attaches the liver to the anterior abdominal wall.
- Umbilical cord: two arteries and one vein in Wharton's jelly. A single umbilical artery is linked with renal and cardiac anomalies.
- Umbilicus: L3–L4 disc in a supine adult of normal build (dermatome T10).
- The Veress needle enters at the umbilical base, where layers are fused with no muscle; the aortic bifurcation (L4) lies close behind, so the needle is aimed towards the pelvis. After midline surgery Palmer's point (3 cm below the left costal margin, midclavicular line) is used.
- Port sites: fascial defects of 10 mm or more must be closed, or bowel wall can be trapped (Richter's hernia: vomiting and a tender lump, sometimes with flatus still passing).
Umbilical defects
| Feature | Omphalocele (exomphalos) | Gastroschisis |
|---|---|---|
| Site | Through the umbilical ring | Right of an intact cord |
| Covering | Membrane sac (amnion and peritoneum), cord inserts on the sac | None; bowel exposed and thickened |
| Cause | Failure of midgut to return from physiological herniation | Wall defect, possibly vascular |
| Associated anomalies | Common: trisomies 13 and 18, cardiac defects, Beckwith–Wiedemann | Uncommon; bowel atresia |
- An omphalocele demands a search for other anomalies and a karyotype.
Incisions
| Incision | Key features |
|---|---|
| Midline | Through linea alba; quick, nearly bloodless, extensible; more incisional hernias |
| Paramedian | Through anterior sheath; rectus retracted laterally; inferior epigastric at risk |
| Pfannenstiel | Curved, 2–3 cm above the symphysis; anterior sheath cut transversely, recti separated, peritoneum opened vertically |
| Joel-Cohen | Straight, about 3 cm below the interspinous line; layers opened by blunt traction |
| Maylard | Recti cut transversely after ligating the inferior epigastric vessels |
- Transverse incisions follow the transverse aponeurotic fibres and Langer's lines: less pain, better scar, fewer hernias.
- Lateral extension of a Pfannenstiel risks the iliohypogastric and ilioinguinal nerves (suprapubic numbness, burning pain).
- The bladder rises out of the pelvis in labour, so the peritoneum is opened high; opening it low near the pubis risks the bladder, especially after a previous caesarean.
- Biggest risk for incisional hernia: wound infection.
Pregnancy and penetrating injury
- Linea nigra: oestrogen, progesterone and melanocyte-stimulating hormone increase melanocyte activity in the linea alba skin (not their number). Striae gravidarum: tearing of dermal collagen and elastin.
- A cyclical painful nodule in a caesarean scar is scar endometriosis.
- A penetrating wound just lateral to the right rectus at the tip of the 9th costal cartilage (transpyloric plane) reaches the fundus of the gallbladder.
Remember Below the arcuate line everything goes in front of rectus; behind it lies only transversalis fascia.
Exam trap The lateral umbilical fold holds the inferior epigastric vessels, not the umbilical artery; that is the medial fold.
Exam trap The umbilicus is at the T10 dermatome but the L3–L4 vertebral level; read which one the question asks.
How FCPS asks about this
- Ligament formed by external oblique between ASIS and pubic tubercle: inguinal ligament; internal oblique and transversus aponeuroses form the conjoint tendon.
- Rectus sheath above the arcuate line: anterior wall external oblique plus half of internal oblique; posterior wall the other half plus transversus.
- Layer immediately deep to transversus abdominis: transversalis fascia, then extraperitoneal fat and peritoneum.
- Stab wound lateral to the lower linea alba with shock: inferior epigastric artery.
- Internal thoracic artery ligated, rectus perfused by: inferior epigastric via the superior epigastric anastomosis.
- Upward flow in abdominal wall veins with leg oedema: IVC obstruction.
- Contents of the female inguinal canal: round ligament of the uterus and ilioinguinal nerve.
Test yourself on this topic
5 of the 84 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A woman having an elective caesarean section under spinal anaesthesia has a Pfannenstiel incision placed in the T12–L1 dermatomes. Why must the sensory block nevertheless reach T4?
Answer: Peritoneal and visceral handling extend higher
Skin incision pain is low, but during a caesarean the peritoneum is opened, the uterus is exteriorised or handled and the bowel is packed, stretching parietal and visceral peritoneum well above the incision. Afferents from these structures enter the cord over many segments, so a block below T4 leaves the woman with pain and nausea during traction. A T4 block (nipple level) covers this.
Block height for caesarean
- Skin incision: T12–L1
- Target block: T4
- Because of peritoneal traction
Remember Caesarean spinal: nipple level, not incision level.
Exam trap Matching the block to the skin incision alone leads to intraoperative pain.
Why the other options are wrong
- The uterine incision is supplied from T4 The uterus is supplied mainly from T10–L1.
- Spinal drugs spread down and recede quickly Not the reason a high block is required.
- The rectus muscles are supplied from T4 The rectus is supplied by T7–T12.
- Sympathetic block must reach the heart Cardiac sympathetic block is unwanted, not the aim.
When closing a Pfannenstiel incision in an obese woman, the surgeon places sutures in a deep, fibrous layer of the subcutaneous tissue below the umbilicus, which holds stitches well. Which layer is this?
Answer: Membranous layer (Scarpa fascia)
Below the umbilicus the superficial fascia has two layers: a superficial fatty layer (Camper) and a deeper membranous layer (Scarpa). Scarpa fascia is fibrous enough to hold sutures and closing it helps reduce dead space and wound separation in thick subcutaneous tissue. It fuses with the fascia lata just below the inguinal ligament and continues into the perineum as Colles fascia and into the labia majora.
Scarpa fascia
- Membranous layer of superficial fascia
- Holds sutures in obese wounds
- Fuses with fascia lata
Remember Camper is fat; Scarpa is the fibrous sheet.
Exam trap Scarpa fascia is distinct from the deep fascia of the muscles.
Why the other options are wrong
- Fatty layer (Camper fascia) Too soft to hold sutures.
- Deep fascia over external oblique Very thin over the muscle; not a subcutaneous layer.
- Anterior rectus sheath Lies deeper and is closed as a separate layer.
- Transversalis fascia Lies deep to the muscles.
At diagnostic laparoscopy for infertility, the surgeon inspects the inner surface of the lower anterior abdominal wall before placing a lateral port. Which structure lies within the lateral umbilical fold and must be avoided?
Answer: Inferior epigastric vessels
Three peritoneal folds are seen on each side of the inner lower abdominal wall. The median umbilical fold covers the urachus, the medial umbilical fold covers the obliterated umbilical artery, and the lateral umbilical fold covers the inferior epigastric vessels. Lateral ports are placed lateral to the lateral fold (and lateral to the rectus edge) to avoid these vessels, whose injury causes a rectus sheath haematoma or heavy bleeding.
Umbilical folds
- Median fold: urachus
- Medial fold: obliterated umbilical artery
- Lateral fold: inferior epigastric vessels
Remember Lateral fold, live vessels: keep lateral ports outside it.
Exam trap The medial fold is a fibrous remnant; only the lateral fold carries live vessels.
Why the other options are wrong
- Obliterated umbilical artery Lies in the medial umbilical fold.
- Urachus Lies in the median umbilical fold.
- Round ligament of the uterus Runs to the deep ring below and lateral, not in this fold.
- Deep circumflex iliac vessels Run along the iliac crest, far lateral.
A 40-year-old woman has a melanoma excised from the skin of her right upper abdomen, 5 cm above the umbilicus. Which nodes are its first lymphatic drainage?
Answer: Right axillary nodes
Superficial lymphatics of the anterior abdominal wall divide at about the level of the umbilicus. Skin above this line drains to the axillary (mainly pectoral) nodes of the same side, while skin below it drains to the superficial inguinal nodes. Deep lymphatics follow the deep vessels to the parasternal, iliac and para-aortic nodes. The watershed is not exact, so sentinel node mapping is used for lesions near the umbilicus.
Abdominal skin lymph
- Above umbilicus: axillary nodes
- Below umbilicus: superficial inguinal
- Watershed at umbilicus
Remember Above the belly button, up to the axilla.
Exam trap Deep and superficial drainage of the wall go to different nodes.
Why the other options are wrong
- Right superficial inguinal nodes Drain skin below the umbilicus.
- Para-aortic nodes Receive deep, not superficial, drainage.
- Right internal iliac nodes Drain pelvic organs.
- Right supraclavicular nodes Not the first station for abdominal skin.
Lower transverse incisions heal with less pain, better cosmetic results and fewer incisional hernias than lower midline incisions. What is the main anatomical reason?
Answer: They follow aponeurotic fibres and Langer lines
The fibres of the flat muscle aponeuroses in the lower abdomen run mainly transversely, and skin tension (Langer) lines are also horizontal here. A transverse incision separates fibres rather than cutting across them, so the tension on the wound pulls the edges together instead of apart, and the scar is fine. A vertical midline incision cuts across these fibres, so muscle pull opens it.
Transverse incisions
- Aponeurotic fibres run transversely
- Langer lines are horizontal
- Wound tension closes the edges
Remember Cut with the grain, heal with less strain.
Exam trap Transverse incisions still risk the iliohypogastric and ilioinguinal nerves when extended.
Why the other options are wrong
- They avoid all the segmental nerves of the wall Lateral extensions can still cut L1 nerves.
- They pass through a single fused aponeurosis The midline also passes through a single fused layer.
- The posterior rectus sheath reinforces them There is no posterior sheath below the arcuate line.
- The recti are cut and heal by strong fibrosis Pfannenstiel separates, not cuts, the recti.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Abdominal wall and breast, Anterior abdominal wall: layers, vessels, nerves and incisions (Pfannenstiel, midline)