Q1 LONG ANSWER 10M ★ High-yield

Describe the layers of the anterior abdominal wall and the formation of the rectus sheath. Add a note on the inguinal canal.

Anatomy · 1st Professional — Pre-Clinical

Layer	Description
Skin	Thin and mobile except over the umbilicus
Superficial fascia	Above the umbilicus a single layer; below it splits into CAMPER (fatty) and SCARPA (membranous)
External oblique	Fibres pass downward and forward ("hands in pockets")
Internal oblique	Fibres upward and forward, at right angles to the external
Transversus abdominis	Fibres transverse
Rectus abdominis and pyramidalis	Vertical; enclosed in the rectus sheath
Fascia transversalis	Continuous lining of the abdominal cavity
Extraperitoneal fat	—
Parietal peritoneum	The innermost layer

Scarpa fascia continues into the perineum as COLLES fascia, and into the scrotum and penis as the dartos and Buck fascia
It is attached to the fascia lata a finger-breadth below the inguinal ligament — the reason extravasated urine spreads into the scrotum and abdominal wall but never into the thigh

The rectus sheath is formed by the aponeuroses of the three flat muscles, and encloses rectus abdominis.

Level	Anterior wall	Posterior wall
Above the costal margin	External oblique aponeurosis only	Absent — rectus lies on the 5th to 7th costal cartilages
Costal margin to the arcuate line	External oblique + anterior lamina of internal oblique	Posterior lamina of internal oblique + transversus abdominis
Below the arcuate line	All three aponeuroses pass in front	Only the fascia transversalis

The arcuate line lies midway between the umbilicus and the pubic symphysis
Contents of the sheath — rectus abdominis, pyramidalis, superior and inferior epigastric vessels, and the terminal parts of the lower six thoracic nerves
The two sheaths meet in the midline as the linea alba

Lower six thoracic nerves (T7–T11) — thoraco-abdominal
T12 — subcostal nerve
L1 — iliohypogastric and ilioinguinal nerves
The nerves run between internal oblique and transversus — the plane used for a tap (transversus abdominis plane) block
Dermatomes — T7 epigastrium, T10 umbilicus, L1 inguinal region

Artery	Origin	Territory
Superior epigastric	Internal thoracic	Upper part, within the sheath
Inferior epigastric	External iliac, just above the inguinal ligament	Lower part; anastomoses with the superior
Deep circumflex iliac	External iliac	Lower lateral part
Lower posterior intercostal and lumbar	Aorta	Lateral part
Superficial epigastric, superficial circumflex iliac, external pudendal	Femoral	Superficial fascia

The superior and inferior epigastric arteries anastomose within the sheath — an important portacaval and caval–caval collateral route

An oblique passage about 4 cm long in the lower part of the anterior abdominal wall, just above the medial half of the inguinal ligament.

Wall	Formed by
Anterior	External oblique aponeurosis; internal oblique laterally
Posterior	Fascia transversalis; conjoint tendon medially
Roof	Arching fibres of internal oblique and transversus
Floor	Inguinal ligament, with the lacunar ligament medially

Deep ring — a defect in the fascia transversalis, 1.25 cm above the midpoint of the inguinal ligament, lateral to the inferior epigastric artery
Superficial ring — a triangular defect in the external oblique aponeurosis, above and medial to the pubic tubercle
Contents — spermatic cord in the male, round ligament of the uterus in the female, and the ilioinguinal nerve in both
Obliquity of the canal — the two rings do not lie opposite each other
Shutter mechanism — the arching fibres of the roof descend on straining and close the canal against the floor
Sphincter action of the internal oblique at the deep ring
Ball-valve action of the cremaster; and the slit-valve action of the crura of the superficial ring

Indirect hernia — through the deep ring, lateral to the inferior epigastric artery; congenital, from a patent processus vaginalis; may reach the scrotum
Direct hernia — through Hesselbach's triangle, medial to the inferior epigastric artery; acquired, in older men; rarely reaches the scrotum and rarely strangulates
Hesselbach's triangle — bounded by the inferior epigastric artery laterally, rectus abdominis medially, and the inguinal ligament below
Surgical incisions — a midline incision through the linea alba is bloodless and avoids nerves; a gridiron (McBurney) incision splits muscles along their fibres; paramedian incisions cut nerves and weaken the rectus
Divarication of recti — stretching of the linea alba after pregnancy or obesity, producing a midline bulge on sitting up

All questions in Abdomen