Abdomen & pelvis · Free article
Inguinal canal anatomy for MRCS Part A: walls, rings, cord coverings, indirect versus direct hernias, the femoral canal, surface landmarks and nerves at risk.
The inguinal canal is an oblique passage about 4 cm long through the lower anterior abdominal wall, immediately above the medial half of the inguinal ligament. It exists because the testis, or the round ligament, passed through the wall during development and dragged a sleeve of each layer with it. Its entrance is the deep (internal) ring, an oval opening in the transversalis fascia about 1.25 cm above the midpoint of the inguinal ligament and immediately lateral to the inferior epigastric vessels. Its exit is the superficial (external) ring, a triangular split in the external oblique aponeurosis resting on the pubic crest, just above and medial to the pubic tubercle, to which its lateral crus is attached. Because the rings are not opposite one another, a rise in intra-abdominal pressure flattens the canal rather than opening it: the arching fibres of internal oblique and transversus drop like a shutter across the deep ring, and the conjoint tendon braces the wall behind the superficial ring. The canal transmits the spermatic cord or the round ligament and, in both sexes, the ilioinguinal nerve, which enters not through the deep ring but by piercing internal oblique part-way along, and leaves through the superficial ring lying on the front of the cord.
| Wall | Formed by | What it means |
|---|---|---|
| Anterior | External oblique aponeurosis throughout; the lowest fibres of internal oblique in front of the lateral third, where they arise from the inguinal ligament | Reinforces the deep ring, the lateral weak point |
| Posterior | Transversalis fascia throughout; the conjoint tendon (falx inguinalis) behind the medial third | Reinforces the superficial ring; the unsupported middle is Hesselbach's triangle, where a direct hernia appears |
| Roof | Arching lower fibres of internal oblique and transversus abdominis | They fuse medially as the conjoint tendon, inserting on the pubic crest and pectineal line |
| Floor | The inguinal ligament, the in-rolled lower edge of the external oblique aponeurosis, with the lacunar ligament at its medial end | The free edge of the lacunar ligament is the medial boundary of the femoral ring |
Everything about the two inguinal hernias follows from where the sac leaves the abdomen relative to the inferior epigastric vessels, which run up from the external iliac artery along the medial edge of the deep ring into the rectus sheath. An indirect hernia enters the deep ring lateral to the vessels, follows the cord down the canal inside its coverings and can pass through the superficial ring into the scrotum. The sac is usually a persistent processus vaginalis, which is why indirect hernias dominate in infants and young men, are commoner on the right (the right testis descends later) and are the commonest groin hernia at every age and in both sexes. A direct hernia bulges forward through the posterior wall medial to the vessels, in Hesselbach's triangle: inferior epigastric vessels laterally, lateral border of rectus abdominis medially, inguinal ligament below. It is an acquired weakness of transversalis fascia in older men, lies outside the internal spermatic fascia, has a wide neck and seldom reaches the scrotum. A pantaloon hernia has a sac on each side of the vessels.
| Row | Indirect | Direct |
|---|---|---|
| Route | Through the deep ring and the full length of the canal | Forward through the posterior wall in Hesselbach's triangle |
| Relation to inferior epigastric vessels | Lateral | Medial |
| Coverings | All three coverings of the cord | Transversalis fascia and, at the superficial ring, external spermatic fascia |
| Reaches the scrotum | Often | Rarely |
| Typical patient | Infants, children and young adults; the commonest hernia in both sexes | Older men with chronic cough or straining |
| Neck | Narrow (the deep ring): obstruction and strangulation more likely | Wide: strangulation uncommon |
| Controlled by pressure over the deep ring | In principle | No |
Below the inguinal ligament the external iliac vessels become the femoral vessels and carry with them a funnel of transversalis fascia in front and iliac fascia behind: the femoral sheath. The artery lies laterally, the vein in the middle, and medially is the femoral canal, about 1.25 cm long, containing fat, lymphatics and Cloquet's node and giving the vein room to expand. The abdominal mouth of the canal is the femoral ring: the inguinal ligament in front, the pectineal ligament of Cooper behind, the crescentic edge of the lacunar ligament medially and the femoral vein laterally. A femoral hernia pushes through the ring, descends the canal and turns forward through the saphenous opening, so the lump lies below and lateral to the pubic tubercle. Three of the ring's four boundaries are ligament or bone and cannot stretch. Bowel that enters is gripped at the neck, loses its venous return and strangulates, often as a Richter's hernia in which only part of the circumference is caught, so there may be no obstruction to warn you. That is why femoral hernias, though far less common than inguinal hernias even in women (in whom they are relatively more frequent), supply a disproportionate share of strangulated groin hernias and are repaired promptly rather than watched. Releasing one means dividing the lacunar ligament medially, where in a minority an aberrant obturator artery from the inferior epigastric, the corona mortis, runs along the free edge.
An open repair passes through skin, the fatty superficial fascia (Camper's) and the membranous layer beneath it (Scarpa's), which fuses with the fascia lata just below the inguinal ligament and continues into the perineum as Colles' fascia, so urine from a ruptured bulbar urethra tracks up the abdominal wall but never down the thigh. Then come the external oblique aponeurosis, opened in line with its fibres from the superficial ring; the cord with the ilioinguinal nerve on it; and the posterior wall of transversalis fascia and conjoint tendon against which the mesh is laid. Deeper still are extraperitoneal fat and parietal peritoneum, through which a laparoscopic (TEP or TAPP) repair reaches the same posterior wall from behind.
Test yourself
A 72-year-old woman taking apixaban develops sudden lower abdominal pain after a bout of coughing. There is a tender, non-pulsatile mass in the right lower quadrant that remains palpable and becomes more tender when she lifts her head from the pillow.
Which vessel is the most likely source of bleeding?
Test yourself
A 40-year-old man has painless bright-red bleeding after defaecation. Proctoscopy shows prolapsing haemorrhoids arising above the dentate line.
Into which vein do the dilated vessels principally drain?
Anterior wall: external oblique aponeurosis, with internal oblique in front of the lateral third. Posterior wall: transversalis fascia, with the conjoint tendon behind the medial third. Roof: the arching fibres of internal oblique and transversus abdominis. Floor: the inguinal ligament and, medially, the lacunar ligament.
By the sac's relationship to the inferior epigastric vessels at operation: an indirect sac passes through the deep ring lateral to the vessels and lies inside the coverings of the cord; a direct sac bulges through the posterior wall medial to the vessels in Hesselbach's triangle. The bedside deep ring occlusion test is unreliable.
Inguinal ligament in front, pectineal ligament behind, lacunar ligament medially and the femoral vein laterally. Three of these are rigid, so bowel entering the narrow ring is quickly trapped; a Richter's hernia can strangulate without obstructing.
The mid-inguinal point is halfway between the anterior superior iliac spine and the pubic symphysis and marks the femoral artery. The midpoint of the inguinal ligament is halfway between the spine and the pubic tubercle and, by UK convention, marks the deep inguinal ring just above it.
Written for revision in the MRCS Part A style under our editorial standards. It is not clinical advice: always confirm against current guidance. Spotted an error? Tell us.
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