Anatomy chapters
Applied anatomy of the trunk
The inguinal canal and hernias, femoral hernia, the rectus sheath and abdominal incisions, referred pain, and the anatomy behind common procedures on the chest, abdomen and pelvis.
- The inguinal canal runs from the deep ring (above the midpoint of the inguinal ligament) to the superficial ring (above the pubic tubercle).
- Indirect hernias enter the deep ring lateral to the inferior epigastric vessels; direct hernias push through Hesselbach's triangle, medial to them.
- Femoral hernias come out below and lateral to the pubic tubercle and strangulate easily.
On this page
Overview
Applied anatomy of the trunk is about using anatomy at the bedside and in theatre: finding hernias, choosing incisions, understanding referred pain and doing procedures safely.
The most tested topic by far is the inguinal canal and inguinal hernias.
The inguinal canal
The inguinal canal is an oblique passage, about 4 cm long, through the lower anterior abdominal wall, just above the medial half of the inguinal ligament.
- Deep (internal) ring: an opening in the transversalis fascia, just above the midpoint of the inguinal ligament, lateral to the inferior epigastric vessels.
- Superficial (external) ring: a triangular gap in the external oblique aponeurosis, just above and medial to the pubic tubercle.
Walls
| Wall | Formed by |
|---|---|
| Anterior | External oblique aponeurosis (all along); internal oblique (lateral third) |
| Posterior | Transversalis fascia (all along); conjoint tendon (medial third) |
| Roof | Arching fibres of internal oblique and transversus abdominis |
| Floor | Inguinal ligament (rolled-up lower edge of external oblique); lacunar ligament medially |
"MALT, two of each": roof, 2 Muscles (internal oblique, transversus); anterior wall, 2 Aponeuroses (external oblique, plus internal oblique laterally); floor, 2 Ligaments (inguinal, lacunar); posterior wall, 2 Ts (transversalis fascia, conjoint tendon).
Contents
- Male: the spermatic cord and the ilioinguinal nerve.
- Female: the round ligament of the uterus and the ilioinguinal nerve.
The spermatic cord contains the vas deferens, testicular artery, pampiniform plexus of veins, the genital branch of the genitofemoral nerve, lymph vessels and the remains of the processus vaginalis.
The ilioinguinal nerve passes through part of the canal and out of the superficial ring, but it does not enter through the deep ring.
Inguinal hernias
Indirect inguinal hernia
How it happens: the processus vaginalis stays open, so abdominal contents enter the deep ring, follow the spermatic cord through the canal and can reach the scrotum.
What you see: a groin swelling that may go into the scrotum. It is the commonest hernia in both sexes and in children. At operation the sac lies lateral to the inferior epigastric vessels.
Direct inguinal hernia
How it happens: weakness of the posterior wall in older men pushes contents straight forward through Hesselbach's (inguinal) triangle.
What you see: a broad swelling that rarely reaches the scrotum. The sac lies medial to the inferior epigastric vessels.
Hesselbach's triangle
| Boundary | Structure |
|---|---|
| Lateral | Inferior epigastric vessels |
| Medial | Lateral border of rectus abdominis |
| Inferior | Inguinal ligament |
The key at operation is the inferior epigastric vessels: indirect lateral, direct medial.
The deep ring occlusion test (pressing over the deep ring above the midpoint of the inguinal ligament) controls an indirect hernia but not a direct one. It is a bedside guide, not exact.
Femoral hernia
The femoral canal is the medial compartment of the femoral sheath. Its upper opening, the femoral ring, is bounded by:
- In front: the inguinal ligament.
- Behind: the pectineal ligament.
- Medially: the lacunar ligament (sharp edge).
- Laterally: the femoral vein.
How it happens: abdominal contents push down through the femoral ring into the canal.
What you see: a swelling below and lateral to the pubic tubercle. It is more common in women (wider pelvis) and is prone to strangulation, because the ring is narrow and rigid.
Inguinal hernia: neck above and medial to the pubic tubercle. Femoral hernia: neck below and lateral to it.
The rectus sheath
The rectus sheath encloses rectus abdominis. It is formed by the aponeuroses of the three flat muscles.
| Region | Front wall | Back wall |
|---|---|---|
| Above the costal margin | External oblique only | Rib cartilages (no sheath) |
| Costal margin to arcuate line | External oblique + anterior layer of internal oblique | Posterior layer of internal oblique + transversus abdominis |
| Below the arcuate line | All three aponeuroses | Transversalis fascia only |
The arcuate line lies about halfway between the umbilicus and the pubic symphysis. Below it, the inferior epigastric vessels enter the sheath.
The linea alba is the midline seam where the sheaths of both sides meet.
Abdominal incisions
| Incision | Where | Notes |
|---|---|---|
| Midline | Through the linea alba | Quick, bloodless, good access; heals with a weaker scar; risk of incisional hernia |
| Paramedian | Through the rectus sheath, rectus pushed aside | Strong closure; now used less |
| Gridiron (McBurney) | At McBurney's point, splitting the muscles along their fibres | For appendicectomy; muscles are split, not cut |
| Kocher (subcostal) | Below and parallel to the right costal margin | For gallbladder and liver; avoid the 9th thoracic nerve |
| Pfannenstiel | Curved, just above the pubic symphysis | Caesarean section and pelvic surgery; good cosmetic scar |
Incisions that split muscles along their fibres and avoid cutting nerves heal strongest. Cutting several intercostal nerves (T7–T12) weakens the muscles they supply.
Referred pain
Visceral pain is felt in the skin supplied by the same spinal segments as the organ's sympathetic pain fibres.
| Organ | Pain felt at |
|---|---|
| Heart | Left chest, left arm (T1–T4) |
| Stomach, duodenum | Epigastrium (T6–T9) |
| Appendix (early) | Umbilicus (T10) |
| Kidney and ureter | Loin to groin (T11–L2) |
| Diaphragm irritation | Shoulder tip (C3–C5) |
Procedures
Urethral catheterisation (male)
The catheter passes through the external urethral meatus, the navicular fossa, the spongy, membranous and prostatic urethra. The narrowest parts are the external meatus and the membranous urethra. The penis is held upward to straighten the first curve.
Suprapubic catheter
Inserted just above the pubic symphysis into a full bladder, which rises above the pubis and pushes the peritoneum up, so the needle avoids the peritoneal cavity.
Pericardiocentesis
A needle is passed from below the xiphoid process, angled up towards the left shoulder, into the pericardial cavity.
Abdominal paracentesis
Fluid is drained from the lower abdomen lateral to the rectus sheath, avoiding the inferior epigastric vessels.
Clinical relevance
Strangulated hernia
How it happens: the neck of the sac squeezes bowel so its blood supply is cut off.
What you see: a tense, tender, irreducible lump with vomiting and signs of bowel obstruction. It needs urgent surgery.
Umbilical and incisional hernias
How it happens: weakness at the umbilicus or in an old scar.
What you see: a bulge at the site. Paraumbilical hernias in adults have narrow necks and can strangulate.
The common questions are the walls and contents of the inguinal canal, the position of the rings, direct vs indirect hernia and the inferior epigastric vessels, Hesselbach's triangle, the femoral ring and why femoral hernias strangulate, the arcuate line, and which incision suits which operation.
Quick revision
- Deep ring above the midpoint of the inguinal ligament; superficial ring above the pubic tubercle.
- Indirect: lateral to inferior epigastric vessels. Direct: medial, through Hesselbach's triangle.
- Hesselbach's triangle: inferior epigastric vessels, rectus, inguinal ligament.
- Femoral hernia: below and lateral to the pubic tubercle; women; strangulates easily.
- Below the arcuate line: all aponeuroses in front of rectus.
- Pfannenstiel for caesarean; gridiron for appendix; Kocher for gallbladder.
Written by The House Job from standard anatomy references (Snell's Clinical Anatomy by Regions is the book named in the UHS PGET syllabus).
FAQ
What is the difference between a direct and an indirect inguinal hernia?
An indirect hernia enters the deep ring lateral to the inferior epigastric vessels. A direct hernia pushes through Hesselbach's triangle, medial to them.
What are the contents of the inguinal canal?
The spermatic cord in men, or the round ligament of the uterus in women, plus the ilioinguinal nerve.
Why does a femoral hernia strangulate easily?
The femoral ring is narrow and its edges, especially the lacunar ligament, are rigid.
What is the arcuate line?
The lower edge of the posterior layer of the rectus sheath, about halfway between the umbilicus and pubis.