Above is visceral and insensitive; below is somatic and painful
Mnemonic
“Above and below the line, everything changes”:
| Above (hindgut) | Below (ectoderm) | |
|---|---|---|
| Epithelium | Columnar | Squamous |
| Artery | Superior rectal | Inferior rectal |
| Vein | Superior rectal to portal | Inferior rectal to systemic |
| Lymph | Internal iliac | Superficial inguinal |
| Nerve | Autonomic, insensitive | Somatic (pudendal), sensitive |
| Pathology | Internal haemorrhoids, painless | External haemorrhoids and fissures, painful |
“Internal haemorrhoids do not hurt; external ones do.” That single fact follows from the nerve supply and is the most useful consequence of the line.
The line is also a portosystemic watershed, which is why portal hypertension can produce rectal varices, and a lymphatic watershed, which determines where anal carcinoma spreads.
Expansion
The pectinate (dentate) line marks the junction of endoderm (hindgut) and ectoderm (proctodeum).
| Above the line | Below the line | |
|---|---|---|
| Epithelium | Columnar | Stratified squamous |
| Innervation | Autonomic - insensitive to pain | Somatic (inferior rectal nerve) - very sensitive |
| Arterial supply | Superior rectal artery | Inferior rectal artery |
| Venous drainage | Superior rectal vein → portal | Inferior rectal vein → systemic |
| Lymphatics | Internal iliac and para-aortic nodes | Superficial inguinal nodes |
The clinical consequences are direct. Internal haemorrhoids arise above the line and are painless; external haemorrhoids and anal fissures are below and are exquisitely painful. A low anal carcinoma spreads to inguinal nodes, while a higher one spreads along the internal iliac chain.