Human Anatomy
Perineum
Perineum anatomy for MBBS: urogenital and anal triangles, anal canal and pectinate line, ischioanal fossa and pudendal nerve, mapped to NMC codes AN49.1 to AN49.5.
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Perineum
Perineum anatomy for MBBS: urogenital and anal triangles, anal canal and pectinate line, ischioanal fossa and pudendal nerve, mapped to NMC codes AN49.1 to AN49.5.
The perineum chapter divides the region into urogenital and anal triangles and covers the anal canal, the ischioanal fossa and the pudendal nerve. It underpins the anatomy of haemorrhoids, perineal tears in childbirth and the pudendal nerve block.
High-yield: Perineum
- The perineum is diamond-shaped and divided by a line between the ischial tuberosities into a urogenital triangle in front and an anal triangle behind.
- The anal triangle contains the anal canal and the ischioanal fossae on either side.
- The ischioanal fossa is filled with fat and allows the anal canal to expand during defaecation.
- The pudendal nerve, from sacral roots two to four, is the main nerve of the perineum.
- The pudendal nerve passes through the lesser sciatic foramen and runs in the pudendal canal along the lateral wall of the ischioanal fossa.
- A pudendal nerve block is given near the ischial spine to provide anaesthesia for childbirth.
- The anal canal has an internal sphincter of smooth muscle and an external sphincter of skeletal muscle.
- The external anal sphincter is supplied by the inferior rectal branch of the pudendal nerve and keeps continence under voluntary control.
- The pectinate line divides the anal canal into an upper part of endodermal origin and a lower part of ectodermal origin.
- Above the pectinate line the canal is insensitive to pain and drains to internal iliac nodes; below it is sensitive and drains to inguinal nodes.
- Internal haemorrhoids arise above the pectinate line and are painless, while external ones below it are painful.
- The perineal body is a fibromuscular node in front of the anal canal into which several muscles insert.
- The perineal body is important in supporting the pelvic floor and is at risk of tearing during childbirth.
- The superficial perineal pouch contains the erectile tissues and the muscles associated with them.
The pectinate line divide
- **Above pectinate line:** Endodermal origin, insensitive to pain, drains to internal iliac nodes. Internal haemorrhoids are painless.
- **Below pectinate line:** Ectodermal origin, pain-sensitive, drains to superficial inguinal nodes. External haemorrhoids are painful.
- **Pudendal nerve:** Sacral roots two to four. Main nerve of the perineum; blocked at the ischial spine for childbirth.
- **Perineal body:** Central fibromuscular node supporting the pelvic floor; at risk in childbirth.
NMC competencies in this chapter
- **AN49.1:** Boundaries and subdivisions of the perineum
- **AN49.2:** Anal canal, sphincters and the ischioanal fossa
- **AN49.3:** Pudendal nerve, perineal pouches and the perineal body
- **AN49.4:** Urogenital triangle and its contents
- **AN49.5:** Applied anatomy of the perineum
Frequently Asked Questions
Why are internal haemorrhoids painless?
They arise above the pectinate line, where the anal canal has autonomic sensation and is insensitive to pain, unlike the somatically innervated area below the line.
Where is a pudendal nerve block given?
Near the ischial spine, where the pudendal nerve loops around before entering the pudendal canal, providing perineal anaesthesia for the second stage of labour.
Why does the pectinate line matter clinically?
It marks a change in origin, sensation, blood supply and lymphatic drainage, so lesions above and below it differ in pain and in the lymph nodes they spread to.
How does this chapter help in NEET-PG?
The pectinate line, pudendal nerve and perineal body appear in surgery and obstetrics, so the perineum is directly high-yield for PG preparation.
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