Learning Objectives

• Identify the sphincter mechanisms responsible for fecal continence.
• Master the Pectinate (Dentate) Line as the critical landmark for neurovascular and embryological transitions.
• Distinguish between visceral vs. somatic innervation and its impact on clinical presentation.
• Understand the anatomical basis of haemorrhoids and their classic clock positions.

Anatomical Structure & Sphincters

The anal canal (4cm) is the terminal segment of the GIT. It is normally collapsed by two concentric sphincters to maintain continence.

• Internal Anal Sphincter: Involuntary smooth muscle (continuation of circular bowel muscle). Surrounds upper 2/3.
• External Anal Sphincter: Voluntary skeletal muscle. Surrounds the lower 2/3 and blends with the puborectalis muscle.
• Anorectal Ring: A palpable muscular ring at the rectum-anal junction formed by the fusion of both sphincters and the puborectalis.

Internal Landmarks: The Pectinate Line

The Pectinate Line is the most important landmark in proctology, representing the site where the hindgut meets the ectoderm.

• Anal Columns: Longitudinal mucosal folds containing terminal branches of the superior rectal vessels.
• Anal Valves & Sinuses: Small horizontal folds and pouches that secrete mucus to aid defecation.

The “Above vs. Below” Rule

This table is high-yield for exams as it dictates where cancers spread and how pain is felt:

Clinical Relevance: Haemorrhoids

Haemorrhoids are prolapsed vascular cushions. Their location is described using a clock-face with the patient in the lithotomy position.

• Classic Positions: 3, 7, and 11 o’clock.
• Internal Haemorrhoids: Occur above the pectinate line. They are usually painless because they are under visceral innervation.
• External Haemorrhoids: Occur below the pectinate line. They are highly painful due to somatic (inferior rectal) nerves.

Activity