Anatomy and morphology of the anus: A review of the form and function of the anal canal

Background/Objective: Despite its small size, the anus is an anatomically complex structure whose characteristic shape results from the interplay of embryological zonation, layered musculature, and specialised vascular cushions. This narrative review aims to systematically summarise the anatomical basis of normal anal form and function, from embryological development to clinical relevance.
Methods: A structured, non-systematic literature search was conducted across PubMed/PMC, StatPearls, and other peer-reviewed and educational anatomical resources, focusing on English-language sources describing the embryology, gross anatomy, musculature, vascular anatomy, and normal variability of the anal canal, searched through July 2026.
Results: The anal canal develops from two embryologically distinct compartments – the endodermal hindgut and the ectodermal proctodeum – joined at the dentate line. Continence and shape are maintained by a three-layered muscular system (internal anal sphincter [IAS], external anal sphincter [EAS], and puborectalis muscle) that generates the anorectal angle, together with three vascular anal cushions that provide fine continence and contribute to the characteristic radial “rosette” pattern of the closed anus. The macroscopic architecture is largely conserved between individuals, while length, crypt number, and cushion distribution show normal variability.
Conclusion: The form of the anus reflects a coordinated anatomical system rather than a simple opening. Distinguishing this conserved basic plan from normal individual variability is relevant both for clinical practice and for general anatomical understanding of a frequently stigmatised body region.