Anal Canal - Gross Anatomy

Overview and Extent

• Surgical (functional) anal canal: from the anorectal ring to the anal verge (~4 cm)
• Anatomical anal canal: from the dentate/pectinate line to the anal verge (~2 cm)

Lining / Mucosal Zones

Key Surface Features (Above the Dentate Line)

• Anal columns (Columns of Morgagni): 8-12 vertical mucosal folds in the upper anal canal, containing terminal branches of the superior rectal artery and the internal rectal (hemorrhoidal) plexus.
• Anal valves: Crescentic transverse folds connecting the lower ends of adjacent anal columns.
• Anal sinuses: Small depressions (pockets) located superior to each anal valve.
• Anal crypts: Recesses at the anal valves where the anal glands open. These glands are the origin of most cryptoglandular abscesses and fistulae.
• Anal papillae: Folds of tissue at the dentate line, between the crypts.

Key Landmarks

• Dentate (pectinate) line: Formed by the row of anal valves. Marks the approximate site of the embryological cloacal membrane and the boundary between endodermal and ectodermal territory. This is the single most important landmark of the anal canal - blood supply, venous/lymphatic drainage, and innervation all change at this line.
• Anocutaneous line ("white line" of Hilton): The lower boundary of the anal pecten, where stratified squamous epithelium transitions to true skin.

Sphincter Complex and Pelvic Floor

Internal Anal Sphincter (IAS)

• A thickening of the circular smooth muscle layer of the rectal wall, extending 2-4 cm.
• Involuntary, supplied by autonomic nerves (sympathetic and parasympathetic - both inhibit contraction, i.e., both promote relaxation).
• Responsible for approximately 85% of resting anal tone.

External Anal Sphincter (EAS)

• Striated (skeletal) voluntary muscle, surrounding and overlapping the IAS.
• Divided into three parts: deep, superficial, and subcutaneous.
• Contiguous superiorly with the levator ani and puborectalis; attached inferiorly to the coccyx via the anococcygeal ligament.
• Supplied by the inferior rectal branch of the pudendal nerve (S2, S3).

Longitudinal Muscle

• The "conjoined longitudinal muscle" lies between the IAS and EAS in the intersphincteric plane. It is formed by extensions of the rectal longitudinal muscle blended with fibres from levator ani. Its terminal fibres fan out as the corrugator cutis ani, which create the puckered appearance of the anus.

Puborectalis and the Levator Ani

• The levator ani muscle (pubococcygeus, puborectalis, iliococcygeus) forms the pelvic floor.
• Puborectalis forms a U-shaped sling around the anorectal junction, passing from the posterior pubis around the back of the rectum. This creates the anorectal angle (~90°), a key mechanism of continence.
• The deep part of the EAS is essentially fused with the puborectalis.

Intersphincteric Space

• The space between the IAS and EAS. This is the most common pathway for the spread of cryptoglandular infection (abscess and fistula formation).

Anorectal Spaces

Blood Supply

• Superior rectal artery (terminal branch of inferior mesenteric artery): supplies the upper anal canal and rectum. Descends posterior to the rectum and bifurcates.
• Middle rectal arteries (from internal pudendal or inferior gluteal arteries, branches of internal iliac): present as a distinct vessel in fewer than 15% of people; supply the lower rectum and upper anal canal.
• Inferior rectal arteries (from internal pudendal artery, within Alcock's canal): traverse the ischioanal space to supply the anal sphincters and lower canal.
• Median sacral artery: provides additional supply to the posterior anal canal and IAS.

• Superior rectal vein → inferior mesenteric vein → portal system
• Middle and inferior rectal veins → internal iliac veins → systemic (caval) system
• This portosystemic communication is the anatomical basis for rectal varices in portal hypertension.
• The haemorrhoidal plexus (internal rectal plexus) is a submucosal plexus deep to the columns of Morgagni - a sinusoidal (not true venous) network draining into all three rectal veins.

Lymphatic Drainage

Nerve Supply

• IAS: autonomic innervation; both sympathetic and parasympathetic fibres inhibit (relax) the sphincter.
• EAS and puborectalis: inferior rectal branch of internal pudendal nerve.
• Levator ani: pudendal nerve + direct branches from S3-S5.

Summary: Clinical Significance of the Dentate Line