[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Pelvis](https://durgical.com/en/anatomy/region/pelvis/) › Anal canal [Digestive system](https://durgical.com/en/anatomy/system/digestive-system/) · [Pelvis](https://durgical.com/en/anatomy/region/pelvis/) # Anal canal The anal canal is the last 3-4 cm of the bowel, from the rectum to the anus; its sphincters keep stool and gas in and open to let the bowel empty. [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=pelvis&variant=male&select=anal_canal&side=r) Opens in the Durgical app, no sign-up needed. ## What it is The last 3-4 cm of the bowel, from the end of the rectum to the anus, surrounded by the internal and external anal sphincters. ## What it does Keeps stool and gas in and opens to let the bowel empty. ## Key points - The pectinate line splits the canal in two: columnar mucosa above (adenocarcinoma, painless lesions) and squamous anoderm below (squamous cell carcinoma, painful lesions) - Internal haemorrhoids arise above the line and bleed painlessly unless they prolapse and strangulate; external haemorrhoids lie below it and hurt, especially when thrombosed - Most anal fissures sit in the posterior midline, where the anoderm has the poorest blood supply; a fissure off the midline suggests Crohn disease, infection or cancer - Anal glands open into the anal crypts at the pectinate line; their infection is the usual start of an anorectal abscess and, later, a fistula ## Key facts **Where** Last 3–4 cm of the gut, from the anorectal junction at the puborectalis sling to the anus; the pectinate line divides an upper (endodermal) and a lower (ectodermal) part **Blood supply** Above the pectinate line: [superior rectal artery](https://durgical.com/en/anatomy/superior-rectal-artery/) (inferior mesenteric); below: inferior rectal artery (internal pudendal); veins form a portosystemic anastomosis **Nerve** Above the line: autonomic, stretch only (internal hemorrhoids painless); below: inferior rectal nerve (pudendal, S2–S4), pain-sensitive; external sphincter also pudendal **Function** Continence by the internal (smooth, resting tone) and external (striated, voluntary) sphincters with puborectalis; lymph above the line to internal iliac nodes, below to superficial inguinal ## Relations **Above** [Rectum](https://durgical.com/en/anatomy/rectum/), at the anorectal junction where the [puborectalis](https://durgical.com/en/anatomy/puborectalis/) sling pulls the gut forward and forms the anorectal angle **In front** [Perineal body](https://durgical.com/en/anatomy/perineal-body/) and, in the male, the bulb of the [corpus spongiosum](https://durgical.com/en/anatomy/corpus-spongiosum-of-penis/) and the [membranous urethra](https://durgical.com/en/anatomy/intermediate-part-of-male-urethra/), at risk in anterior anorectal surgery **Behind** Anococcygeal body, a fibrous band between the canal and the [coccyx](https://durgical.com/en/anatomy/coccyx/) **Lateral** Ischioanal fossae, filled with fat and crossed by the inferior rectal vessels and nerves; abscesses spread into them and can pass behind the canal from one side to the other (horseshoe abscess) ## How to find it Examined with the person lying on the left side, hips and knees bent. Inspection shows the anal verge, [skin](https://durgical.com/en/anatomy/skin/) tags, fissures or external openings; on digital rectal examination the finger passes the sphincters and meets the anorectal ring (puborectalis) at the top of the canal. ## Examination - Digital rectal examination: resting tone reflects mainly the internal sphincter and voluntary squeeze the [external anal sphincter](https://durgical.com/en/anatomy/external-anal-sphincter/) and puborectalis; weak tone points to sphincter damage or a [pudendal nerve](https://durgical.com/en/anatomy/pudendal-nerve/) or sacral root lesion - Anal wink (anocutaneous reflex): stroking the perianal skin makes the external anal sphincter contract; an absent response suggests a lesion of S2–S4 or the pudendal nerve - Anoscopy: shows internal haemorrhoids, the base of a fissure and the internal opening of a fistula ## On imaging Endoanal ultrasound shows the internal sphincter as a dark (hypoechoic) ring and the external sphincter as a brighter outer layer, and finds sphincter defects. Pelvic MRI is the reference for mapping complex fistulas and their relation to the sphincters. ## Common problems - Haemorrhoids (swollen anal cushions). - Anal fissure (a small, painful tear). - Abscess and fistula. ## How it is diagnosed Examination; anoscopy. ## Treatment - Fibre, fluids and creams; small procedures or surgery when needed. ## Clinical notes - Internal haemorrhoids are graded by prolapse: I none, II prolapse that returns by itself, III needs pushing back, IV stays out - Fistulas are classed by their path through the sphincters (Parks): intersphincteric, transsphincteric, suprasphincteric, extrasphincteric; the more sphincter crossed, the greater the risk of incontinence after fistulotomy - Goodsall rule: an external opening behind a transverse line through the anus usually tracks in a curve to the posterior midline; one in front usually runs straight to the nearest crypt - Anal cancer below the pectinate line spreads to the [superficial inguinal nodes](https://durgical.com/en/anatomy/superficial-inguinal-nodes/), so the groin is examined; tumours above drain to the internal iliac nodes ## Development The upper canal comes from the endodermal hindgut (cloaca) and the lower from the ectodermal proctodeum; the anal membrane between them breaks down in the embryo and its line persists as the pectinate line. Failure of this process gives imperforate anus and other anorectal malformations. ## Related conditions - [Anal fissure](https://durgical.com/en/conditions/anal-fissure/) - [Anal cancer](https://durgical.com/en/conditions/anal-cancer/) - [Anorectal malformations (imperforate anus)](https://durgical.com/en/conditions/anorectal-malformations/) - [Syphilis](https://durgical.com/en/conditions/syphilis/) ## Test yourself Think of the answer, then tap the question to see it. ## How does the pectinate line divide the anal canal? Above: columnar mucosa, autonomic stretch-only nerves (adenocarcinoma, painless lesions). Below: squamous anoderm, pudendal pain fibres (squamous cancer, painful lesions). ## Where do most anal fissures lie, and what does a fissure off the midline suggest? In the posterior midline, where blood supply is poorest; off the midline suggests Crohn disease, infection or cancer. ## How are internal haemorrhoids graded? By prolapse: I none, II prolapses and returns by itself, III needs pushing back, IV stays out. ## What does Goodsall rule predict about anal fistulas? An external opening behind a transverse line through the anus usually curves to the posterior midline; one in front runs straight to the nearest crypt. ## Where does lymph drain from the anal canal above and below the pectinate line? Above: internal iliac nodes. Below: superficial inguinal nodes, so the groin is examined in anal cancer. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/anatomy/anal-canal/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** canalis analis **Official Spanish (DeCS)** Canal anal **English (TA2)** Anal canal ## Related - [Rectum](https://durgical.com/en/anatomy/rectum/) - [External anal sphincter](https://durgical.com/en/anatomy/external-anal-sphincter/) - [Puborectalis](https://durgical.com/en/anatomy/puborectalis/) - [Internal pudendal artery](https://durgical.com/en/anatomy/internal-pudendal-artery/) Sources: [Anatomy, Abdomen and Pelvis: Anal Canal](https://www.ncbi.nlm.nih.gov/books/NBK554531/). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/anal-canal/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/anal-canal/