The Pectinate Line: The Most Important Centimetre in the Body
There is a line inside you that no scan will ever show, that has no thickness, and that you cannot feel with a finger — and yet on one side of it a lesion the size of a grape can bleed for months and never hurt, while on the other side a tear no wider than a paper cut can make a grown adult dread going to the toilet. The line is four centimetres inside the anus. It is not a wall, a valve or a muscle. It is a scar: the exact place where, in the eighth week of embryonic life, the gut tube coming down from inside met the skin pushing up from outside, and the membrane between them dissolved. Everything that arrives at that meeting point in a developing body — the epithelium, the artery, the vein, the lymphatic and the nerve — changes there and never changes back. Understand that one line and the whole of anorectal disease stops being a list to memorise.
Two patients are booked into the same clinic on the same morning, and both have come because of blood on the toilet paper. The first is a man of fifty-two. He describes bright red blood that drips into the pan after he passes stool, sometimes enough to colour the water, and he has been living with it for the better part of a year. Does it hurt? He looks puzzled by the question. Not at all, he says — that is precisely why he ignored it so long. The second patient is a woman of thirty-one, three months after the birth of her first child. She has almost no bleeding, a streak at most. But she has not slept properly in five weeks, she has started eating less so that she will need the toilet less often, and she describes the pain as being like passing broken glass, followed by an hour of deep burning afterwards. On examination the man has a soft, congested swelling arising above an invisible landmark, and the woman has a small linear tear beginning just below it. The two lesions are perhaps a centimetre apart. One is on the gut's side of an old embryonic border; the other is on the skin's side. That single centimetre is the entire difference between a nuisance and a torment.
A four-centimetre tube, and it does not point where you think
The canal begins where the rectum stops being a reservoir and starts being a door. The ANAL CANAL is the terminal segment of the gastrointestinal tract: roughly 4 cm long in the adult, running from the ANORECTAL JUNCTION above — the point where the rectum described in the rectum passes through the pelvic floor — to the ANAL VERGE below, where its lining becomes ordinary perianal skin. Its most under-appreciated feature is its direction. It does not continue the line of the rectum downwards; it turns and runs downwards and BACKWARDS, so that its axis points towards the coccyx, while the rectum above it is angled forwards over the pelvic floor. That single fact governs every instrument that has ever been passed into it: a proctoscope, a rigid sigmoidoscope, a thermometer or a suppository introduced straight upwards along the axis of the body will jam against the anterior wall, while the same instrument aimed towards the umbilicus for the first few centimetres and then swung backwards passes without resistance. Unlike almost every other part of the gut, the canal at rest is not a patent tube at all: it is held firmly CLOSED, flattened into an anteroposterior slit by the tone of the surrounding sphincters and sealed at the top by three soft vascular pads. Laterally it is separated from the ISCHIOANAL FOSSA on each side — the fat-filled wedge described in the perineum — by the sphincter complex, and it is through that fat that infection so readily spreads from one side of the canal to the other.
The line itself: where the gut met the skin
In the embryo, the hindgut ends blindly in a chamber called the CLOACA, which is partitioned by the urorectal septum into a urogenital sinus in front and an anorectal canal behind. That endodermal tube is separated from the outside world by a thin ANAL MEMBRANE. Meanwhile the surface ectoderm dimples inwards towards it, forming the PROCTODEUM — an ingrowth of skin. Around the eighth week the anal membrane breaks down and the two tubes become one continuous passage. But the join leaves a mark. The PECTINATE LINE — also called the DENTATE LINE, and lying at the level of the ANAL VALVES — is the anatomical remnant of that anal membrane: the visible watermark of where endoderm ended and ectoderm began. It sits roughly at the midpoint of the canal, about 2 cm above the anal verge and about 4 cm from the skin surface along the canal's axis. Nothing about it is mechanical. It is a border in the political sense: on either side of it, the tissue was supplied by a different developmental system, and every supply line drawn to it — arterial, venous, lymphatic and nervous — respects the border to this day.
This is the table that the rest of the article, and the rest of anorectal medicine, hangs from. ABOVE THE PECTINATE LINE the canal is HINDGUT, derived from ENDODERM. Its epithelium is simple COLUMNAR (mucosa, like the rest of the bowel). Its artery is the SUPERIOR RECTAL artery, the terminal branch of the inferior mesenteric artery. Its veins drain by the superior rectal vein into the inferior mesenteric vein and so into the PORTAL system — making this one of the classic sites of PORTOSYSTEMIC ANASTOMOSIS described in the portal system and inferior vena cava. Its lymph drains upwards to the INTERNAL ILIAC nodes. And its nerve supply is AUTONOMIC, from the inferior hypogastric (pelvic) plexus — visceral afferents that register STRETCH and nothing else. Because there is no somatic sensation at all, a lesion above the line is PAINLESS: this is where INTERNAL HAEMORRHOIDS arise, and it is why they can bleed for a year without ever announcing themselves. BELOW THE PECTINATE LINE the canal is PROCTODEUM, derived from ECTODERM — it is, developmentally, skin. Its epithelium is STRATIFIED SQUAMOUS (non-keratinised at first, becoming true keratinised skin at the verge). Its artery is the INFERIOR RECTAL artery, a branch of the internal pudendal artery from the internal iliac artery. Its veins drain by the inferior rectal vein into the internal pudendal vein and the internal iliac vein — the SYSTEMIC (caval) circulation. Its lymph drains downwards and outwards to the SUPERFICIAL INGUINAL nodes, in the groin. And its nerve supply is SOMATIC, by the INFERIOR RECTAL branch of the PUDENDAL nerve — the same nerve described in the pudendal nerve — carrying touch, temperature and pain from an epithelium as richly innervated as the fingertip. A lesion below the line is therefore exquisitely PAINFUL: this is the territory of ANAL FISSURES and EXTERNAL HAEMORRHOIDS.
Think of a border between two countries that were united a long time ago. The frontier posts came down, the road runs straight through, and a traveller walking the road notices nothing at all. But the two halves were built by different administrations, and they never merged their infrastructure: the water mains on the north side come from one reservoir and on the south from another; the north drains into one river system and the south into a different one; post from the north goes to one sorting office and post from the south to another; and — most consequential of all — the north was wired for a cheap party-line telephone that can report only that something heavy is pressing on the ground somewhere, while the south was wired with a full alarm network that reports precisely what happened and exactly where. Set a fire on the north side and the capital receives a vague murmur. Set an identical fire ten metres south and every siren in the country goes off. Nothing about the fire changed. Only the wiring did.
Every treatment decision in the anal canal is really a question about which side of the line you are on. A rubber band applied to an internal haemorrhoid is placed ABOVE the pectinate line, on insensate columnar mucosa, and the patient feels a dull fullness at worst — place the same band a centimetre lower, on squamous epithelium supplied by the pudendal nerve, and it is unbearable. An injection of sclerosant, a stapled haemorrhoidopexy, a diathermy point: all of them are safe above the line and cruel below it. Turn the rule around and it also becomes a diagnostic tool. A patient with painless bleeding has disease above the line; a patient with pain on defecation has disease below it. And when anal canal carcinoma spreads, ask which lymphatics served the tumour's origin: above the line it goes to the internal iliac nodes deep in the pelvis, invisible to the examining hand, while below the line it goes to the superficial inguinal nodes, which you can palpate in the groin in an outpatient clinic in thirty seconds.
Columns, valves, sinuses — and the glands that cause the trouble
Look inside the upper half of the canal and it is not smooth. Six to ten longitudinal ridges of mucosa run down it, the ANAL COLUMNS (columns of Morgagni), each carrying within it a terminal branch of the superior rectal vessels. The lower ends of adjacent columns are joined by small crescentic folds of mucosa, the ANAL VALVES, and it is the row of these valves that draws the scalloped, tooth-like line for which the DENTATE line is named. Behind each valve is a small pocket opening upwards: the ANAL SINUS, or crypt. Into these crypts open the ducts of the ANAL GLANDS — four to ten branched, mucus-secreting glands whose bodies burrow outwards and downwards through the internal sphincter to lie in the INTERSPHINCTERIC PLANE, the potential space between the internal and external sphincters. This tiny piece of anatomy is responsible for a large fraction of colorectal surgical practice. A crypt becomes blocked, the gland behind it becomes infected, and pus has nowhere to go except along the plane the gland already occupies. That is the CRYPTOGLANDULAR THEORY: essentially all perianal abscesses, and the fistulas that follow them, begin in an anal gland at the level of the pectinate line. Which is also why the internal opening of an anal fistula is almost always found exactly there — and why draining the pus without dealing with that crypt simply buys a few months before it returns.
Two sphincters, two nervous systems, two jobs
One holds the door shut all day without being asked; the other slams it when you tell it to. The INTERNAL ANAL SPHINCTER is not a separate muscle at all: it is the terminal thickening of the CIRCULAR SMOOTH MUSCLE of the gut, continuing down from the rectum and ending about a centimetre above the external sphincter's lower edge, so that the step between them can be felt as the intersphincteric groove. It is INVOLUNTARY, supplied by autonomic nerves, and it is unusual among smooth muscles in that it is held in continuous TONIC CONTRACTION — sympathetic fibres from L1–L2 maintain that tone, while parasympathetic input relaxes it. It contributes most of the canal's RESTING pressure, on the order of 70–85%, which is to say that the thing keeping you continent while you sleep, or read, or forget entirely that the question exists, is a strip of smooth muscle acting entirely without you. The EXTERNAL ANAL SPHINCTER is skeletal muscle, arranged as a cylinder around the canal in three imperfectly separable parts — SUBCUTANEOUS (encircling the lowest canal, below the internal sphincter), SUPERFICIAL (elliptical, anchored to the perineal body in front and the anococcygeal ligament and coccyx behind), and DEEP (a thick ring blending above with puborectalis). It is VOLUNTARY, supplied by the INFERIOR RECTAL branch of the PUDENDAL nerve (S2–S4), and it generates SQUEEZE pressure — the extra clamp you apply for the thirty or sixty seconds it takes to reach a toilet. It cannot be held indefinitely: like any skeletal muscle it fatigues, which is precisely why the involuntary sphincter must do the long shift. Between the two runs the CONJOINT LONGITUDINAL MUSCLE, the continuation of the rectal longitudinal muscle mixed with fibres from levator ani, which fans out to anchor the canal and divides the ischioanal fat into compartments; it is this fibrous fan that guides tracking pus into its characteristic fistula pathways. Above and behind, the PUBORECTALIS sling of the pelvic floor blends with the deep part of the external sphincter, so the two work as one functional unit.
The cushions, and the real work of continence
A sphincter closing a tube leaves a small gap, the way a purse-string never quite seals a bag. The body's solution is the ANAL CUSHIONS: three specialised vascular pads of submucosa in the upper canal, containing arteriovenous communications from the superior rectal vessels, held in place by smooth muscle and connective tissue (Treitz's muscle) and positioned — in the lithotomy position, conventionally — at 3, 7 and 11 o'clock. They engorge and act as a compressible washer, contributing perhaps 15–20% of resting closure and, crucially, providing the fine seal that keeps flatus and liquid in. They are NORMAL structures present in everyone from birth. This is the single most important correction to make about haemorrhoids: they are not varicose veins of the anus. They are anal cushions that have become enlarged, congested and displaced downwards as their supporting tissue degenerates with straining and age. Continence itself is a layered system, and no single component carries it: the resting tone of the internal sphincter; the ANORECTAL ANGLE, about 80–90 degrees at rest, maintained by the puborectalis sling pulling the junction forwards so that the rectum's own contents press against the front wall of the canal like a kink in a hose; the seal of the cushions; the reservoir compliance of the rectum; and the voluntary squeeze of the external sphincter in reserve. Layered on top is a beautiful piece of reflex physiology. When stool enters the rectum, distension triggers the RECTOANAL INHIBITORY REFLEX — the internal sphincter relaxes automatically. If nothing else happened, that would be incontinence. But the same distension simultaneously provokes a reflex CONTRACTION of the external sphincter, and in the moment the internal sphincter opens, a tiny amount of content is allowed up into the sensitive zone just below the pectinate line, where somatic epithelium performs the SAMPLING REFLEX: it identifies whether the arrival is solid, liquid or gas, and reports the answer to consciousness. That is the mechanism by which you know, without looking and without thinking, whether it is safe to pass wind. Defecation then reverses everything voluntarily: a posture that straightens the anorectal angle, a Valsalva effort that raises intra-abdominal pressure, relaxation of puborectalis to open the angle, and relaxation of the external sphincter — the only step in the whole sequence you truly command.
- The anal canal is about 4 cm long, from the anorectal junction to the anal verge, directed downwards and BACKWARDS towards the coccyx — the reason every instrument must be aimed towards the umbilicus first and then swung posteriorly. At rest it is closed, not patent.
- The PECTINATE (dentate) line lies at the level of the anal valves and is the remnant of the ANAL MEMBRANE — the junction of endodermal hindgut above with ectodermal proctodeum below.
- ABOVE the line: columnar epithelium · superior rectal artery (from the inferior mesenteric) · superior rectal vein → PORTAL · internal iliac nodes · AUTONOMIC (inferior hypogastric plexus), stretch only → lesions are PAINLESS → internal haemorrhoids.
- BELOW the line: stratified squamous epithelium · inferior rectal artery (from the internal pudendal) · inferior rectal vein → SYSTEMIC/caval · superficial inguinal nodes · SOMATIC (inferior rectal branch of the pudendal nerve) → lesions are PAINFUL → fissures and external haemorrhoids.
- The superior rectal (portal) and inferior rectal (systemic) veins meet here, making the anal canal a classic site of PORTOSYSTEMIC ANASTOMOSIS.
- Anal COLUMNS (of Morgagni) are joined below by anal VALVES; behind each valve is an anal SINUS (crypt) receiving the duct of an ANAL GLAND whose body lies in the INTERSPHINCTERIC PLANE — the origin of nearly every perianal abscess and fistula.
The canal in the clinic
Almost every anorectal complaint is the pectinate line telling you where it is. INTERNAL HAEMORRHOIDS are enlarged anal cushions above the line: painless bright red bleeding that drips or streaks, sometimes prolapse, mucus discharge and pruritus — graded 1 (bleeding only, no prolapse), 2 (prolapse on straining, reduces spontaneously), 3 (prolapse requiring manual reduction) and 4 (irreducible). Because the mucosa there is insensate, they are treated in the clinic: rubber-band ligation, sclerotherapy or infrared coagulation, all applied ABOVE the line where the patient cannot feel them — placing a band below it is an error that produces immediate severe pain and demands removal. EXTERNAL haemorrhoids lie below the line, covered by squamous epithelium supplied by the pudendal nerve, and are painful when they thrombose: an acutely tender blue-black lump at the anal margin. ANAL FISSURE is a longitudinal tear of the squamous epithelium below the line, and in over 90% of cases it sits in the POSTERIOR MIDLINE — the point where the external sphincter's fibres decussate, leaving the mucosa least supported, and where the terminal blood supply is poorest. Its pathophysiology is a self-perpetuating loop: pain triggers spasm of the internal sphincter, spasm raises resting anal pressure, high pressure further reduces perfusion to the already ischaemic posterior midline, and poor perfusion prevents healing — so treatment is directed not at the tear but at the spasm, with topical glyceryl trinitrate or diltiazem to relax the internal sphincter chemically, and lateral internal sphincterotomy when it fails. A fissure that is lateral, multiple or atypical should raise suspicion of Crohn's disease, tuberculosis or malignancy.
A man attends with a small discharging opening in the perianal skin, two centimetres from the anus and behind it. This is a FISTULA-IN-ANO — a chronic track that began as an infected anal gland at the pectinate line, tracked through the intersphincteric plane, discharged as a perianal abscess, and left a tube joining an internal opening in a crypt to an external opening in the skin. Fistulas are classified by their relationship to the sphincters (intersphincteric, transsphincteric, suprasphincteric and extrasphincteric), because the surgeon's problem is always the same: reach the track without dividing enough external sphincter to make the patient incontinent. To find the internal opening, GOODSALL'S RULE is the classic guide. Draw an imaginary transverse line across the anus. An external opening ANTERIOR to that line usually connects to the canal by a short, straight, radial track. An external opening POSTERIOR to it usually curves and opens into the midline posteriorly. The reason is anatomical, not magical: posterior tracks follow the curved intersphincteric planes back towards the posterior midline crypts, where most glands are concentrated. Note also what the rule assumes — a simple cryptoglandular fistula. In Crohn's disease, tuberculosis, malignancy, or after obstetric injury, the tracks obey the disease, not the rule.
ANAL CANAL CARCINOMA obeys the line as faithfully as anything else. Tumours arising ABOVE the pectinate line drain to the internal iliac (and thence para-aortic) nodes and are typically adenocarcinoma near the transitional zone; tumours arising BELOW it are usually squamous cell carcinoma and drain to the SUPERFICIAL INGUINAL nodes — which is why an inguinal lymph node is part of every anal cancer examination and staging scan, and why the same finding would be irrelevant for a rectal tumour a few centimetres higher. OBSTETRIC SPHINCTER INJURY is the other great clinical theme: the anterior perineum and the perineal body are directly in the path of the presenting fetal head, and a third-degree tear involves the anal sphincter complex (3a under 50% of the external sphincter, 3b over 50%, 3c involving the internal sphincter) while a fourth-degree tear breaches the anal epithelium as well. Because the internal sphincter carries resting tone, an unrecognised 3c injury classically presents years later as passive faecal soiling and urgency, a course explored in continence and pelvic organ prolapse. Finally, DIGITAL RECTAL EXAMINATION uses all of this at once. The examining finger, introduced along the canal's backward axis, first meets resting tone — firm, unyielding, and a property of the involuntary internal sphincter; then the patient is asked to squeeze, and the voluntary external sphincter tightens around the finger, testing the pudendal nerve and its S2–S4 roots. Loss of resting tone points to the internal sphincter or its autonomic supply; loss of squeeze points to the external sphincter or the pudendal nerve. A tender, boggy, fluctuant swelling is an abscess; a hard, irregular, fixed mass at the anorectal junction requires biopsy, not reassurance.
- The INTERNAL anal sphincter is a thickening of circular SMOOTH muscle, INVOLUNTARY, held in tonic contraction by sympathetic (L1–L2) tone, and provides about 70–85% of RESTING anal pressure.
- The EXTERNAL anal sphincter is SKELETAL muscle in subcutaneous, superficial and deep parts, VOLUNTARY, supplied by the inferior rectal branch of the PUDENDAL nerve (S2–S4), and provides SQUEEZE pressure; the conjoint longitudinal muscle lies between the two and PUBORECTALIS blends with the deep part.
- The ANAL CUSHIONS at 3, 7 and 11 o'clock (lithotomy) are NORMAL vascular structures contributing to the fine seal of continence; haemorrhoids are their pathological enlargement and downward displacement, NOT varicose veins.
- Continence = internal sphincter resting tone + the anorectal angle held by puborectalis + the cushions + rectal compliance + voluntary squeeze in reserve. The rectoanal inhibitory reflex relaxes the internal sphincter on distension; the SAMPLING reflex lets the sensitive zone below the line identify solid, liquid or gas.
- Internal haemorrhoids: painless bleeding, graded 1–4, banded ABOVE the line. External haemorrhoids: painful when thrombosed. ANAL FISSURE: posterior midline in over 90%, exquisitely painful, driven by a pain → sphincter spasm → ischaemia → non-healing loop — treated by relaxing the internal sphincter.
- Anal cancer above the line spreads to INTERNAL ILIAC nodes; below the line to SUPERFICIAL INGUINAL nodes — so the groin is examined in every anal carcinoma. Obstetric third- and fourth-degree tears involving the internal sphincter (3c) classically present years later with passive soiling.
- Calling haemorrhoids varicose veins of the anus. They are enlarged, displaced ANAL CUSHIONS — normal vascular structures present in everyone — and they contain arteriovenous communications, which is why haemorrhoidal bleeding is bright red arterial blood, not dark venous blood.
- Assuming that painless rectal bleeding is 'just haemorrhoids'. Painlessness only localises the lesion to ABOVE the pectinate line — and a rectal or upper anal carcinoma is also above it. The absence of pain is an anatomical statement, never a reassurance.
- Treating the tear in an anal fissure instead of the spasm. The lesion is maintained by a loop of pain, internal sphincter spasm and ischaemia of the poorly perfused posterior midline; creams that soothe without relaxing the internal sphincter do not heal it.
A 44-year-old man has a painless, bright red rectal bleed and is found to have a second-degree internal haemorrhoid. He is treated with rubber-band ligation and feels nothing during the procedure. Which single set of features correctly describes the tissue on which the band was placed?
- The anal canal is about 4 cm long, runs from the anorectal junction to the anal verge, is directed downwards and BACKWARDS towards the coccyx, and is normally held closed by its sphincters and anal cushions.
- The PECTINATE (dentate) line at the anal valves is the remnant of the anal membrane. ABOVE: hindgut/endoderm, columnar epithelium, superior rectal artery, portal venous drainage, internal iliac nodes, autonomic innervation → PAINLESS internal haemorrhoids. BELOW: proctodeum/ectoderm, stratified squamous epithelium, inferior rectal artery, systemic drainage, superficial inguinal nodes, somatic pudendal innervation → PAINFUL fissures and external haemorrhoids.
- The anal columns end in valves; behind each valve an anal sinus receives an anal gland lying in the intersphincteric plane — the origin of nearly all perianal abscesses and fistulas (Goodsall's rule). The internal sphincter (smooth, involuntary) supplies ~70–85% of resting tone; the external sphincter (skeletal, pudendal S2–S4) supplies squeeze; puborectalis holds the anorectal angle.
- Clinically: band internal haemorrhoids ABOVE the line only; anal fissure is posterior-midline and is treated by relaxing the internal sphincter; anal cancer above the line goes to internal iliac nodes and below it to superficial inguinal nodes; and a missed obstetric internal sphincter tear shows up years later as passive faecal soiling.
- Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students — Pelvis and Perineum: the anal canal, pectinate line and anal sphincters.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Anal canal: arterial supply, venous and lymphatic drainage, innervation above and below the pectinate line; haemorrhoids.
- Netter FH. Atlas of Human Anatomy — Coronal section of the anorectum; anal columns, valves and sphincter complex.
- Last RJ. Last's Anatomy: Regional and Applied — The anal canal, anal glands and the ischioanal fossa.
- Snell RS. Clinical Anatomy by Regions — Anal fissure, perianal abscess, fistula-in-ano and Goodsall's rule.
- TeachMeAnatomy — The Anal Canal; The Anal Sphincters and the Anorectal Angle.
