Atlas of Human Anatomy by Netter

from ectoderm (proctodeum)

Inferior half lined by nonkeratinized squamous epithelium (skin)

Vascular, nerve and lymphatic supply of these two regions different as a result of different embryologic origin

Vascular, Lymphatic, and Nerve Supply to the Anal Canal
Supply Above Pectinate Line Below Pectinate Line
Arterial Superior rectal artery Inferior rectal arteries
Venous Internal plexus drains to superior rectal vein and portal
system

Internal venous plexus drains to inferior rectal veins and

caval system

LymphaticTo internal iliac and common iliac and lumbar nodes To superficial inguinal nodes

Nerve From inferior hypogastric (pelvic) plexus (both sympathetic

and parasympathetic; sensitive to stretching only)

From inferior rectal nerves, branches of the pudendal

nerves (sensitive to pain, temperature and touch)

Has two sphincters: external (voluntary) and internal (involuntary)

External sphincter

Described as having superficial, subcutaneous, and deep parts

Parts are not readily distinguishable

Fibers attach to the central perineal tendon (body) anteriorly and anococcygeal ligament posteriorly

Internal sphincter: internal circular muscular layer continuous from rectum around upper two thirds of anal canal

Anorectal Musculature

Ischiorectal (Ischioanal) fossae
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Fat filled, wedge-shaped recesses either side of anal canal

Communicate with each other posteriorly over the anococcygeal ligament

Bounded by

Laterally: ischium and obturator internus muscle and fascia

Medially: anal canal surrounded by external anal sphincter

Anteriorly by external sphincter urethrae and deep transverse perineal muscles

Superiorly by pelvic diaphragm

Inferiorly by superficial perineal fascia and skin

Filled with fat spanned by fibrous bands to support anal canal but compressible during defecation

On lateral walls are found internal pudendal vessels and pudendal nerve within pudendal (Alcock's) canal in the fascia of the internal

surfaces of obturator muscles

Extends anteriorly superior to perineal membrane as anterior recesses of ischiorectal (ischioanal) fossae

Important site abscess formation