Overview Anal canal divided into upper 2/3 (visceral portion) that belongs to the intestines and lower 1/3 (somatic portion) that belongs to the perineum upper 2/3 is derived from the hindgut, endodermal in origin lower 1/3 is derived from the proctodeum, ectodermal in origin pectinate (dentate) line is the junction between endoderm and ectoderm and is located at the inferior limit of the anal valves pectinate line divides the visceral and somatic portion Upper 2/3 above pectinate line simple columnar epithelium upper 2/3 anal canal is the site of adenocarcinoma arterial supply via the superior rectal artery, a continuation of IMA venous drainage superior rectal vein → inferior mesenteric vein → portal system lymphatic drainage to the internal iliac lymph nodes internal hemorrhoids occur above the pectinate line and are covered by the mucous membrane receive visceral innervation and therefore are not painful may be a sign of portal hypertension Lower 1/3 below pectinate line stratified squamous epithelium lower 1/3 of the anal canal is the site of squamous cell carcinoma arterial supply via the inferior rectal arteries, branches of the internal pudendal arteries venous drainage inferior rectal vein → internal pudendal vein → internal iliac vein → IVC lymphatic drainage to the superficial inguinal lymph nodes external hemorrhoids occur below the pectinate line and are covered by the skin receive somatic innervation by the inferior rectal nerve, a branch of the pudendal nerve, and therefore are painful