Perineum – Anal Triangle Pelvis & Perineum Unit Lecture 6 د. حيدر جليل الأعسم

Perineum It is part of the pelvic cavity below pelvic diaphragm. Perineum is diamond shaped and is bounded anteriorly by symphysis pubis, posteriorly by tip of coccyx, and laterally by ischial tuberosities. It is divided by imaginary line between the ischial tuberosities into: Anterior urogenital triangle Posterior anal triangle

Perineum - Anal triangle Anal triangle is bounded behind by tip of coccyx and on each side by ischial tuberosity and sacrotuberous ligament, overlapped by border of gluteus maximus muscle. Anus, or lower opening of anal canal, lies in midline, and on each side is ischiorectal fossa. Skin around anus is supplied by inferior rectal (hemorrhoidal) nerve. Lymph vessels of skin drain into medial group of superficial inguinal nodes.

Perineum - Anal triangle Anal Canal It is about 1.5 in. (4 cm) long and passes downward and backward from rectal ampulla to anus. Except during defecation, its lateral walls are kept in apposition by levatores ani muscles and anal sphincters. Relations Posteriorly: anococcygeal body, which is a mass of fibrous tissue lying between anal canal and coccyx Laterally: ischiorectal fossae Anteriorly: In male, perineal body, urogenital diaphragm, membranous part of urethra, and bulb of penis. In female, perineal body, urogenital diaphragm, and lower part of vagina.

Structure of Anal Canal Mucous membrane of upper half of anal canal: is derived from hindgut endoderm & has following features: 1. lined by columnar epithelium. 2. thrown into vertical folds (anal columns), joined together at their lower ends by small semilunar folds (anal valves) 3. Nerve supply is like rectal mucosa by (hypogastric plexuses) & it is sensitive only to stretch. 4. Arterial supply: Superior rectal artery, a branch of (IMA). 5. Venous drainage: Superior rectal vein, tributary of (IMV). 6. Lymphatic drainage is along superior rectal artery to pararectal nodes

Structure of Anal Canal Mucous membrane of lower half of anal canal: is derived from ectoderm of proctodeum & has following features: 1. It is lined by stratified squamous epithelium, which gradually merges at anus with perianal epidermis. 2. There are no anal columns. 3. Nerve supply is from inferior rectal nerve; sensitive to pain, temperature, touch & pressure. 4. Arterial supply: inferior rectal artery, a branch of internal pudendal artery. 5. Venous drainage: inferior rectal vein, a tributary of internal pudendal vein, which drains into internal iliac vein. 6. Lymph drainage: medial group of superficial inguinal nodes. Pectinate line: is upper half of anal canal joins lower half.

Structure of Anal Canal Muscle Coat: outer longitudinal & inner circular layer of smooth muscle. Anal Sphincters: part of circular muscle coat and has an involuntary internal sphincter & a voluntary external sphincter. A. Internal sphincter: is formed from a thickening of smooth muscle of circular coat at upper end of anal canal. It is enclosed by a sheath of striped muscle that forms voluntary external sphincter. B. External sphincter can be divided into three parts: 1. Subcutaneous part: encircle lower end of anal canal & has no bony attachments 2. Superficial part: attached to coccyx behind & perineal body in front 3. Deep part: encircles upper end of anal canal and has no bony attachments Puborectalis of two levatores ani muscles blend with deep part of external sphincter.

Structure of Anal Canal Longitudinal smooth muscle of anal canal is continuous above with that of rectum. It forms a continuous coat around anal canal and descends in interval between internal and external anal sphincters. Some of longitudinal fibers are attached to mucous membrane of anal canal, whereas others pass laterally into ischiorectal fossa or are attached to perianal skin. At junction of rectum and anal canal, internal sphincter, deep part of external sphincter and puborectalis muscles form distinct ring (anorectal ring) which can be felt on rectal examination.

Anal Canal Blood Supply & Lymphatics of Anal Canal Arteries: upper ½ by Superior rectal artery (branch of IMA) lower ½ by inferior rectal artery (branch of Internal pudendal artery) Veins: Upper ½ is drained by superior rectal vein into inferior mesenteric vein Lower ½ is drained by inferior rectal vein into internal pudendal vein. Lymph Drainage: Upper ½ drains into pararectal nodes then inferior mesenteric nodes. Lower ½ drains into superficial inguinal nodes.

Nerve Supply of Anal Canal Mucous membranes: upper ½ is sensitive to stretch and is innervated by hypogastric plexuses Lower ½ is sensitive to pain, temperature, touch, and pressure and is innervated by inferior rectal nerves. Involuntary internal sphincter is supplied by sympathetic fibers from inferior hypogastric plexuse. Voluntary external sphincter is supplied by inferior rectal nerve, a branch of pudendal nerve, and perineal branch of 4th sacral nerve.

Anal triangle - Ischiorectal Fossa Ischiorectal fossa (ischioanal fossa) It is a wedge-shaped space located on each side of anal canal. Base of wedge is superficial and formed by skin. Edge of wedge is formed by junction of medial and lateral walls. Medial wall is formed by sloping levator ani muscle and anal canal. Lateral wall is formed by lower part of obturator internus muscle, covered with pelvic fascia. Contents of Fossa: 1. Dense fat: supports anal canal. 2. Pudendal nerve & internal pudendal vessels: are embedded in a fascial canal, pudendal canal, on lateral wall of ischiorectal fossa, on medial side of ischial tuberosity. 3. Inferior rectal vessels & nerve: cross fossa to reach anal canal.

Perineum - Nerves Pudendal Nerve: branch of sacral plexus, leaves main pelvic cavity through greater sciatic foramen & then enters perineum through lesser sciatic foramen. Nerve passes forward in pudendal canal. Branches Inferior rectal nerve: runs medially across ischiorectal fossa & supplies external anal sphincter, mucous membrane of lower ½ of anal canal & perianal skin. Dorsal nerve of the penis (or clitoris) Perineal nerve: supplies muscles in urogenital triangle & skin on posterior surface of scrotum or labia majora.

Perineum - Vessels Internal Pudendal Artery: It is a branch of internal iliac artery & passes from pelvis through greater sciatic foramen & enters perineum through lesser sciatic foramen. Branches: Inferior rectal artery: supplies lower ½ of anal canal. Branches to the penis in male & Branches to labia & clitoris in female Internal pudendal vein: receives tributaries that correspond to branches of internal pudendal artery.

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