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Perineum – Anal Triangle
Pelvis & Perineum Unit
Lecture 6
‫ حيدر جليل األعسم‬.‫د‬
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:
A. Anterior urogenital triangle
B. 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.
Thank You