DISSECTION 30
The Perineum, Anal Triangle
References: M1 402-406, 409-418, 453-459, 461-464, 567; N 340-346, 359-361, 363, 365-369, 376,
384-385, 391, 393, 477, 484-485; N352-359, 377-380, 382-386, 395, 404-405, 411, 413,
495, 502-503; R 338-341, 350-354
AT THE END OF THIS LABORATORY PERIOD YOU WILL BE RESPONSIBLE FOR THE
IDENTIFICATION AND DEMONSTRATION OF THE STRUCTURES LISTED BELOW:
1. Bones and related features: symphysis pubis, inferior pubic ramus, ischial ramus, ischial
tuberosity, ischial spine.
2. Features of external genitalia: scrotum and scrotal raphe, prepuce, frenulum, glans, body,
external urethral orifice, perineal raphe, mons pubis, labium majus, anterior and posterior labial
commissures, body of penis, body of clitoris, labium minus, vaginal vestibule, vaginal orifice.
3. Fasciae and spaces of the perineum: ischiorectal (ischioanal) fossa and its anterior recess,
central tendon of the perineum.
4. Muscles: levator ani, gluteus maximus, external anal sphincter, obturator internus, piriformis.
5. Nerves: pudendal, posterior femoral cutaneous and its perineal branch, inferior rectal.
6. Vessels: internal pudendal, inferior rectal artery and vein.
7. Ligaments: sacrotuberous, sacrospinous.
YOU SHOULD ALSO BE ABLE TO DO THE FOLLOWING THINGS:
1. Give the bony landmarks which delineate the perineum and divide it into anal and urogenital
triangles.
2.
Demonstrate on the cadaver the boundaries of the ischiorectal (ischioanal) fossa including its
anterior and posterior recesses.
3. Identify on the cadaver the nerves and arteries passing through the ischiorectal fossa and passing
in its walls.
4. Discuss the arterial supply and venous drainage of the rectum and anal canal.
5. Distinguish between internal and external hemorrhoids.
Although the term perineum has two
meanings, an anatomical and a clinical, it will be
used here in the anatomical sense to mean the
region external to the inferior pelvic outlet. This
region is bounded by the arch and inferior rami of
the pubic bones, the tuberosities of the ischia,
medial borders of the gluteus maximus muscles,
and the tip of the coccyx. Superiorly, it is
bounded by the pelvic diaphragm and inferiorly
by the skin between the thighs and buttocks. A
line drawn transversely from one ischial
tuberosity to the other divides the region
arbitrarily into an anterior, urogenital triangle,
and a posterior, anal triangle. The anal triangle is
similar in the two sexes, but the urogenital
triangle is different by reason of the associated
Dissection 30, Perineum, Anal Triangle
urogenital organs (G3.42; N363; N380). Before
beginning the dissection of the perineum, identify
the following features of surface anatomy:
Male
(N 365, 369; N382, 386)
PENIS
PREPUCE
and its FRENULUM
GLANS
Page 2
Remove the skin from the perineum. In the
male incise the skin in the midline along the
scrotal raphe, and bluntly separate the right and
left portions of the scrotum. An incision should
encircle the anus to leave it in situ, with the
vulvar region similarly treated in the female.
Complete removal of the skin avoids bothersome
flaps in this restricted area (G3.44, 54; N344, 346;
N357, 359; A344, 346, 370). Since the anal
triangle will be dissected first, it is well to finish
the skin removal with the cadaver in the prone
position.
BODY
EXTERNAL URETHRAL ORIFICE
SCROTUM and SCROTAL RAPHE
Female
(N359; N377)
MONS PUBIS
LABIUM MAJUS (PLURAL, LABIA MAJORA)
LABIAL COMMISSURES, ANTERIOR and
POSTERIOR
CLITORIS
PREPUCE and
its FRENULUM
GLANS
To better view important neurovascular
structures entering the region of the anal triangle,
begin with a superficial exposure of the gluteal
region. Remove any remaining skin and tela
subcutanea of the buttock laterally to the
midaxillary line and inferiorly to include the skin
of the upper thigh. In these regions the tela
subcutanea can be reflected effectively only by
sharp dissection. Cutaneous nerves need not be
retained but be aware of them and note from atlas
illustrations their wide origin -- superior cluneal
nerves (posterior primary rami (L1, L2, L3), middle
cluneal nerves (posterior primary rami S1, S2, S3)
and inferior cluneal nerves (anterior primary rami
S1, S2, S3) from the POSTERIOR FEMORAL
CUTANEOUS NERVE.
Find and preserve the
POSTERIOR
BODY
LABIUM MINUS (PLURAL, LABIA MINORA)
EXTERNAL URETHRAL ORIFICE
VAGINAL VESTIBULE
VAGINAL ORIFICE
Male and Female
(N 340-342; N352-354)
FEMORAL
CUTANEOUS
NERVE
(A426; G5.3B; N476, 477, 522; N494, 495, 540).
It emerges from beneath the gluteal fold deep to
the fascia lata (the deep muscle fascia of the
thigh), which must be split to expose the nerve.
The PERINEAL BRANCH of the POSTERIOR
FEMORAL CUTANEOUS NERVE (N393; 485,
N413, N503) contributes to the innervation of the
cutaneous area of the perineum and is in a fascial
plane that must be taken into account by
gynecologists in effective anesthesia of this area.
SYMPHYSIS PUBIS
INFERIOR PUBIC RAMUS
ISCHIAL RAMUS
ISCHIAL TUBEROSITY
PERINEAL RAPHE
Complete the removal of fascia from the
GLUTEUS MAXIMUS. Free its upper margin and
transect the muscle through its middle at right
angles to the direction of its fibers. Be careful not
to damage the SACROTUBEROUS LIGAMENT
(G5.15; N477, 485; N495, 503; A427). By blunt
dissection reflect the muscle and separate it from
Dissection 30, Perineum, Anal Triangle
the gluteus medius. Use your scalpel to reflect
the gluteus maximus from the surface of the
sacrotuberous ligament (G5.24, 25; N477; N495;
A430).
Gluteal arteries and nerves entering the deep
surface of the muscle may be detached and
identified in detail in a later dissection. Identify
the PIRIFORMIS MUSCLE, inferior to which many
structures (including the large sciatic nerve) enter
the gluteal region. Deep to the sacrotuberous
ligament, locate the ISCHIAL SPINE and
SACROSPINOUS LIGAMENT.
Identify the
PUDENDAL NERVE and INTERNAL PUDENDAL
ARTERY where they cross the sacrospinous
ligament (G5.25; N485; N503; A430). The
pudendal nerve usually is located medial to the
internal pudendal vessels.
Start the dissection of the anal triangle by
removing the fat of the superficial fascia from the
ISCHIORECTAL (ISCHIOANAL) FOSSA (N344,
346; N357, 359). These fossae are bilateral spaces
separated medially by the anus and lower end of
the rectum, and by the anococcygeal raphe.
Extend your removal of fat anteriorly to
demonstrate a blind prolongation of the
ischiorectal fossa superior to the urogenital
diaphragm known as the ANTERIOR RECESS.
Other boundaries are:
superomedially, the
LEVATOR ANI MUSCLE and its inferior fascial
layer; posterolaterally, the GLUTEUS MAXIMUS
MUSCLE; anteriorly, the body of the pubis;
laterally, the obturator internus. As the fat is
removed, watch for the INFERIOR RECTAL
VESSELS and NERVES (N391, 393; N411, 413).
When found, trace them to their site of emergence
Page 3
from the lateral wall of the fossa, where they arise
from larger, pudendal structures which traverse
the pudendal (Alcock's) canal in the fascia of the
OBTURATOR INTERNUS MUSCLE.
Open the
fascia of the pudendal canal and follow the
pudendal nerve and internal pudendal vessels
posteriorly to their emergence from deep to the
sacrotuberous ligament. The pudendal canal ends
anteriorly at the posterior border of the urogenital
diaphragm and the PUDENDAL NERVE and
INTERNAL PUDENDAL VESSELS pierce the fascia
and enter the urogenital diaphragm. As they do
so they give rise to superficial nerves and arteries
to the perineum, named perineal nerves and
arteries and posterior scrotal (or labial) nerves
and arteries. Initially they are in the superficial
perineal space between the superficial perineal
fascia and the inferior fascia of the urogenital
diaphragm. These fascial layers will be clarified
in the next dissection of the urogenital triangle.
Identify the EXTERNAL ANAL SPHINCTER
(G3.46, 55; N344, 346; N357, 359; A344, 346,
370-371), a muscle that surrounds the anus at a
subcutaneous level, and trace its posterior
attachment to the coccyx. Note that its anterior
attachment blends with a fibrous mass, called the
CENTRAL TENDON OF THE PERINEUM or
"perineal body". (The obstetrical definition of the
term perineum is restricted to this structure.)
Examine the thickened part of the smooth muscle
that surrounds the inferior 2.5 cm of the anal
canal, and which is in contact with the external
sphincter. This band of muscle is the internal
anal sphincter. This may be seen more clearly
when the pelvis is split in dissection 33.
______________________________________________________________________________________________
STUDY QUESTIONS
1.
What are the boundaries of
the perineum?
1.
The boundaries of the perineum are:
1) the symphysis pubis,
2) the inferior pubic rami,
3) the ischial rami,
4) the ischial tuberosities,
5) the sacrotuberous ligaments, and
6) the coccyx.
Dissection 30, Perineum, Anal Triangle
Page 4
2.
What kind of muscle is the external
anal sphincter skeletal or smooth?
2.
It is skeletal muscle.
3.
How is the external anal sphincter
innervated?
3.
It is innervated by the perineal branch of the
fourth sacral nerve and by the inferior rectal
nerves. The cell bodies of the motoneurons which
supply it are located in the ventral gray column of
the mid sacral spinal cord, especially S4.
4.
From what artery does the inferior
rectal artery arise?
4.
Internal pudendal artery.
5.
What other arteries supply the
rectum?
5.
The middle and superior rectal arteries.
6.
What is the venous drainage of
the rectum?
6.
The venous drainage follows the arteries. A
plexus of veins surrounds the rectum and drains
chiefly into the superior rectal vein, which is a
tributary to the inferior mesenteric vein. The
paired middle and inferior rectal veins drain into
the internal iliac veins.
7.
What are hemorrhoids?
7
Hemorrhoids are varicose dilations of veins in the
rectal submucosal venous plexus.
Distinguish between internal
and external hemorrhoids.
Internal hemorrhoids lie above the pectinate line
and drain into the superior rectal vein. External
hemorrhoids are below the pectinate line and
drain into the inferior rectal veins.
8.
What are the boundaries of the
ischiorectal (ischioanal) fossa?
8
Medially--levator ani
Laterally--obturator internus
Posteriorly--gluteus maximus and sacrotuberous
ligament
Inferiorly--urogenital diaphragm and skin
9.
How far anteriorly does the anterior
recess of the ischiorectal (ischioanal)
fossa reach?
9.
To the body of the pubis.
10. What is the pudendal canal?
10. A passageway for the internal pudendal artery and
vein and the pudendal nerve in the obturator
internus fascia on the lateral wall of the
ischiorectal fossa (spine to tuberosity of ischium).
Dissection 30, Perineum, Anal Triangle
Page 5
11. Describe the course of the pudendal nerve and
internal pudendal artery proximal to the pudendal
canal.
11. The pudendal nerve and internal pudendal artery
exit the pelvis and enter the gluteal region through
the greater sciatic foramen inferior to the
piriformis. They exit the gluteal region and enter
the perineum by passing through the lesser sciatic
foramen. In this course they pass superficial to
the sacrospinous ligament and deep to the
sacrotuberous ligament.
12. Consider some of the structures that
could be damaged by an incision to
drain an abscess in the ischiorectal fossa.
12.
13. What does the inferior rectal
nerve supply?
13. External anal sphincter muscle and skin adjacent
to anus.
14. What nerve supplies the levator ani?
14. Nerve to the levator ani on its superior surface
direct from the sacral plexus (chiefly S4).
15. What nerves supply the scrotum?
15. Anterior scrotal nerves from the ilioinguinal
nerve, posterior scrotal from the pudendal nerve;
genital branch of genitofemoral supplies the
cremaster muscle as well as the skin, and laterally
the perineal branch of the posterior femoral
cutaneous.
16. Do the two ischiorectal (ischioanal)
fossae communicate?
16. Yes. The two fossae communicate with each
other posterior to the anal canal above a part of
the external anal sphincter that extends backward
to attach to the coccyx. Here an abscess in one
ischiorectal fossa may extend across to the other
fossa. (M1 416 and H793-794).
LJ:bh
revised
06/19/09