Session 1B: The Pelvis, Rectum, Pelvic Floor And Perineum

Question 1

What are the functions of bony the pelvis?

Answer 1

The bony pelvis has numerous functions including supporting the spine, torso, and upper body; locomotion; and housing and protecting the pelvic viscera.

Question 2

What is the bony pelvis made up of?

Answer 2

It is made up of three bones: the sacrum and the left and right hip bones. The hip bone is composed of three smaller bones that fuse together: the ilium, ischium and pubis.

Question 3

What are the five articulations of the pelvis?

Answer 3

• Hip joint - between the head of the femur and the acetabulum (socket) of the pelvis. The ilium, ischium, and pubis of the hip bone fuse at the acetabulum.
• Sacroiliac joint – between the sacrum and the ilium of the hip bone. This joint is very stable and strong and is supported by many ligaments.
• Pubic symphysis – between the two pubic bones at the front of the pelvis. Very little movement is permitted at the pubic symphysis.
• Lumbosacral joint – between the 5th lumbar vertebra and the sacrum (an intervertebral disc lies between the two).
• Sacrococcygeal joint – between the sacrum and the coccyx.

Question 4

What are the bony landmarks of the pelvis?

Answer 4

• Iliac crest - when palpating the iliac crests from the back, a line drawn between them at their highest point (the intercristal line) marks the level of the L4/L5 disc space. This is useful as this is a position where a lumbar puncture can be performed, or an epidural injection given.
• Anterior superior iliac spine (ASIS) - this is the most anterior point of the ilium and is palpable in almost all patients.
• Iliac tubercle - this is the most lateral point of the ilium. A line drawn between the iliac tubercles (the ‘intertubercular line’) marks the division of the lower third of the abdomen from the middle third, thereby delineating the suprapubic region from the umbilical, and the iliac fossae from the flanks.
• Pubic tubercle - again palpable in most people, this is the most medial point of the pubic bone.
• Inguinal ligament - runs from the anterior superior iliac spine to the pubic tubercle.
• Mid-inguinal point - the mid-point of a line drawn from the anterior superior iliac spine to the pubic symphysis (not the pubic tubercle). The femoral artery is palpable here.

Question 5

What are the differences between the male and female bony pelvis?

Answer 5

There are several differences between male and female pelves that you may be able to identify on specimens or models in the MTU:
• Wider, circular pelvic inlet in females (for childbirth) and narrower, heart-shaped pelvic inlet in males.
• Obtuse (>90˚) angle formed by the inferior pubic rami in females, acute angle (<90˚) in males.
• Wider and shorter sacrum in females; a narrower, longer sacrum in males.

Question 6

What are the contents of the pelvis?

Answer 6

The pelvis contains organs of the gastrointestinal and genitourinary systems, including the bladder and rectum in both males and females, the uterus, ovaries and vagina in females and the prostate and seminal vesicles in males. It also contains numerous arteries, veins and nerves that supply the pelvic organs.

Question 7

What is the pelvic floor?

Answer 7

The pelvic floor is the collective name for several muscles that support the pelvic organs from below. The muscles of the pelvic floor separate the pelvis from the perineum.

Question 8

What are the primary functions of the pelvic floor muscles?

Answer 8

The pelvic floor muscles are shaped like a bowl or funnel and have three primary functions:
1. Prevent herniation of the pelvic organs inferiorly, out of the pelvis.
2. Control continence of urine and faeces by providing a sphincter action on the urethra and rectum respectively.
3. Aid in increasing intra-abdominal pressure.

Question 9

What are the muscles of the pelvic floor?

Answer 9

The pelvic floor is made up of two muscles: the levator ani and coccygeus. Levator ani is itself composed of three smaller paired muscles

Question 10

What are the muscles that compose the levator ani?

Answer 10

Levator ani is itself composed of three smaller paired muscles:
1. Puborectalis - this U-shaped muscle attaches to the pubic bones anteriorly and forms a sling around the rectum. When it contracts it pulls on the rectum so that a sharp angle is formed between the rectum and anal canal, preventing defecation. When it relaxes, the path from the rectum to the anal canal straightens and faeces can pass through. It contributes to control of micturition in a similar fashion. Puborectalis is the most anterior levator ani muscle.
2. Pubococcygeus - this muscle lies posterior and lateral to puborectalis. It attaches to the pubic bone anteriorly, and the coccyx and sacrum posteriorly.
3. Iliococcygeus - this muscle lies lateral to pubococcygeus. Its name is slightly misleading, as it attaches to the spines of the ischium (not the ilium) and the coccyx.

Question 11

What is the innervation of the levator ani?

Answer 11

Levator ani is innervated by a branch of the S4 nerve and by some branches of the pudendal nerve (from S2-S4).

Question 12

What is the arterial supply of the pelvis?

Answer 12

The arterial supply to the pelvis is via the left and right internal iliac arteries. The internal iliac arteries give rise to several branches in the pelvis that supply the pelvic viscera. Some branches of the internal iliac arteries exit the pelvis to supply the perineum and gluteal region.

Question 13

What are the key branches of the iliac arteries ?

Answer 13

Key branches of the internal iliac arteries are the:
• Vesical arteries - supply the bladder in both sexes, and the prostate and seminal vesicles in males.
• Uterine and vaginal arteries in females.
• Middle rectal artery - supplies the rectum.
• Internal pudendal artery - exits the pelvis to supply the perineum.
• Superior and inferior gluteal arteries - exit the pelvis to supply the gluteal region.
• Obturator artery – exits the pelvis to supply the lower limb.

Question 14

What is the venous drainage of the pelvis?

Answer 14

Veins of the Pelvis
The pelvis contains several venous plexuses which drain the pelvic organs. These plexuses unite and mostly drain into the internal iliac veins.

Question 15

What is the innervation of the pelvis?

Answer 15

The nerves of the pelvis are complex. Structures in the pelvis are supplied by somatic, parasympathetic, and sympathetic nerves. In the pelvis, the sacral spinal nerves come together to form the sacral plexus.

Question 16

What nerves arise from the sacral plexus?

Answer 16

Key nerves arising from the sacral plexus are the:
• Sciatic nerve - formed by the lower lumbar and sacral spinal nerves L4 - S3. It exits the pelvis and supplies the lower limb. We’ll come back to it when we study the lower limb.
• Pudendal nerve – this somatic nerve is derived from spinal nerves S2 - S4. It exits the pelvis and is the major nerve of the perineum.
• Superior and inferior gluteal nerves - these nerves exit the pelvis and innervate the gluteal region.
• Pelvic splanchnic nerves – these nerves carry parasympathetic fibres from the S2 - S4 spinal cord segments to the pelvic viscera.

Question 17

Where are sympathetic fibres derived from?

Answer 17

Sympathetic fibres that innervate the pelvic viscera are derived from the lumbar splanchnic nerves.

Question 18

What is the rectum?

Answer 18

The rectum is the terminal part of the large intestine. Its name derives from Latin for ‘straight intestine’. This is misleading as the rectum has two notable flexures: the sacral flexure anteriorly and the anorectal flexure posteriorly. The rectum is approximately 12cm long in an adult and its primary purpose is the storage of faeces prior to defecation.

Question 19

What is the blood supply of the rectum?

Answer 19

The rectum is supplied by superior, middle, and inferior rectal arteries. The superior rectal artery is a branch of the inferior mesenteric artery, the middle rectal artery is a branch of the internal iliac artery, and the inferior rectal artery is a branch of the internal pudendal artery (itself a branch of the internal iliac artery).

Question 20

What is the Venous drainage of the rectum?

Answer 20

Venous blood from the rectum enters the superior, middle, and inferior rectal veins. Branches of these veins anastomose with each other to form a venous plexus around the rectum and anus, making this a site of portosystemic anastomoses. In portal hypertension, these veins may become varicose.

Question 21

Where is the perineum?

Answer 21

Immediately superficial and inferior to the pelvic floor is the perineum, the superficial region between the pubic symphysis and the coccyx, and between the medial surfaces of the thighs.

Question 22

What can the perineum be split into?

Answer 22

Viewed from below, the perineum is roughly diamond-shaped and can be split into two triangles by drawing an imaginary line between the ischial tuberosities:
• The urogenital triangle (anteriorly) - this area is complicated and has several layers.
• The anal triangle (posteriorly) - contains the anus and the external anal sphincter.

Question 23

What does the urogenital triangle contain?

Answer 23

The urogenital triangle contains the urethral and vaginal openings, the erectile tissues that unite to form the clitoris or penis, and the muscles overlying them.

Question 24

What are the layers of the urogenital triangle contain?

Answer 24

Skin - the urethra and vagina open out through the skin
Perineal fascia - continuation of the fascia overlying abdominal walls
Superficial perineal pouch
Perineal membrane

Question 25

What is the superficial perineal pouch and perineal membrane?

Answer 25

Superficial perineal pouch - A potential space that contains the erectile tissues of the penis or clitoris and three muscles: ischiocavernosus, bulbospongiosus and the superficial transverse perineal muscles.

Perineal membrane - A strong fibrous membrane that provides support for the attachment of ischiocavernosus and bulbospongiosus muscles. • The urethra and vagina pass through it.

Question 26

What does teh anal triangle contain?

Answer 26

The anal triangle contains the anal canal and anus. The anal canal is the most distal part of the gastrointestinal tract and is approximately 4cm long in the adult. It extends from the distal rectum to the anus.

Question 27

What controls defecation?

Answer 27

Two anal sphincters control defecation. The internal anal sphincter is composed of smooth muscle and is involuntarily controlled, whilst the external anal sphincter is composed of skeletal muscle and is voluntarily controlled. The pectinate (dentate) line divides the anal canal into superior and inferior parts.

Question 28

What is the superior part of the anal canal?

Answer 28

The superior part of the anal canal, above the pectinate line, is part of the hindgut and derived from endoderm. Columnar epithelium (i.e. intestinal mucosa) lines the lumen. It is supplied by the inferior mesenteric artery via the superior rectal artery.

Question 29

What is the venous drainage of the anal canal like?

Answer 29

Venous drainage is via the portal venous system towards the liver. It is encircled by the internal anal sphincter which is innervated by sympathetic fibres (originating from the sympathetic trunk) and parasympathetic fibres (via the pelvic splanchnic nerves).

Question 30

What is the inferior part of the anal canal?

Answer 30

The inferior part of the anal canal, below the pectinate line, is derived from ectoderm. Stratified squamous epithelium lines the lumen. It is supplied by the middle and inferior rectal arteries which originate from the internal iliac arteries. Venous blood enters the systemic venous system and does not pass through the portal system. The inferior anal canal is encircled by the external (voluntary) anal sphincter which is innervated by the pudendal nerve (a somatic nerve).

Question 31

What is the perineal body?

Answer 31

Between the two triangles of the perineum sits the perineal body. This dense mass of fibrous tissue and muscle sits in the centre of the perineum and acts as an attachment for almost all the perineal and pelvic floor muscles, including levator ani, the external anal sphincter, the external urethral sphincter, and muscles associated with the erectile tissues of the perineum.

Question 32

What is pelvic floor weakness?

Answer 32

The pelvic floor muscles support the pelvic organs, but they can be stretched, damaged, or weakened by childbirth, ageing, straining or obesity. Pelvic floor weakness can lead to incontinence of urine or faeces. It can also lead to pelvic organ prolapse where the uterus prolapses into the vagina, or the rectum prolapses out of the anus, because they are no longer supported.

Question 33

What is an episiotomy?

Answer 33

During childbirth, the perineal body may be damaged due to stretching or tearing of perineal tissues. This can lead to incontinence or pelvic floor weakness. During labour, a pre-emptive incision - an episiotomy - can be made just lateral to the perineal body. This avoids injury to the perineal body or an uncontrolled tear through the anal sphincter.

Question 34

What are haemorrhoids?

Answer 34

The walls of the anal canal are lined with an abundance of veins that form ‘cushions’ which aid faecal continence. These cushions are known as haemorrhoids, or ‘piles’. They can become swollen and inflamed and, depending on whether they become inflamed above or below the pectinate line, they may be painless (above the pectinate line) or painful (below pectinate line).

If they are pathological, they often bleed when passing stool. If they are painless haemorrhoids, the presence of blood on wiping or blood dripping into the toilet after defecation may be the first sign that they are present. Increased intra-abdominal pressure, for example, in excessive straining, constipation, squatting or pregnancy, increases pressure in the pelvic veins and can lead to haemorrhoids becoming swollen and problematic.