Abdomen 4

Question 1

What is the perineum defined as?

Answer 1

The anatomical region which is located inferiorly (below) the main pelvic cavity.

Question 2

How is the perineum separated from the main pelvic cavity?

Answer 2

Pelvic diaphragm (the levator ani muscles and the coccygeus muscles)

Question 3

Why must you be cautious when discussing anatomical directions relating to the perineum?

Answer 3

For the perineum, inferior structures are described as being more superficial as they are closer to the skin of the perineal region- e.g. the perineum is inferior to the pelvic diaphragm.

INFERIOR = SUPERFICIAL

Question 4

What types of structures does the perineum contain?

Answer 4

Structures of the genitourinary and GI systems.

Question 5

What shape is the perineum when observed inferiorly with the thighs abducted?

Answer 5

Diamond

Question 6

What 2 triangles can the diamond shaped perineum be divided into?

Answer 6

An anterior urogenital triangle and a posterior anal triangle.

Question 7

Describe the anatomical borders of the perineum.

Answer 7

Anterior Border: Pubic symphysis

Lateral Border: Ischiopubic rami and sacrotuberous ligament

Inferior Border: Tip of coccyx

Question 8

Describe the surface borders of the perineum.

Answer 8

Anterior Border: Mons pubis

Lateral Border: Medial surface of thigh

Inferior Border: Intergluteal cleft

Question 9

Describe the location, function and significance of the perineal body.

Answer 9

In the very centre of the perineum, in between the urogenital triangle and anal triangle is the perineal body.

This is a small, fibromuscular structure located either between the bulb of the penis and the anus, or between the vagina and the anus.

It is important for the structural support and functions of the perineum as it is a site of attachment for the muscles and fascia of the perineum.

Question 10

What is the puborectalis and where is it located?

Answer 10

The puborectalis muscle is a muscular sling that wraps around the lower rectum as it passes through the pelvic floor. It serves an important role in helping to maintain fecal continence and also has an important function during the act of having a bowel movement.

It is a muscle of the levator ani muscle group.

(BLENDS WITH UPPER MOST PART OF EXTERNAL ANAL SPHINCTER TO FORM ANORECTAL RING)

Question 11

Detail what the urethra has to pass through.

Answer 11

The urethra has to pass through the pelvic floor and into the perineum.

In the perineum it passes through the deep perineal pouch, perineal membrane and superficial perineal pouch before opening into the vestibule in front of the vagina, or it travels through within the penis where it opens via the external urethral orifice in the glans.

Question 12

List the layers of the urogenital triangle from deep to superficial.

Answer 12

• (NOT in the perineum but for orientation purposes) Pelvic diaphragm muscles (levator ani and coccygeus)
• Deep perineal pouch- containing the deep transverse perineal muscles and the external urethral sphincter. It also contains the urethra, and the vagina (female) and bulbourethral glands (male).
• Perineal membrane- provides attachment sites for the roots of the external genitalia
• Superficial perineal pouch- containing the ischiocavernosus, bulbospongiosus and superficial transverse perineal muscles. It also contains the erectile tissue for the penis or clitoris, and the greater vestibular glands (female).
• Deep perineal fascia
• Superficial perineal fascia
• Skin (openings for the urethra and vagina)

Question 13

Describe the deep perineal pouch, including its structures and innervation.

Answer 13

The deep perineal pouch is the first layer of the perineum that the urethra passes through.

The main components of the deep perineal pouch are a group of skeletal muscles which primarily act as urethral sphincters:
- Female - external urethral sphincter, sphincter urethrovaginalis, compressor urethrae
- Male - external urethral sphincter

The deep transverse perineal muscle is believed to stabilise the perineal body.

They are all innervated by the PUDENDAL NERVE (S2-4).

In the male, the bulbourethral glands are also present in the deep perineal pouch. These pea-sized glands sit one either side of the urethra and secrete a mucous like substance to lubricate the urethra

Question 14

Describe the function of the perineal membrane.

Answer 14

The perineal membrane is a thickened fascia which separates the deep and superficial perineal pouches.

It serves as an attachment point for the roots of the external genitalia and the muscles of the SUPERFICIAL PERINEAL POUCH.

The perineal membrane also transmits the urethra (and vagina if present) to the superficial perineal pouch below (remember, inferior = superficial).

Although considered to be part of the perineum, the perineal membrane assists with supporting the pelvic floor muscles due to its thickness.

Question 15

What are the main structures of the superficial perineal pouch?

Answer 15

Erectile tissue (penis or clitoris) and muscles.

Question 16

Detail the female erectile tissue of the superficial perineal pouch.

Answer 16

Clitoris (and the bulb of the vestibule)

The 2 CORPORA CAVERNOSA (crura) of the clitoris are the attached (to the bone) parts of the clitoris, they converge near to the pubic symphysis to form the BODY of the clitoris, which is not attached to the bone.

Additionally, there are the 2 VESTIBULAR BULBS in the superficial perineal pouch, which are also erectile tissue, and they converge to form the GLANS of the clitoris.

The glans of the clitoris is the only visible part of the clitoris in the perineum, however the body of the clitoris can be palpated under the skin.

Additionally, there are the GREATER VESTIBULAR GLANDS in the superficial perineal pouch, which secrete mucous into the vestibule of the vulva and around the opening of the vagina.

Question 17

Detail the male erectile tissue of the superficial perineal pouch.

Answer 17

Penis

The penis is also formed by 2 corpora cavernosa as well as an additional corpus spongiosum (the equivalent to the vestibular bulbs and glans of the clitoris).

The corpora cavernosa form the dorsal side, whilst the corpus spongiosum forms the ventral side of the penis.

THE CORPUS SPONGIOSUM IS NOT ERECTILE TISSUE as it contains the penile urethra (if this was erectile tissue it would compress the urethra inside it and prevent ejaculation).

The corpus spongiosum expands distally to form the glans of the penis.

The penis is made up of two regions:
1. the root of the penis = fixed ends of the corpora cavernosa and the corpus spongiosum
2. the body of the penis = covered by skin, free parts of the corpora cavernosa and the corpus spongiosum.

Question 18

Detail the muscles of the superficial perineal pouch.

Answer 18

The muscles found in both the male and female perineal pouch are:

• Ischiocavernosus (associated with root of penis/clitoris)- moves blood from CRURA of penis/clitoris into the BODY of the penis/ clitoris during erection
• Bulbospongiosus (associated with root of penis/clitoris)- move blood into the GLANS of the clitoris or penis and assists with movement of residual urine/ semen from the penile urethra
• Superficial transverse perineal muscle (associated with perineal body)- stabilise the perineal body

Question 19

Detail the perineal fascia (deep and superficial) and what they cover and are continuous with.

Answer 19

The perineal fascial layers are continuous with layers of abdominal fascia. The deep perineal fascia covers the muscles and erectile tissue of the superficial perineal pouch. The superficial perineal fascia is continuous with the superficial fascia of the anterior abdominal wall.

Question 20

Highlight the superficial features of the female external genitalia.

Answer 20

VULVA and associated structures

The clitoris and vestibular bulbs along with connective tissue and skin folds form the vulva.

VESTIBULE = The area where the urethra and vagina open

LABIA MINORA = either side of vestibule, two folds of (WITHOUT HAIR) skin. Singular = labium minus.

The labia minora unite anteriorly around the body and glans of the clitoris to form the FRENULUM (under, rarely visible) and PREPUCE (above) OF THE CLITORIS. Posteriorly the labia minora unite to form the frenulum of the vagina.

Lateral to the labia minora are the LABIA MAJORA which are larger folds of skin (WITH HAIR).

The labia majora unite anteriorly at the MONS PUBIS, and they do not unite posteriorly forming the posterior commissure (a surface marking for the perineal body lying deep to this area).

Paraurethral glands drain via their ducts at the urethral opening whilst the greater vestibular glands drain at the opening of the vagina.

Question 21

Highlight the superficial features of the male external genitalia (including developmental features of the testes)

Answer 21

Scrotum and associated structures

The SCROTUM is homologous to the labia majora in terms of their embryonic development, however during development the scrotum fuses in the midline to form a single scrotum into which the testes and their coverings and neurovasculature descend.

The line of fusion in the scrotum forms the RAPHE in the midline extending from the anus, over the scrotal sac and to the inferior aspect of the penis.

The EXTERNAL URETHRAL ORIFICE is located on the GLANS of the penis. The base of the glans forms a raised circular margin called the CORONA of the glans. The depression just below the corona is called the NECK of the glans and it is from here a fold of skin usually arises which covers the glans, the PREPUCE of the glans (aka the FORESKIN).

The TESTES are the gonads which reside inside the scrotum.
- During embryonic development, they descend from the posterior abdomen, through the inguinal canal and into the scrotal sac. As they descend, they take a covering of peritoneum from the abdominal wall which becomes the TUNICA VAGINALIS (parietal and visceral layers). The testes themselves are encased in a thick covering called the TUNICA ALBUGINEA.

The EPIDIDYMUS is a highly convoluted tubule which connects the testes to the ductus deferens.

Question 22

What are the main contents of the anal triangle?

Answer 22

Anal canal
Anal sphincter muscles (internal and external)
Ischioanal fossae
- Pudendal nerve
- Internal pudendal artery and vein

Question 23

What is the anal canal continuous with?

Answer 23

The inferior rectum above (a pelvic organ)

Question 24

Describe the structure, function and innervation of the different divisions of the anal canal.

Answer 24

The anal canal is divided into two parts, the upper and lower halves, they are divided by the PECTINATE line of the anus which is formed by the collection of anal valves which secrete mucous.
- The upper half, similar to the proximal part of the abdominal GIT (e.g. the colon) is innervated by the autonomic nervous system (via the hypogastric plexus) and is therefore only sensitive to stretch.
- The lower half of the anal canal is innervated by the somatic nervous system (via the inferior rectal nerve of the sacral plexus) and is therefore sensitive to pain, temperature touch and pressure.

Question 25

Describe the type of muscle and innervation of the internal and external anal sphincters.

Answer 25

The internal anal sphincter muscle is smooth muscle and is therefore involuntary and controlled by the autonomic nervous system.

The external anal sphincter muscle is skeletal muscle, an is therefore voluntarily controlled by the somatic nervous system (PUDENDAL NERVE S2-4).

Question 26

Describe the structure and function of the anal sphincters.

Answer 26

The internal anal sphincter is formed by a thickening of the smooth muscle of the wall of the anal canal itself. Around the internal anal sphincter is a layer of skeletal muscle- the external anal sphincter muscle.

The upper most part of the external anal sphincter blends with the puborectalis muscle of the pelvic floor/levator ani to form the anorectal ring- a strong ring of muscle which assists with maintaining faecal continence by creating the anorectal junction (an acute angle between the anal canal and the rectum).

The external anal sphincter also uses the perineal body as a point of attachment.

Question 27

What is the name of the two wedge shaped spaces that sit either side of the anal canal? Describe their function and what travels through them.

Answer 27

Ischioanal fossae

They are mainly fat filled spaces that allow the anal canal to expand during defecation.

Importantly, the PUDENDAL NERVE (S2-4) and the INTERNAL PUDENDAL ARTERY (branch of internal iliac artery) and VEIN travel within the ischioanal fossae.
- These neurovascular structures are wrapped in a fascia forming the PUDENDAL CANAL and they lie in the lateral aspect of the ischioanal fossae.

Question 28

Describe the journey of the pudendal nerve and what it supplies.

Answer 28

The pudendal nerve is a branch of the sacral plexus (S2-4).

Once it leave the plexus it exits the pelvis via the GREATER SCIATIC FORAMEN. It supplies a number of structures in the pelvis namely the [LEVATOR ANI] muscles before travelling into the perineum via the ISCHIOANAL FOSSA.

Once in the perineum, the pudendal nerve supplies the [EXTERNAL ANAL SPHINCTER MUSCLE] (inferior rectal nerve branch) and the [MUSCLES OF THE PERINEUM] (perineal nerve branch and dorsal nerve of penis/ clitoris branch) (covered in urogenital triangle chapter), and the [SKIN OVERLYING THE PERINEUM].

BRANCHES (in the perineum):
1) Inferior rectal nerve –> External anal sphincter
2) Perineal nerve –> Muscles of perineum/Skin overlying perineum
3) Dorsal nerve of penis/clitoris –> Muscles of perineum

Question 29

What is the pudendal artery a branch of?

Answer 29

The anterior division of the internal iliac artery (the vein drains into the internal iliac vein).

Question 30

What does the internal pudendal artery give rise/branches to?

Answer 30

The internal pudendal artery gives rise to the INFERIOR RECTAL ARTERY to supply the anal canal and also gives BRANCHES to the external genitalia.
- IPA supplies CORPORA CAVERNOSA of penis/clitoris (erectile tissue and muscles of superficial perineal pouch)

(REMEMBER, superior rectal artery is a branch of inferior mesenteric artery)