Where is the perineum and what does it contain?
Perineum: anything below the pelvic floor between the thighs and bound by the pelvic outlets.
Contains the outflow tracts of pelvic systems: vagina, urethra, anal canal.
The perineum is often described as having a diamond shape (when looked at from below)
Describe this diamond and its boundaries
- The A-P limits of the diamond are the pubic symphisis anteriorly to the tip of the coccyx posteriorly
- The Lateral points of the diamond are the ishial tuberosity on either side.
The diamond is also further divided by an imaginary line between the two tuberosities
The diamond is subdivided by the horizontal lines between the isheal tuberosities.
What are the two triangles and how do they differ between genders?
- anal triangle posteriorly (same in both sexes)
- urogenital triangle in front which contains the the urethra in males and the urethra and vagina for females
The urogenital triangle is further subdivided, how is this done?
ie. what is the dividing structure?
There is a Urogenital/perineal membrane that fills in the urogenital triangle extending from one ishiotuberosity to another.
It is in the horizontal plane (transverse of the body) dividing the urogenital triangle into two horizontal cavities (one on top of the other)
What are the two cavities of the urogenital triangle called and what are their boundaries?
Deep perineal pouch is deep to the urogenital diaphragm or perineal membrane between perineal membrane and levator ani/pelvic floor
Superficial perineal pouch between the membrane and the skin and superficial fascia.
Describe the urogenital (perineal) membrane
It is a thick, triangular and arching fibrous tissue membrane that spans from one pubic ramus to another.
It has a free posterior edge.
The midline is thickened into a dense nodular structure called the perineal body.
What is the importance of the perineal body of the urogenital membrane?
Lots of muscles attach into this thickened nodule (also called the central tendon of perineum).
These muscles are particularly important as many of them have sling and/or sphincter functions.
There are a pair of muscles on either side of the perineal body.
That are these muscles called?
On what surface of the urogenital membrane are they?
- Superficial transverse perineal muscles extending sideways reinforcing the membrane.
- On the other side is an identical called the deep transverse perineal muscles.
Thus a pair is present on both sides of the membrane (superficial and deep)
What lies on the superficial surface of the fascia of the urogenital membrane?
The roots of the external genitalia (penis and clitoris) are also attached (anchor point).
What are important structures that lie in the deep perineal pouch?
A series of sphincters for the outflow tract attached
What makes up the side walls of the anal triangle?
The lower hald of the obturatory internus muscle just as it is about to pass through the obturator canal (because the pelvic floor muscles cut the obturator muscle in half)
There is a specialised canal located in the fascia overlying obturator internus.
In the fascia overlying obturator internus is a horizontal split in the fascia that creates a canal called the pudendal canal that transmits the pudendal neurovascular bundle for the perineum
Is the remainder of the anal triangle hollow?
What is it called
No, the rest of the anal triangle is full of fat = ischiorectal fossa (collection of fibrofatty tissue).
The anal canal has a very clear and distinct line of demarcation between the upper two thirds and the lower third. What happens at this line of demarcation? 
- Change in lining tissue from mucosa to skin
- Change in arterial supply from the superior rectal artery to the inferior rectal artery (both reinforced by middle rectal).
- Venous drainage changes from superior rectal to the portal system into la branch of ileac system (into the systemic veins). Thus the rectum is a site of porto-systemic anastomosis: why it is a site affected in portal hypertension.
- Change in nerve supply from visceral nerve supply (dull pain) to the pudendal nerve (a somatic nerve). Any pathology here will by sharp, severe and localised.
What is the difference between the external and internal muscle sphincters of the rectum?
Internal is a thickening in smooth muscle wall (autonomic innervation). External sphincters which are somatic muscle.
Describe the structure and components of the external anal sphincter
External anal sphincter is made up of 3 distinct annular components.
These are rings stacked one above the other.
- Closest to levator ani is deep
- The sphincter ring in the middle is the superficial
- The one related to the skin is subcutaneous.
Describe the arrangment/organisation of the three external anal sphincter muscles
- Deep muscle fuses/blends with puborectalis of the pubococcygeus levator ani
- Superficial is anchored and fixed posteriorly to tip of coccyx and anteriorly to the perineal body.
- Subcutaneous is a flat circular band immediately deep to the skin
Where does the urethra span to?
What is the difference between a male and a female?
Urethra begins at the base of the bladder and ends by opening to the exterior via the perineum.
In the female it is short, 4cm long and has a slightly curved course through pelvic floor, through deep perineal pouch through perineal membrane into the vestibule associated with the female external genitalia. This short course makes it easier to catherterise a female but the disadvantage is predisposition to UTI.
Much more complex, longer and divided into 4 main parts traversing along the system also receiving secretions from the reproductive system to exit from the penis.
The male urethra is much more complex than the female.
Describe the four major parts of the male urethra
- Preprostatic (not usually described as one of the parts) - related to neck
- Prostatic part: through the prostate and impacted on medial lobe and receives from prostatic duct and ejaculatory ducts.
- Membranous part: through deep perineal pouch and perineal membrane
- Spongey/penile: through the penis
Describe urethral sphincters in males
In the male there is an internal urethral sphincter at the very start related to the neck of the bladder.
= Involuntary smooth muscle.
The membranous part of the urethra is surrounded by external sphincter (on deep surface of perineal membrane) which is voluntary skeletal muscle
Describe the urethral sphincters in females
In the female there is no internal sphincter (mucosal arrangement functions as a sort of internal sphincter but without smooth muscle).
The external sphincter is related to the deep surface of the perineal membrane.
In the female the external sphincter is part of a larger sphincter for the vaginal structures.
What are the major muscular structures attaching to the perineal body?
What is a clinical implication of this?
The perineal body: sphincters associated with the urethra insert into it, the middle part of the external anal sphincter, deep and transverse perineal muscles.
The Perineal membrane and associated structures can tears through the perineal body in pregnancy thus all sphincters lose their connection point. Tear in AP direction.
Describe the erectile tissue of the penis
In the root of the penis is the left and right crura, and the bulb of the penis. The bulb is situated in the midline of the penile root, and is traversed by the urethra. The left and right crura are located laterally, attached to the ipsilateral ischial ramus.
The erectile tissues continue into the body of the penis. The left and right crura move anteriorly into the dorsal part of the penis – they form the twocorpora cavernosa. They are separated by the septum of the penis, although often incompletely. The bulb forms the corpus spongiosum, which lies ventrally. The male urethra runs through the corpus spongiosum – to prevent it becoming occluded during erection the corpus spongiosum fills to a reduced pressure.
Distally, the corpus spongiosum expands to forms the glans penis.
There are three cylindrical structures that contribute to the penis in the male and the clitoris in the female.
What are these?
What nerve conveys the majority of sensory, sympathetic and somatic motor fibres to the perineum (not parasympathetic)
Describe this nerve
The pudendal nerve
It is a branch of sacral plexus S2-4. The sacral pelvis is on the side wall of the pelvis. It doesn’t penetrate the pelvic floor, it exits the pelvis with piriformis muscle (out of the greater sciatic foramen). It slips behind the isheal spine and tracts in back under the pelvic floor running along the side wall of the pelvis in the pudendal canal (split in fascia overlying obturator internus).
Describe how the pudendal nerve supplies the major viscera of the perineum
It comes in the back of anal triangle from the side wall and heads to the front of the urogenital triangle and sends branches out on its path.
- Inferior rectal nerve is the first main branch to the external anal sphincter
- Once the nerve reaches the urogenital triangle (complex due to the membrane) it gives off a perineal nerve and its branches which arrange on either side of the membrane to supply the structures here: motor branches, cutaneous branches and branches relating to midline viscera.
- As it hits the apex subpubically it changes its name to dorsal nerve of the clitoris or penis and supplies that structure.