Video: Female perineum

I bet you’re all remember that dreaded sex education class at school – condoms, bananas, and all that talk about the birds and the bees. Perhaps the most stressful part being labeling the female anatomy – trying to figure out which way is up while the boys are in shock that pee and babies definitely do not come from the same place. It's a disaster. But fear not, in today's tutorial, we will cover everything that you need to know about the female perineum.

So, what exactly will we learn today? First, we'll be looking at the borders and boundaries of the perineum. Next up, we'll see the two areas the perineum is divided into by what's known as the interischial line – the anal and urogenital triangles. We'll talk about the borders and the contents of both triangles as well as the further subdivision of the urogenital triangle into superficial and deep perineal pouches. We'll also look at the surface anatomy of the urogenital triangle. And, finally, we'll finish up with some clinical notes.

In this tutorial, we won't be discussing the neurovascular structures associated with the perineum – that's a whole other story – but, of course, we have a dedicated video tutorial on our website if you need to learn about this. I hope you're ready because we're about to get started with the borders and boundaries of the female perineum.

The good news about the perineum is that the most difficult part is getting your head around its location. Once you've done that, the rest will be a breeze. In the images on the screen now, you can see the trunk and the pelvic girdle from anterior and lateral views. The perineum is a compartment located inferior to the pelvis. It is bound superiorly or internally by the pelvic diaphragm and inferiorly or externally by the perineal skin and fascia. Let's take a look at these boundaries from what's known as the lithotomy position.

The lithotomy position refers to an individual lying on their back with their legs abducted and their knees bent. First up, let's look at the superior, or internal boundary, also known the roof of the perineum. We're now looking at the pelvic diaphragm, also known as the pelvic floor, from an inferior view. It is formed by the paired levator ani and coccygeus muscles, which function to provide support for the pelvic organs, maintain optimal intraabdominal pressure, and are involved in urinary and fecal continence.

There are two openings in the pelvic diaphragm – the round rectal canal, which provides a passage for the rectum, and a U-shaped urogenital hiatus, which allows the passage of the urethra and the vagina.

The external boundary of the perineum is quite simply skin – well, skin and fascia, to be more precise. The skin of the perineum is continuous with the skin over the thighs and finishes at the gluteal folds.

Now, let's have a little chat about the borders of the perineum. First up, marking its anterior point, we have the pubic symphysis, which you can see highlighted now. Right opposite it at the posterior apex, you'll find the tip of the coccyx attached to the sacrum. The lateral boundaries are formed by the ischial tuberosity of each hip bone.

The structures connecting the pubic symphysis and ischial tuberosities are the ischiopubic rami, and they form the anterolateral borders. Finally, the perineum is limited by the sacrotuberous ligaments.

If you look at the borders of the perineum, you'll notice that it is roughly diamond-shaped. It is split into two separate divisions by a horizontal line between the ischial tuberosities known as the interischial line, and these divisions are known as the anal and urogenital triangles.

Bang in the middle of the interischial line and, therefore, in the center of the perineum, we find the perineal body. It is located anterior to the anus and posterior to the vulval vestibule. It is a fibromuscular structure that functions as the attachment site for quite a few muscles found in the perineum which we'll review in just a bit.

Let's talk about these divisions in more detail starting with the anal triangle. The anal triangle is sometimes known as the anorectal triangle. As we've already seen, the edges of this triangle are formed by the interischial line and the sacrotuberous ligaments, while its apices are at the tip of the coccyx and ischial tuberosities.

The female anal triangle is significantly wider than the male anal triangle. This is due to a wider transverse diameter or distance between the ischial tuberosities in women as well as a longer anteroposterior distance between the pubic arch and the tip of the coccyx. This sexual dimorphism accommodates the passage of a baby through the pelvic cavity during childbirth.

Now let's take a quick look at the contents of the anal triangle. So, if we look at the anal triangle from an external perspective, the only thing we'll see other than the skin is the anus which is the opening of the anal canal. If the skin in the fascia are removed, we can see this little structure here known as the anococcygeal ligament stretching between the margin of the anus and the coccyx.

If we now take a look at a coronal section through the anal triangle, we can see some of the deeper structures including the anal canal which is surrounded by the internal anal sphincter and the external anal sphincter. On either side of the anal canal, we find an ischioanal fossa or the ischiorectal fossa. It is a paired space filled with adipose tissue occupying most of the anal triangle. You can find out more about these structures in our rectum and anal canal video.

Now let's move over to the anterior part of the perineum – the urogenital triangle. As we saw earlier in the tutorial, the edges of this triangle are formed by the interischial line and the ischiopubic rami, while its corners are at the pubic symphysis and ischial tuberosities.

The urogenital triangle is not quite as straightforward as the anal triangle as it is further subdivided into deep and superficial pouches. But before we look at these pouches, we first need to talk about the structure which is known as the perineal membrane. You are now looking at the pelvic floor in the lithotomy position with the location of the perineal membrane indicated. It is attached anteriorly to the pubic arch and has a free posterior edge. The perineal membrane is a tough layer of connective tissue which closes off the urogenital hiatus of the pelvic floor. You can visualize this better if you look down at the pelvic floor from the superior aspect. There are two openings in the perineal membrane – the urethral and vaginal orifices.

You're now looking at a coronal section through the urogenital triangle. In this illustration, we can see how the perineal membrane divides the perineum into a deep perineal pouch and a superficial perineal pouch. Now let's look at these divisions of the perineum starting with the deep perineal pouch.

The deep perineal pouch is bounded inferiorly by the perineal membrane, superiorly by the inferior fascia of the pelvic floor, and laterally by the fascia of the obturator internus muscle. The contents of the deep perineal pouch are fairly straightforward, beginning first with the deep transverse perineal muscle, which stabilizes the perineal body. Also, in the deep perineal pouch are parts of the urethra and the vagina. Around the urethra is the external urethral sphincter which is responsible for voluntary control of the flow of urine. Finally, in the deep perineal pouch, we see the anterior extensions of the adipose tissue found in the ischioanal fossa.

Now we're moving on to the other compartment of the urogenital triangle – the superficial perineal pouch. It is bordered superiorly by the perineal membrane; inferiorly by the perineal fascia – the layer of connective tissue just deep to the perineal skin and fat; and laterally by the ischiopubic rami.

In the superficial perineal pouch, we again find a few muscles. Highlighted now, you can see the bulbospongiosus muscles which extend either side of the vaginal and urethral orifices, from the perineal body to the pubic symphysis. We also have the ischiocavernosus muscles which extend from the ischial tuberosity and along the ischiopubic ramus on either side. The final muscle of the superficial perineal pouch is the superficial transverse perineal muscle, which you can see highlighted now.

Also, in the superficial perineal pouch, we find the greater vestibular glands, also known as Bartholin's glands, which lubricate the vagina and also the most distal part of the urethra.

Our final structure of interest in the superficial perineal pouch is the clitoris, which is, in fact, the female equivalent of the penis in that it is the primary erectile sexual organ in females and has a substantial blood and nerve supply. However, it doesn't play a role in reproduction. The lateral portions of the clitoris are formed by two sets of paired structures – the crura which are attached to the inferior pubic rami and the perineal membrane, and also the vestibular bulbs which extend on either side of the vagina.

Both the crura and vestibular bulbs contain erectile tissue that swells with the blood during sexual arousal. The crura course anteromedially and merge to form a wishbone-shaped structure known as the body of the clitoris, which contains the corpora cavernosa of the clitoris. This is an erectile tissue continuous with that of the crura. At the midline, the body of the clitoris bends anteriorly away from the pubis and terminates as the glans clitoris which we'll look at in a few moments.

There are two types of fascia in the urogenital triangle – the superficial perineal fascia and deep perineal fascia – both continuous with the fascia of the anterior abdominal wall.

The deep perineal fascia covers the superficial perineal muscles and the clitoris, whereas the superficial perineal fascia is composed of two layers. The superficial layer is continuous with Camper’s fascia. The deep layer, also known as Colles’ fascia, is continuous with Scarpa’s fascia.

At this point, what else is there left to do, but look at the surface anatomy of the urogenital triangle. The first structure we're going to look at is the labium majus, or rather, the labia majora, as it is a paired structure. The labia majora refer to the prominent folds of skin on either side of the vulval vestibule. They extend from the mons pubis, which is a fatty eminence anterior to the pubic symphysis.

The external aspects of the labia majora are usually covered in pubic hair and skin similar to the rest of the body, while the internal aspect has a smoother, moister, and hairless skin which merges into the mucous membrane of the underlying structures. The majority of their substance is formed by the fatty subcutaneous tissue layer of the perineum.

The anterior aspects of the labia majora are thickened and come together to form a small cleft known as the anterior labial commissure, whereas posteriorly, they join to form a small ridge overlying the subcutaneous perineal body known as – surprise, surprise – the posterior labial commissure.

Every time we come across a structure which has major in the title, we know we're also bound to see the minor counterpart. That's exactly the case here as just medial to the labia majora, we find the labia minora or labia minus. Unlike the labia majora, labia minora do not contain fatty tissue, but rather have spongy connective tissue with erectile tissue towards their base. The location of the labia minora corresponds pretty accurately to the bulbospongiosus muscle underlying it. Overlying this is a type of skin known as a mucous membrane, which is kept moist by specialized cells of the epithelium. It is rich in sebaceous glands, nerve endings, and blood vessels.

There are two anterior folds of the labia minora. The medial fold extends to the base of the glans of the clitoris where it becomes the frenulum of the clitoris, whereas the lateral fold extends anteriorly to form the prepuce of the clitoris. The prepuce is also known as the clitoral hood and corresponds to the foreskin of the penis in men. Posteriorly, the labia minora are sometimes connected by a transverse fold known as the frenulum of the labia minora or the fourchette. The labia formed the borders of the vulval vestibule – a smooth triangular area with the glans of the clitoris at its apex – the labia forming its lateral sides and the frenulum of the labia minora forming its base.

The vulval vestibule contains the vaginal orifice – the opening of the vagina – and on either side of it are the openings of the greater vestibular glands. As I briefly mentioned before, these glands secrete mucus which lubricates the vaginal opening during sexual arousal.

Also within the vulval vestibule is the external orifice of the urethra – the exit point for urine. On either side of it is an opening of a paraurethral gland. The paraurethral glands are the equivalent of the male prostate.

Now, we've already briefly mentioned it, but let's take a closer look at the glans clitoris, which you can see highlighted in green now. It is located where the labia minora meet anteriorly and is normally covered by the prepuce, which we've already seen. It is the most densely innervated part of the clitoris, and therefore, the most sensitive to stimulation.

The last external structure to mention when talking about the perineum is the perineal raphe. It is the small raised edge stretching between the labia minora anteriorly and the anus posteriorly. It is superficial to the perineal body and is a result of fusion during embryological development.

That wraps up our anatomy of the perineum. But what about its clinical significance?

Although the female reproductive system is adapted for bearing children, there's a variety of things that can go wrong. In the case of the perineum, the most common issue is perineal tears. In fact, it is the most common obstetric injury altogether. Perineal tears refer to the tearing of perineal skin and underlying soft tissues. The trauma is caused due to the strain on the posterior vaginal wall during vaginal childbirth. The severity of perineal injury varies and is classified into four categories.

First-degree injuries refer to tears of perineal skin and the frenulum of the labia minora. Second-degree tears include injuries to the perineal skin, the frenulum of the labia minora, and the perineal muscles, but do not include the anal sphincters. Third-degree tears involve injury to the previously mentioned structures as well as the vaginal mucosa and the anal sphincters. Fourth-degree tears include all of the following: the perineal skin, the frenulum of the labia minora, the perineal muscles, both anal sphincters, the vaginal mucosa, and the anal mucosa.

These tears can lead to a weakened pelvic floor, which in turn can result in a prolapsed bladder or uterus. Sometimes, an incision is made in the inferoposterior wall of the vagina to try and minimize tearing. It is either directed posteriorly right in the midline and is known as a median episiotomy, or posterolaterally, it is known as a medio-lateral episiotomy.

A median episiotomy cuts through the perineal body. Some argue that the healing is better due to a neat scar rather than a tear and that it may limit the tear. Others, however, believe that if anything, a median incision directs the tears to the midline and, therefore, the anal sphincters. The reasoning behind a medio-lateral episiotomy is that even if a tear occurs, it is directed away from the anal sphincters.

And that concludes today's tutorial. But before you run off, let's have a quick recap of everything that we learned today.

We started our tutorial by defining the location of the perineum as well as its borders and boundaries. It is limited internally by the pelvic diaphragm and externally by perineal skin. It is bordered anteriorly by the pubic symphysis, posteriorly by the tip of the coccyx, laterally by the ischial tuberosities, anterolaterally by the ischiopubic rami, and finally, posterolaterally, by the sacrotuberous ligaments.

We saw that it was roughly diamond-shaped with the interischial line dividing it into anal and urogenital triangles. Here we took a quick look at the perineal body which many structures in both triangles attach to. We then moved on to explore the contents of the anal triangle. Here we found the anus, the anococcygeal ligament, the anal canal, the internal and external anal sphincters, and the ischioanal fossa. We then looked at the urogenital triangle, which was a tad more complicated due to its division into deep and superficial pouches by the perineal membrane.

We started with the deep perineal pouch which contains the deep transverse perineal muscles, the external urethral sphincter, as well as the anterior extensions of adipose tissue from the ischioanal fossae. Contained in the superficial perineal pouch, we saw the bulbospongiosus, the ischiocavernosus, and superficial transverse perineal muscles, as well as the greater vestibular glands and the different components of the clitoris.

We then looked at the surface anatomy of the urogenital triangle which included the labia majora and its anterior and posterior labial commissures, followed by the labia minora with its medial fold which becomes the frenulum of the clitoris, its lateral fold which forms the prepuce of the clitoris, and finally, we saw the frenulum of the labia minora. We then took a look at the vulval vestibule, which contains the vaginal and external urethral orifices. Associated with these structures, we saw the openings of the greater vestibular and paraurethral glands. We finally took a closer look at the glans of the clitoris. The last structure that we mentioned was the perineal raphe.

Finally, in our clinical notes section, we explored the perineal tears and how they may be reduced by a surgical procedure known as episiotomy.

That's it, folks. We hope you enjoyed this tutorial and see you next time.