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Hey everyone! It’s Nicole from Kenhub, and today we're going to be talking about the anatomy of the female pelvis as seen from a superior view. The female pelvis differs from the male pelvis in many ways such as the size and shape of the bony pelvis, but also by internal viscera due to the presence of the uterus, the ovaries, and the superior vagina. This, of course, has implications for the layout of the pelvis which must be understood for anyone studying anatomy and how it can be interpreted in medical imaging.
Therefore, in this tutorial, we're going to be breaking down the different components of the female pelvis by comparing them in the following order. So first we're going to begin with the visible osteology of the pelvis then the structures of the abdominal wall which includes the muscles and the fascial layers, and then we'll move on to the pelvic viscera and associated structures and the vasculature, and finally, we'll discuss the clinical correlates towards the end of the tutorial.
Before we continue with the tutorial, I'd like to introduce you to the illustration that we'll be using throughout. In this illustration, we're looking down into the pelvic cavity. The peritoneum that would normally form the roof has been removed to enable us to see the underlying structures. So first off, let's make sure that we understand the orientation of the image. So the top of the image is the anterior pelvis, the bottom is the posterior pelvis, and then we have the left and the right sides of the image.
As mentioned before, we're going to start with the osteology of the pelvis and this will allow us to build the anatomical structures onto this and give us a better appreciation of the view and the pelvis as a whole.
So the first osteological components that we're going to be looking at is the sacrum. So here we can see the sacrum from the anterior view of the pelvis and, as you can see, it is an irregularly-shaped bone and, although it looks like a single bone, the sacrum is actually made up of five fused vertebrae and links the spinal column to the iliac bones of the pelvis. So over here we see the sacrum from a posterior view and it's a really good landmark to orientate yourself when looking at illustrations or medical images. Easily identifiable on its posterior aspect are its spinous processes and sacral canal. So going back to the superior view of the bony pelvis, we can see the triangular sacrum posteriorly articulated with the other bones of the pelvis.
So the next bones articulate with the sacrum as we just saw and these are the Ilia or the iliac bones, and these broad bones make up a large portion of the bony pelvis and it can be seen in green on the left covering the posterior part of the pelvis. On the right image, we can see the right ilium joining with the sacrum and also the femur sitting in the acetabulum which the ilium contributes to.
The final bones that we are going to mention are the pubic bones, and these are also known simply as the pubis. So in this illustration on the left, you can see the pubic bone highlighted as seen from the superior view of the pelvis. And as you can see, only a small part of the pubis is visible from this perspective as it is covered by other structures, but on the right, we can appreciate that it completes the pelvic inlet anteriorly and unites in the midline via a cartilaginous joint known as the pubic symphysis.
On the illustration that now appears on the right side of the screen, we can see the pubic symphysis as seen from the superior view of the pelvis, and in this illustration, the urinary bladder has been retracted slightly to allow us a view of the structure from this perspective.
All three of the previously mentioned bones on each side form the pelvic brim or the linea terminalis and the linea terminalis is a bony line which encircles the pelvic inlet into the true pelvis where the pelvic organs are housed – and we can see a component of this line on our left-hand Image – and on the right, you can see the whole line and each bony component.
So now that we've covered the bony pelvis, it's time to add in the soft tissue structures which form the bowl-like pelvic cavity and houses the pelvic viscera. These structures include muscles and their overlying fascial layers.
So the psoas major will be the first muscle that we’ll identify and this powerful muscle is a strong flexor of the hip joint. From the superior view, we can see that it lies just anterior to the joint between the ilium and the sacrum - the sacroiliac joint. After that, it courses anteriorly across the pelvic cavity to pass inferiorly under the inguinal ligament and on this image on the right, we can see that this muscle extends down from the lumbar spine then courses through the pelvis to reach the lesser trochanter of the femur where it has its insertion.
This next muscle, the iliacus muscle, actually works with the psoas major as a flexor of the hip and will combine with the muscle fibers from the psoas major to form the iliopsoas muscle. And here we can see that the iliacus muscle is located just lateral to the psoas major and anterior to the wing of the ilium which is its proximal attachment site. And this attachment is better seen here where its relationship with the psoas major and the ilium is pretty clear. But like the psoas major, it courses anteriorly in the pelvic cavity inferior to the inguinal ligament and inserts onto the lesser trochanter of the femur.
Overlying the iliacus muscle, there's a layer of connective tissue called the iliac fascia, and this can be seen here highlighted in green. And this layer of connective tissue just covers the muscle and will join with the fascia of the psoas major.
Okay so let's now discuss the anterolateral abdominal wall, and this wall is composed of three muscle layers laterally and one muscle anteriorly at the midline. The most superficial muscle of the lateral aspect of the abdomen is the external oblique, and this muscle contributes to increasing intra-abdominal pressure and assists in lateral flexion as well as rotation of the trunk. So it's worth noting that the muscle fibers of the external oblique run from superolaterally to inferomedially similar to the orientation of your fingers when your hands are in your pockets which can be seen on the right-hand image.
So as we move deep within the abdominal wall, the next muscle of the lateral side is the internal oblique and fibers of this muscle run perpendicular to the fibers of the external oblique. These muscles also work with the external oblique muscles to allow for stabilization, rotation, and lateral flexion of the trunk while also contributing to increasing intra-abdominal pressure.
So the deepest of the three layers of the lateral abdominal wall is the transverse abdominis which is seen here on the left in green and it lies deep to the two previously mentioned muscles, and this muscles’ fiber orientation, like the name suggests, run in a transverse manner crossing the body from medial to lateral creating a corset effect.
This central muscle mass located on the anterior abdominal wall is called the rectus abdominis, and this is the muscle that, when well-defined, would give us a so-called six-pack. However, this muscle is comprised of rectangular pairs of muscle tissue connected by three or more tendinous intersections and divided at the midline by the linea alba which will be discussed later in this tutorial, and the rectus abdominis muscle allows us to bend forward flexing our trunk and is attached to the pubic symphysis and pubic crest proximally and the xiphoid process and the fifth to seventh costal cartilages distally.
The rectus abdominis muscle is encased in something we call the rectus sheath. Now, this fibrous sheath is formed by the aponeurotic tendons of the external and internal oblique and the transversus abdominis muscles. On the image, we can see that the tendons also known as aponeurosis of these muscles come together before splitting around the muscle belly of the rectus abdominis to form anterior and posterior leaflets. And this configuration occurs above a point called the arcuate line.
And we should note that below this level, all aponeurotic layers move anteriorly to the rectus abdominis leaving no posterior contribution, however, this is not seen at this level.
So let's talk now about the linea alba. So this was mentioned earlier as being the divide between the paired muscle masses of the rectus abdominis and if you look on the image on the right highlighted in green, we can see the linea alba between the bellies of the rectus abdominis attaching to the xiphoid process superiorly and the pubic symphysis inferiorly. And now looking at the left image, we can see the linea alba in cross-section and that the layers of the rectus sheath on the right and the left come together and fuse.
As we move deep to the rectus abdominis and the layers of the rectus sheath, we have another layer and this layer is the transversalis fascia. It’s situated between the rectus sheath and the parietal peritoneum continuous superiorly with the fascias of the inferior aspect of the diaphragm and the fascia of the ilium and the pelvis.
The transversalis fascia has a thickening just behind the site of the inguinal ligament and this is called the iliopubic tract, and the iliopubic tract arches over the external iliac vessels which make their way into the thigh from the pelvis. And this structure is an important landmark in laparoscopic hernia repair.
Another fascial layer that can be seen from this view is the obturator internus fascia and this fascial layer overlies the obturator internus muscle which is seen highlighted in green on the right, and this muscle contributes to the lateral pelvic wall. On the image on the left, we can see the small area of visible fascia highlighted as the bladder is reflected out of the way.
So if we look slightly inferiorly to the fascia of the obturator internus on our superior view of the pelvis, we can see another facial layer and this layer covers the superior portion of a muscle called the levator ani. The levator ani contributes to the pelvic diaphragm and contracts to maintain the location of the pelvic organs in the pelvic cavity. And we can only see a small portion of this on the left image because the organ sit on top of it and prevent us from viewing it as a whole.
So the levator ani muscle only attaches to bone laterally at its anterior and posterior tips while the middle section does not, but it does attach to a thickening of the obturator internus fascia which we saw previously to form what we call the tendinous arch of levator ani.
The last feature of the body wall that we’ll point out is the deep inguinal ring, and the deep inguinal ring is an important feature of the human body particularly for males as it marks the beginning of the inguinal canal allowing passage of the ductus deferens from the testes to the pelvic cavity. However, in females, it allows for the round ligament of the uterus to enter the inguinal canal which we can see in this image.
So now let's look at the viscera in the female pelvis which allows for waste excretion and reproduction to occur, and these structures include the rectum, the cervix and the uterus, and the urinary bladder. The pelvic viscera have many associated structures that support their function and assists in maintaining their location in the bony and muscular housing of the pelvis. And let's work from posterior to anterior when discussing these structures.
The most posterior visceral structure of the pelvis that we're going to mention is the rectum, and we can see the rectum on the left in green lying anterior to the sacrum. The rectum marks one of the final components of the gastrointestinal tract and it is a continuation of the sigmoid colon as we can see on the image on the right and takes a turn inferiorly towards the anal canal. The point at which the sigmoid colon becomes the rectum is the third sacral vertebra and will end when it passes through the levator ani.
Surrounding the rectum, there are a few structures and spaces that we should be aware of, and the first of these is the rectal fascia, and this layer of fascia surrounds the rectum as you can see here highlighted in green. The rectal fascia has two projections attaching the rectum to the lateral walls of the bony pelvis and are known as the rectosacral fascia, and these bilateral projections anchor the rectum in place and reduce the chance of it shifting.
As we follow it laterally, we can see that it connects with a connective tissue that lies just anterior to the sacrum and this is the presacral fascia. And this layer is also known as Waldeyer’s fascia and expands laterally to be continuous with the parietal fascia of the pelvis. However, between the presacral fascia and the more anterior rectal and rectosacral fascia lies a potential space which is the presacral space, and this space houses some of the neurovascular structure that we'll be discussing later in this tutorial.
The rectum and its associated connective tissue structures make up the posterior aspect of the female pelvic viscera so let's now move slightly anteriorly and discuss the next major visceral structure.
So in green, we can see the cervix positioned anterior to the rectum on the left side of the image and at this level it is the cervix that we see, however, if dissection were more superior, we would see the body or even the fundus of the uterus which can be seen on the right. The cervix is a component of the female reproductive system connecting the upper vagina with the uterine cavity. The cervix is held securely in place in the pelvic cavity by a series of fascia and ligaments, however, these are not the ligaments that we think of when describing the structures of the musculoskeletal system.
So these ligaments are condensations of fascia or peritoneum and the one structure that we can see in this superior view is the transverse cervical ligament, also referred to as the cardinal ligament, and this ligament extends laterally from the cervix to the lateral pelvic wall in the area of the obturator fascia which we mentioned earlier in the tutorial.
Okay so when moving more anteriorly from the cervix, we are met by the urinary bladder, and in this image, we can see the urinary bladder and the urinary bladder is responsible for the storage of urine excreted from the kidneys and deposited in the bladder by the ureters. In the floor of the bladder in this view, we can see an opening and this is the urethral orifice where urine is released from the bladder during micturition. In this superior view, there are numerous connective tissue structures associated with the bladder visible and let's run through them now.
So here we see the lateral ligament of the bladder and this connection is a condensation of the endopelvic fascia extending from the lateral bladder to the wall of the pelvic cavity, and it's a pathway for neurovascular structures to pass towards the bladder. Next up is the medial pubovesical ligament which passes anteriorly from the neck of the bladder to the arch of the pubis holding it in place. And centrally and anteriorly, we have the median umbilical ligament, and this structure is a remnant of an embryological structure – the urachus – which drains the bladder of the fetus and runs through the umbilical cord.
And if we move slightly away from the midline, we'll come across the medial umbilical ligament, and this structure, much like the median umbilical ligament, runs superiorly along the inner abdominal wall that is a remnant of the fetal umbilical artery which becomes obliterated and serves no function after birth.
And finally the last connective tissue structure that we're going to discuss is the umbilical prevesical fascia, and this thin layer extends from the medial umbilical ligament on one side to the corresponding ligament on the other side.
Alright so now that we've looked at the bones, the walls of the pelvis, and the visceral structures, let's move on to the final aspect of the anatomy in this tutorial which is the vasculature. So as we discuss the vasculature, try to think back to the structures that we've already mentioned as this will help you bring the information of this tutorial together and boost your learning.
So this small artery which is seen a little bit better on the image on the right is a branch of the aorta roughly in the region where it bifurcates, and this is the median sacral artery. The median sacral artery runs inferiorly along the anterior midline of the sacrum and on the left, if we look closely, we can see it running in the presacral space.
So remember that potential space between the presacral fascia and the rectal and the rectosacral fasciae? You might also notice on the left that paired up with the artery is a vein with the same name.
So let's now look at what might be the most obvious blood vessels of the superior view and that would be the external iliac arteries and veins. On the image on the left, we can see the artery highlighted in green and a little bit of the vein. The external iliac artery is a branch of the common iliac artery as seen on the right and it courses anteriorly under the inguinal ligament. The inguinal ligament is a connective tissue structure connecting the anterior superior iliac spine to the pubic tubercle and as the vessels pass under the inguinal ligament, they change their name to the femoral artery and vein supplying and draining the lower limb.
So its partner who arises from the common iliac vessels are the internal iliac artery and vein, and these are the most important vessels for the pelvis and its viscera as they supply and drain blood from this region. You might notice that the internal iliac is particularly small in comparison to the external iliac vessels and this is because as the internal iliac artery branches, it gives off numerous arteries to supply the various regions of the pelvis.
We’re now going to look at these two major arteries that travel through the pelvis and name some of their branches which can be seen from this view. So starting with the external iliac artery, the first branch to name is the deep circumflex iliac artery, and this vessel arises just before the external iliac artery passes under the inguinal ligament. It courses laterally around the inside or the deep aspect of the ilium curving back posteriorly which gives justification for its name and it supplies the ilium and the muscles that lie in the iliac fossa as well as the iliacus muscle and we mentioned these muscles earlier in the tutorial. Paired with this artery, we can also see the deep circumflex iliac vein just below the artery.
The next branch of the external iliac artery is the inferior epigastric artery and it arises at roughly the same place as the circumflex iliac artery, but takes a different course. It travels superomedially anterior to the transverse fascia up towards the rectus abdominis muscle and is the major blood supply to the rectus abdominis muscle and is the major blood supply of that muscle. And this vessel is also an important clinical landmark which we'll discuss at the end of the tutorial.
A small branch of the inferior epigastric artery descends inferomedially along the pubis and can anastomose with the obturator artery and supplies the medial compartment of the thigh.
Okay so we’re now going to move on to discuss the branches of the internal iliac artery seen from this superior view of the female pelvis. And the first branch that we're going to see is the umbilical artery seen here highlighted in green and this artery arises from the internal iliac artery and courses anteriorly eventually becoming the medial umbilical ligament, which we've already mentioned. And this as part of the artery stays patent and open because branches from it supply organs in the region. However, its distal portion is unnecessary and therefore becomes fibrous.
The branches from the umbilical artery are called the superior vesical arteries and they're usually a number of small vessels which supply the superior portion of the bladder, and we can see these vessels on the left side of the superior view through the condensed pelvic fascia.
The next major branch of the internal iliac artery is the obturator artery and if you know your musculoskeletal anatomy, you'll remember that the obturator artery supplies the medial compartment of the thigh. However, it originates in the pelvis and courses anteriorly passing through the obturator canal to reach the lower limb.
This next artery is extremely important in the female pelvis as it supplies the uterus, which is rightly called the uterine artery. And this artery will travel within the transverse ligament of the cervix and then give off branches both superiorly and inferiorly to supply the upper reproductive tract. And in this image, we can see the uterine arteries on both sides following this course.
The next artery is the middle rectal artery, and this artery arises from the internal iliac artery and then courses inferiorly to reach the middle third of the rectum and anastomoses with the superior and inferior rectal arteries to provide blood to the area. And we can see this artery magnified in green passing through the pelvic fascia like many of the other vessels.
So in addition to the branches of the internal and external iliac arteries, blood can reach the pelvis through other routes, one of which is the ovarian artery. These arteries arise from the aorta on both sides at the level of the L2 vertebra and travel down into the pelvis to supply the ovaries and the fallopian tubes. On the left image, we can see it traveling through the ovarian ligament to reach the area of where the ovary would be in this view, and on the right, we can see its course from the aorta down into the area of the pelvis.
The final vascular structure that we're going to mention is the deep dorsal vein of the clitoris, and this vein can be seen anterior to the bladder before it enters the urogenital triangle to drain blood from the clitoris. And it drains this blood into the vesicular plexus of veins which surrounds the inferior aspect of the bladder.
Okay, so, earlier in this tutorial, we mentioned that the inferior epigastric artery is an important clinical landmark and this is because it's the defining border before a direct and indirect inguinal hernia. You might be asking what's an inguinal hernia? Well, an inguinal hernia is a perforation of the bowel through the anterior abdominal wall, and when it occurs lateral to the inferior epigastric artery, it would be classified as indirect and when it occurs medial to that vessel, it would be classified as direct.
So we're going to be focusing on the direct inguinal hernia as the perforation passes through an area called Hesselbach’s triangle. And the boundaries of Hesselbach’s triangle, are laterally, the inferior epigastric artery; medially, it's the lateral border of the rectus abdominal muscle; and inferiorly, it's the inguinal ligament.
Alright so we've now come to the end of the tutorial. Thanks for sticking with me. It’s quite a long one. Before I let you go, let's quickly recap what we've learned.
So we started out by looking at the osteology of the female pelvis looking at the bones such as the sacrum, the ilium, and the pubis, and then we also looked at the linea terminalis which outlines the pelvic inlet. Next, we looked at the structures of the abdominal wall particularly the muscles – the psoas major and the muscles of the anterior abdominal wall which include the external oblique, the internal oblique, and the transverse abdominis muscles. We've also looked at the rectus abdominis muscle as well as some abdominal fascia, the rectus sheath, the linea alba, the transversalis fascia, the obturator internus fascia, the superior fascia of the levator ani muscle, and the deep inguinal ring.
We then moved on to look at the viscera of the pelvis and the associated structures and we looked at the rectum and the connective tissue of the rectum, the cervix, the urinary bladder, and the ligaments associated with the cervix and the bladder. Next, we looked at the vasculature of the female pelvis looking at the main arteries such as the median sacral artery, the external iliac artery, and its branches as well as the internal iliac artery and its branches, and we also looked at additional vessels such as the ovarian artery and the deep vein of the clitoris. And, finally, we looked at the boundaries of Hesselbach’s triangle with regards to defining hernias.
I hope you enjoyed this tutorial and have learned what structures can be seen and identified from a superior view of the female pelvis. Thanks for watching and happy studying!