The Pelvic Girdle - Online Tutor, Practice Problems & Exam Prep

The pelvic girdle connects the legs to the body and consists of two coxal bones, which are formed by the fusion of the ilium, ischium, and pubis. The ilium is the upper, flattish part, while the ischium forms the lower posterior region, and the pubis is the lower anterior region. The acetabulum serves as the hip socket. Notably, male and female pelvises differ in shape, with males having a narrower, heavier pelvis and females having a wider, lighter pelvis adapted for childbirth.

Pelvic Girdle

We've previously said that the girdles are how the appendicular skeleton connects to the axial skeleton. Well, now we're going to talk about the pelvic girdle, and the pelvic girdle is how your legs connect to the core of your body. The pelvic girdle is just made of 2 bones, the right and the left coxal bones. Now, those coxal bones, though, are also part of what we call the pelvis, and the pelvis is going to be the coxal bones plus the sacrum. And when you think of the pelvis, that's that entire ring of bone that sort of surrounds your pelvic cavity. You can also think of it as how the weight of your body, that's carried by the vertebral column carried by your spine gets transferred into your hips and then down into your legs. Alright. But now we're just focusing on the coxal bone because that makes up the pelvic girdle, and so the coxal bone is also going to be called the 'os coxae'. 'Os coxae', that's the Latin term for it. You should be familiar with that. You can probably use them interchangeably on a test. They're both technically correct. Now the os coxae or the coxal bone, that's actually going to be formed by 3 fused bones. So even though the coxal bone is just one bone, we're going to go into some detail about the 3 bones that fuse to make it up. Before we do that, let's look at our image. We have the full pelvis here shown with the sacrum there and bone color in the back with the color-coded coxal bones on either side. We're color-coding the different bones that met that fuse to make up that bigger coxal bone. We also, on the right here, have a side view of that coxal bone, again color-coded.

So the first bone that makes up the coxal bone is going to be the ilium. The ilium is going to be this superior kind of flattish region, and we can see that in purple here labeled on this drawing. I also have a coxal bone here, a right coxal bone, and you can see that this upper part is sort of this kind of flattish plate. Now the bone itself isn't flat. A lot of times it looks kind of flat in drawings. You have sort of this flattish plate on the top, but then there's a real sort of twist in the bone to get to the two bones on the bottom, which we'll talk about in a second. But this flat part on the top, that's the ilium. And the thing that you'll notice about it is it has this big sort of round crest on it that you can see here. We've labeled it in these two pictures here, that round crest. That round crest is the iliac crest. And if you put your hands on your hips, you feel your hip bones right there, you're touching your iliac crest. You can feel that it comes to the front of your body and sort of wraps around into your back, and it's this big attachment point for muscles.

So as we move down, we're going to talk about the ischium. The ischium is going to be the lower posterior region, the region in the back. These are the bones you sit on. Sometimes people call them your sit bones. If you got a bony butt, these are the bones we're talking about. You can feel those bones in the back. And the way I remember the ischium is that it's kind of round because, well, your rear end is round. And so the roundish ischium is in your rear, and we can see that here in pink. That's going to be the ischium, and here on the side, you can see how it's nice and round. We can look at the bone here. This is that roundish ischium in the back. It's nice and rough if you feel the real bone because it has all these muscle attachments there. Then you'll probably notice that there's this hole in the coxal bone. That hole, we're just going to say here, is called the obturator foramen. And it's just formed. It's just a hole between the ischium in the back and the pubis in the front. It doesn't have a lot of functions. There are some nerves and blood vessels that pass through it, but beyond that, it's just the hole that's pretty noticeable when you look at the coxal bone.

So now we're going to move over and look at the pubis, also just called the pubic bone sometimes. And well, if the ischium is the posterior region, well, then the pubis is going to be the lower anterior region. It's the part of the coxal bone that is in the front. You can feel this as well. It's the bone right in front, the pubic bone that's sort of just above the region of the genitals, and it comes together. The 2 coxal bones come together and meet at the pubic symphysis. Right? That pubic symphysis is where these 2 come together, and there's going to be a cartilage that forms a joint right there. So the pubic symphysis, you can see here, is marked. The way I remember the pubis or the pubic bone: well, I say that pubis is pointy. So the ischium, I have that roundish ischium, and the pointy pubic bone.

The last thing that you're probably really going to notice about this coxal bone is this round socket right here, and we have this round socket here, circled. That socket is the acetabulum, and we're going to say here that it is the hip socket. The hip socket is actually formed by all three bones. So when you look over here, we see that the dividing line between where these bones came together is sort of in the middle of that acetabulum. Now, this is where the femur comes in and joins, so you have that big round hit on the femur and it fits in there really nicely. The way I remember and I orient the bones here is that I say, well, if the pubis is pointy, that's facing toward the front. Well, for the femur to come in here, that has to be facing out to the side. So if I'm holding this bone, I put the hips on the top, the pointy pubis in the front, and the acetabulum out to the side, then this would fit on the right side of my body, so this must be the right coxal bone.

The last thing just about these joints, if you remember when we talked about the pectoral girdle, we said how mobile that shoulder joint was because you're able to not just move the bone around in there a lot, but you can also move the girdle. Well, now you'll notice that the acetabulum is really deep. So that just on its own limits a little bit how much this femur can move in it, but also the other joints here. Well, I can take the sacrum and I can line it up. Now this joint that you make there, that's technically a movable joint, but it doesn't move much. It's a really firm joint because the purpose here is to be transferring weight that's carried by the sacrum down the spinal column and into those hips. Likewise, at the pubic symphysis in the front, well, that's a cartilaginous joint, so there's a little bit of flex there, but not much. Now during pregnancy, some hormones will loosen that up a bit. It'll actually get a little bit wider and give it a little bit more flex, but still not much. So compared to the pectoral girdle, this pelvic girdle has very little movement in it. We're going to be talking about this a little bit more going forward, especially we're going to look at some differences that are regular between male and female pelvises, and I'll see you there.

The Pelvic Girdle Example 1

Our example says on the image below, label the 3 smaller bones that make up the coxswain and identify whether the left or right coxal bone is shown. Alright. And we have a picture of a bone here, a real bone, and we're clearly looking at the side view of a coxal bone. So the first thing that I'm going to do to label the bones is I'm going to draw the dividing line of where those bones meet. So do you remember where that dividing line goes? Well, when I draw it in, I'm going to draw it in and remember it meets in the acetabulum here. And so this is roughly where this dividing line is going to be, dividing that bone on the top and then dividing the 2 bottom the bones on the bottom, we're just going to sort of draw roughly straight down from there. Then we have this obturator foramen, this hole in the bone, but this dividing line is going to cross over and it's going to divide it down there as well. Alright. So now we have our 1, 2, 3 bones. Let's try and name them. Alright. So what's this one on the top? This is that sort of larger kind of plate like bone. Remember? Well, when I look at it, I had that kind of creepy memory tool where I said, I like your iliac crest because this is that big iliac crest there. And iliac crest, that's your hips. And so the ilium, that refers to the ilium is the bone. Alright. So that big plate like bone on the top, the ilium. Next, I'm going to look down and well, do you remember what this bone down here would be? Well, as I look at that, I see how round that is, and I well, I know that my rear end is round, like that bone, and I had that memory tool where I said, well, the roundish ischium makes up my rear end. So the roundish ischium. Ischium is the bone. And then finally, I'm going to look at this bone here. Do you remember what bone is left? What are we going to name that? Well, this is pointy. I see that sort of point on the bone there, and I said that the pubis is pointy. Alright. So that's going to be the pubis there. Now we have to determine, is this the right or the left? Well, as I do that, I can see here that I'm looking at the acetabulum. So that means that the hip, the femur, is going to fit into the hip socket there. So this has to sort of be on the outside of the body. And then I said, well, also, the pubis is pointy and it's pointing to the front. So I always try to imagine how are these bones going to fit in my body, and if I think about this, the pubis is in the front and then the femur has to fit into this acetabulum. Well, if I were to line that up on my body, the way that would work is if it were on the left. All right. So this is a left coxal bone that we're looking at here with the ilium, the ischium, and the pubis. Alright. Like always, more problems to follow. Give them a try.

On which bone is the acetabulum found?

• A. Ilium.
• B. Ischium.
• C. Pubis.
• D. A-C are all correct.

When comparing the pelvic and pectoral girdles, which statement is incorrect?

• A. The articulation between the pectoral girdle and the axial skeleton is smaller than the articulation between the pelvic girdle and the axial skeleton.
• B. The socket of the hip joint is much deeper than the socket of the shoulder joint.
• C. There are more individual bones in the pectoral girdle than the pelvic girdle.
• D. The pelvic girdle allows for a greater range of motion than the pectoral girdle.

Differences Between the Male and Female Pelvis

When learning about the pelvis and the pelvic girdle, you are very likely to see questions about differences between the male and female pelvis, so we're going to go over that now. This all comes down to the fact that the shape of the pelvis tends to differ between males and females. With all things in anatomy, there's variation around this, but an experienced anatomist can identify the sex of a skeleton correctly about 97% of the time just by looking at the pelvis. There are no other bones that show that regular sex difference. So how do they do it? First off, you can look at the general appearance. In males, the pelvis is a little bit taller, and it tends to be more narrow when you're looking at it in that direction. We're going to say that it's narrow, and the bones are also just kind of heavier. By that, we just mean they're thicker. Now in comparison, we look at the female pelvis. You can see it's kind of shorter and wider here, and so I'm going to write here wider. Also, if the male pelvis is heavier, the female pelvis is a little lighter. The bones are just a little bit thinner.

Next, we can look at the angle of the pubic arch, which is right here where the coxal bones come up and meet at the pubic symphysis. You can see that this angle is different between males and females. In males, this angle tends to be a little bit sharper, or we'll say a little bit more acute. And in females, we can see that this angle is going to be wider, or at least it tends to be wider. We can also look at the shape of the pelvic inlet, which is best seen from a top-down view. But if you draw a circle around where the pubic symphysis is, it's sort of that narrowest part of the pelvis where it makes a ring. In males, we say it doesn't make a ring, it's a little bit more heart-shaped—more heart-shaped and just kind of closer together, a little bit pointier in the front and narrower. Whereas in females, when you look at that shape, it's just much rounder, and we're going to say more of an oval shape. What gives it that heart shape is that in males, the sacrum and the coccyx curve underneath the pelvis a bit more. In females, the sacrum and the coccyx point a little bit more downward; they don't curve under the pelvis nearly as much, giving it that more round appearance.

These differences are largely believed to be because the female pelvis is adapted for birthing. During pregnancy, a significant bony structure known as a fetus or a baby is developed here. Then during birthing, it has to pass through this pelvis, and that bony structure is going to pass through the pelvis much easier if it's a rounder, wider pelvis rather than a narrower pelvis. We're going to practice looking at these differences more in the example to follow, so give it a try.

The Pelvic Girdle Example 2

Our example says that below, four pelvises are shown. Two are shown from the front and two are shown from above. In each view, one pelvis is from a female and one is from a male. We need to identify which pelvises we believe to be male and which we believe to be female. Then we need to indicate at least one anatomical feature that allowed us to differentiate the two by marking it on the picture. Alright. So we have two images here. The image on the left clearly shows two pelvises as we're looking down on them, and the image on the right, we have two pelvises from a front view or an anterior view.

Let's start with these two on the left. Which one do you think is more likely a male pelvis, and which one do you think is more likely a female pelvis? Well, when I look at it, the thing I cue in on is this shape here. As I look down through the pelvis, I'm going to circle this, and that's the shape of the pelvic inlet, that ring around the pelvis there. And this one on the left, it looks kind of round and kind of wide, especially when I compare it to this pelvis on the right. This pelvis on the right has that sacrum sort of coming underneath. As I draw that shape, it comes out much more kind of heart-shaped there. Right? That heart-shaped pelvic inlet, that's more likely to be part of a male pelvis, and that bigger, rounder, more oval-shaped pelvic inlet is more likely to be part of a female pelvic inlet. So what's the anatomical feature we use to identify that? We use the pelvic inlet.

Let's look at these two on the right. Take a look. Which one do you think is more likely male? Which one is more likely female? Well, when I take a look, what I notice is that this pelvis on the left, it's pretty tall, and it's also relatively narrow, at least for how tall it is. Well, this one on the right, that's pretty short, and it's very wide, again, at least for how short it is. And I remember that a tall, narrow pelvis is more likely to be a male pelvis, and a shorter, wider pelvis is more likely to be a female pelvis. But the thing that really cues me in here is something else. And that's when I look at this, I look at this angle right here. That's the angle of the pubic arch. And do you see on that pelvis? It's a real sharp angle. It's an acute angle. Whereas on this pelvis, it's very wide, or rather, it's an obtuse angle. All right. That sharper angle is more likely to be part of a male pelvis, whereas the wider or obtuse angle is more likely to be part of a female pelvis. And the anatomical structure that I use to identify that is the shape or rather the angle of the pubic arch.

With that, we've answered the question. More problems to follow. Give them a try.

How does the angle of the pelvic arches differ in male and female pelvises?

• A. The angle of the male pelvic arch tends to be more acute than the female pelvic arch.
• B. A female's pelvic arch tends to be rounder to accommodate the head of the baby.
• C. The female pelvic arch tends to be more heart shaped than the male pelvic arch.
• D. The pelvic arch is not a reliable indicator of sex.

Here’s what students ask on this topic:

What bones make up the pelvic girdle?

The pelvic girdle is composed of two coxal bones, also known as os coxae. Each coxal bone is formed by the fusion of three bones: the ilium, the ischium, and the pubis. The ilium is the upper, flattish part, the ischium forms the lower posterior region, and the pubis is the lower anterior region. These bones come together to form the acetabulum, which serves as the hip socket.

What are the differences between the male and female pelvis?

The male pelvis is generally taller, narrower, and heavier, with a more acute pubic arch angle and a heart-shaped pelvic inlet. In contrast, the female pelvis is shorter, wider, and lighter, with a wider pubic arch angle and a more oval-shaped pelvic inlet. These differences are primarily due to the female pelvis being adapted for childbirth, allowing for easier passage of the baby through the birth canal.

What is the function of the acetabulum in the pelvic girdle?

The acetabulum is the hip socket in the pelvic girdle, formed by the fusion of the ilium, ischium, and pubis. It serves as the point of articulation for the head of the femur, allowing for the movement of the hip joint. The acetabulum is deep, which provides stability to the hip joint and helps in transferring the weight of the body from the spine to the legs.

How does the pelvic girdle connect to the axial skeleton?

The pelvic girdle connects to the axial skeleton through the sacrum, which is part of the vertebral column. The sacrum articulates with the ilium of each coxal bone at the sacroiliac joints. This connection helps transfer the weight of the upper body from the spine to the lower limbs, providing stability and support for movement.

What is the pubic symphysis and its function?

The pubic symphysis is a cartilaginous joint located between the left and right pubic bones at the front of the pelvis. It provides slight flexibility and movement, which is particularly important during childbirth. The joint is reinforced by ligaments and allows for limited movement to absorb shock and maintain the integrity of the pelvic ring.