Hey everyone! This is Nicole from Kenhub, and welcome to another tutorial on cross-sections. In this tutorial, we'll be looking at a cross-section at the level of the male urinary bladder towards the distal end of the coccyx. We will be looking at a lot of structures that we can see at this level on the axial plane which is sometimes called the transverse plane. These structures include organs, bones, muscles and blood vessels.
Knowing which structures you can see at this level in cross-section is especially important when looking at CT and MRI scans and is a fundamental clinical skill. Just a reminder that when we look at a cross-section or CTs or MRIs, the patient is positioned lying on their back and the viewer is looking superiorly from their feet. Therefore, this side of the image is the anterior or the front of the body while this side is the posterior. This is the right-hand side of the body and this is the left-hand side of the body.
So when we study cross-sections, always keep in mind, the right side of the screen corresponds to the left side of the body and the left side of the screen corresponds to the right side of the body. Also note that although we featured an image of an anatomical woman just now, the image we'll be looking at is of a male at the level of the urinary bladder around the distal end of the coccyx.
To begin highlighting the features of this cross-section, let's start by building a framework to reference from. This can easily be done by outlining the osteological features of the cross-section. We'll start by looking at the head of the femur, the greater trochanter, the ischium, and the pubis.
In this image, we can see the head of the femur highlighted in green sitting within the acetabulum of the innominate bone. The head of the femur is the rounded ball component of the hip joint while the acetabulum forms the socket. However, the acetabulum is formed by three bones – the ischium, the ilium and the pubis – and these unite at the acetabulum. We can see two of them in this cross-section – the ischium down here and the ilium up here.
Another feature of the femur is the greater trochanter highlighted in green. This is the large lateral projection of bone that is involved in the attachment for many muscles associated with the movement of the hip. Here we can see the ischium forming the posterior aspect of the acetabulum around the head of the femur. If you look at the image on the right, it's clear what part of the acetabulum is formed by the ischium.
Let's now look at which bone forms the anterior aspect of the acetabulum. This part is formed by the pubic bone. This anteriorly located bone forms a third of the acetabulum and unites with the opposite side at the pubic symphysis. The one bone that forms the innominate bone that we don't see in this cross-section is the ilium. The reason it can't be see is because it'd be seen in a more superior section.
Now let's move on and talk about the pelvic viscera which includes the urinary bladder, the prostate, and the rectum. In this cross-section, we can see three midline pelvic structures and we'll be working through these from anterior to posterior.
The first of these is the urinary bladder. The urinary bladder receives urine from the kidneys and stores it until voiding occurs. You can see in both the cross-sections on the image on the right that the bladder is the most anterior structure. Just posterior to the urinary bladder in the cross-section is the prostate. The prostate is located inferior to the bladder and wraps itself around the urethra. This relationship can be seen on the image on the right. The rectum is the most posterior structure within the pelvic viscera being situated just anterior to the coccyx. Also visible on either side of the rectum is the pelvic floor consisting of the levator ani.
Now that we've covered the osteological and visceral components of this section, we'll move on to the muscular structures that can be seen. The easiest way to do this is to split it into regions. The two regions we can see in this section are the gluteal region and the anterior compartment of the thigh.
Let's start with the gluteal region which is located posteriorly in the cross-section. The structures we'll look at include the gluteus maximus, the gluteus medius, the quadratus femoris, the obturator internus, and the sciatic nerve.
The first muscle we're going to look at is the gluteus maximus. This is one of the largest muscles in the region and works to extend the thigh at the hip joint. You can see on the right image which is a posterior view of the gluteal region that the gluteus maximus is a large quadrangular-shaped muscle.
The next muscle is the gluteus medius. This muscle works to abduct the thigh at the hip joint and lies deep to the gluteus maximus which you can see overlaying the top of the gluteus medius just here. You'll notice that although we can see the thickness of the muscle up here, on a cross-section, we're actually looking at the thinner inferior portion just here which is the part that attaches to the greater trochanter. Therefore, on a cross-section, it looks quite lateral.
The next muscle is one of four muscles involved in the lateral rotation of the hip. This is the quadratus femoris and here we can see it running laterally from the ischium deep to the gluteus maximus and traveling to the attachment on the medial side of the greater trochanter. If we look over to our posterior view image, we can see the whole quadrangular shape of the muscle as it bridges the two attachment sites.
Another muscle involved in lateral rotation is the obturator internus. This muscle originates inside the pelvis then projects out wrapping around the ischium to attach to the greater trochanter. We can see this when looking at the cross-section but also when looking at the image on the right. Note that once out of the pelvis, it is mainly tendinous and runs superiorly to the previously mentioned quadratus femoris.
Before we move on to the next compartment, I just want to point out another really important structure – the sciatic nerve. This is the largest nerve in the body and it supplies many muscles of the lower limb. We can see it in a cross-section deep to the gluteus maximus but posterior to the obturator internus and quadratus femoris muscles.
Let's now move on to the anterior compartment of the leg which is anterior to the femur and the acetabulum. The muscle we're going to look at here include the iliopsoas, the rectus femoris, the tensor fascia latae, the sartorius, and the pectineus.
The first muscle we're going to look at is the iliopsoas. The iliopsoas is a combination of three muscles, two of which can be seen on the right image – the psoas major which is the smaller bulk of muscle and the iliacus which is the larger bulk of muscle and the more lateral of the two muscles. However, on the cross-section, these separate muscles can be distinguished and therefore represented as the iliopsoas. Its function is to produce flexion of the hip which brings the knee in front of you. It's also the strongest muscle which does this.
The muscle slightly lateral of the iliopsoas is the rectus femoris. This muscle is one of the four muscles that make up the quadriceps femoris group which you can see overlaying the rectus femoris. It is the only one of the group that crosses both the knee and the hip joints. This relationship can be seen when looking at the image on the right, for example, the hip joint is up here and the knee joint is down here.
The next muscle immediately lateral to the rectus femoris and anterior to the gluteus medius is the tensor fascia latae highlighted in green. This muscle is very superficial and inserts into the iliotibial tract, a dense connective tissue tract which runs down the lateral aspect of the thigh and attaches to the tibia.
The sartorius muscle is the longest muscle in the body and crosses from its lateral superior attachment on the innominate bone to a medial inferior attachment on the tibia. Therefore, in cross-section, its relationship will change as you go from the level of the hip down through the thigh towards the knee. As we mentioned, this cross-section is at the level of the distal part of the coccyx so you can see it anterior to the rectus femoris and the iliopsoas.
If we were to end back to the location of the iliopsoas muscle and look at the muscle just medial to this, you'll see the pectineus muscle highlighted in green. This muscle is located in the anterior compartment of the thigh and makes up a component of the floor of the femoral triangle.
Next, let's talk about the neurovascular structures of the femoral triangle outlined in blue. These include the femoral nerve, the femoral artery, and the femoral vein. We'll take a look at the orientation. It's important to note that pectineus and the iliopsoas make up the floor of the femoral triangle, therefore, the neurovascular contents will lie anterior to those muscles. Also, the sartorius forms the lateral boundary of the femoral triangle, therefore, the neurovascular structures are going to be medial to this muscle.
The first neurovascular structure we'll discuss is the femoral nerve. As you can see, the femoral nerve is also the most lateral structure in the femoral triangle. This can be seen by looking at the image on the right and looking at how the femoral nerves are in relationship to the other structures. The femoral nerve courses inferior to the anterior compartment of the thigh and supplies the muscles and the skin of the area.
Just medial to the femoral nerve is the femoral artery. This can be seen as a round lumen on a cross-section due to the strong muscular wall of the arteries. This wall also allows you to distinguish them from veins. This artery and its branches are the sole blood supply to the lower limb.
Visible to the femoral triangle, the most medial structure of the femoral triangle is the femoral vein. Note that its lumen isn't as round as the artery. This is because veins do not have muscular walls of the same caliber.
There are a couple of additional structures that are worth pointing out on cross-section – the spermatic cord and the rectus abdominis. The spermatic cord is formed from the layers of the abdominal wall during the descent of the testes from the inguinal canal. At this level of cross-section, you can see the spermatic cord embedded in the subcutaneous tissue around the pubic region.
The final structure to point out is the rectus abdominis muscle. This muscle is located anteriorly to the midline of the trunk and extends inferior to attach to the pubic bone. Here on the left, we can see two muscle bellies side by side in the midline.
Now that we've come to the end of the tutorial, let's take a couple of minutes to summarize what we've talked about. In our cross-section at the level of the male urinary bladder and the distal end of the coccyx, we started with looking at the osteological features. These included the head of the femur, the greater trochanter, the ischium, and the pubis. Next, we looked at the pelvic viscera including the urinary bladder, the prostate, and the rectum.
In the gluteal region, we located the gluteus maximus, the gluteus medius, the quadratus femoris, the obturator internus, and the sciatic nerve. Then we moved to the anterior compartment of the thigh and viewed the iliopsoas, the rectus femoris, the tensor fascia latae, the sartorius, and the pectineus. We also looked at the femoral triangle which included the femoral nerve, the femoral artery, and the femoral vein. And, lastly, we found the spermatic cord and the rectus abdominis.