Muscles and bones of the thigh at the level of the gracilis muscle.
Meet the mighty lifting machines of our body – the muscle powerhouse that are your thighs. A few parts of the human body are built and more specialized for strength and power as the thigh. And although this part of the lower limb contains just a single bone, that bone that happens to be the heaviest, longest and strongest bone in the entire body, and surrounding this bone are several muscle groups which collectively can produce amazing force and strength making us able to lift and push heavy loads just like our friend here. Although, if you're like me, you have a couple more days in the gym to do before you'll be lifting like this.
If you're wondering how are the structures of the thigh arrange and relate to one another, you're in luck. Because in this tutorial, we're going to be cutting this thigh into a cross-section and exploring the anatomy involved here. Let's get started with our tutorial on the cross-section of the lower limb at the level of the gracilis muscle.
In this short tutorial, we are going to be looking at the bones, also the muscles, and vascular structures at this level. Now keep in mind that we will not go into great detail regarding the structures which we will be seeing. Instead, we will be focusing on the anatomical relationships that are displayed within this section. Now if you would like to learn more about the anatomical features of a particular structure and you can do so here at Kenhub by entering the name of the structure you're interested here using our search function. In the meantime, let's keep going with our tutorial beginning with finding out what a cross-section actually is and how to study a cross-section.
So, a cross-section is a section of a human body on the axial plane. You may remember from our tutorials in the basic section that there are three planes which we use to study the human body. These are the coronal plane, the sagittal plane, and the axial plane which is also referred to as the transverse or cross-sectional plane. The axial plane is perpendicular to the long axis of the body and separates the body into superior and inferior parts. This plane is also relevant in radiology because CT scans and MRIs are generated in the axial plane.
Now another key point which may help you better understand a cross-section is to remember that cross-sectional scans are generated with the patient lying down on his back so the lower parts of your cross-section corresponds to the posterior side of the body. So we will find the anterior side of the thigh over here near the top of the image and the posterior side over here near the bottom of the image.
Another key point to keep in mind is that when we are looking at cross-sections, we should imagine that we are observing from a viewpoint of the patient's feet and looking towards the patient's head. This means that the right side of the patient is to our left and the left side of the patient is to our right. Now that we have reviewed some basic principles about cross-sections, let's dive right into our cross-section of the thigh starting with this central structure in our cross-section which is, of course, the femur.
Here you can see the femur in cross-section. The femur is also the most proximal bone of the free lower limb as well as the longest bone in the entire body. It is also the only bone in the thigh. As you can see, the femur has a proximal end and a feature here, the head of the femur, which articulates with the hip bone forming then the hip joint. It also has a distal end which articulates with the tibia and patella forming then the knee joint. Our cross-section today runs through the body also known as the shaft of the femur.
Now let's take a look at the muscles which surround the femur beginning with those of the anterior compartment of the thigh. First up is the quadriceps femoris. This collection of muscles work as the great extensor of the knee and form the largest part of the mass of the anterior thigh. It's also subdivided into four distinct muscles which broadly share the same function. The first of these four muscles is the rectus femoris muscle. At this level, it is only a thin band as it tapers off into the quadriceps tendon.
In addition to extension of the leg at the knee joint, the rectus femoris muscle is also involved in the flexion of the thigh at the hip joint. You can see that the rectus femoris is surrounded by three muscles and these also belong to the quadriceps group. The muscle lateral to the rectus femoris is the vastus lateralis muscle. As you can see, it covers almost all of the anterolateral aspect of the front of the thigh. On the other side of the rectus femoris, we will find the vastus medialis which defines the anteromedial aspect of the thigh.
Now between the vastus lateralis and vastus medialis and posterior to the rectus femoris, we will find the fourth member of the quadriceps muscle hugging around the anterolateral aspect of the femur. This is the vastus intermedius muscle.
The next muscle we will be looking at is the sartorius which is the strap-like muscle which crosses diagonally across the thigh. At this level of cross-section, the sartorius can be found medial and posterior to the vastus medialis muscle. Important to note is that the sartorius muscle is not one of the quadriceps femoris muscles. The sartorius acts mainly as a synergist muscle, meaning rather than performing any action on its own, it assists other muscle groups in completing motions. This muscle helps with flexion, weak abduction and lateral rotation of the thigh at the hip joint as well as flexion and medial rotation of the leg at the knee joint.
We're going to continue now to the medial side aspect of the thigh and focus our attention on the adductor group of muscles, and as their name would suggest, this group of muscles are primarily working to adduct the thigh at the hip joint. There are normally six adductor muscles located across the length of the medial thigh, however, at this level of cross-section, we can only see two of these muscles.
The first we will be looking at is the gracilis muscle. The gracilis is a long and slender muscle. At the level we're looking at, the gracilis can be seen directly posterior to the sartorius muscle lying superficially on the posteromedial aspect of the thigh. The gracilis muscle is involved in adduction and flexion of the thigh at the hip joint as well as internal rotation and flexion of the leg at the knee joint.
The other adductor muscle visible in cross-section is the adductor magnus muscle. The adductor magnus muscle is the largest and most posterior member of the adductor group. It is generally described to have two parts which we can see in our cross-section here. These are the adductor or pubofemoral part and the hamstring or ischiocondylar part. The adductor part inserts into the linea aspera of the femur and we can see the most distal part of this attachment in this cross-section. The hamstring part of the adductor magnus muscle inserts into the medial condyle of the femur. The adductor magnus muscle is mainly involved with then adduction, flexion and lateral rotation of the thigh at the hip joint.
We are almost, almost there. Next up on our list will be the final group of muscles today which, of course, are the muscles of the posterior compartment of the thigh. In the posterior compartment of the thigh, we will be examining three muscles. We will see three muscles in this compartment all of which contribute to then flexion of the knee joint. The first muscle we will be looking at in this region is the biceps femoris. As its name suggests, this muscle has two heads which are the long and the short heads. Collectively, these two heads occupy the posterolateral portion of the thigh.
If we look medial to the biceps femoris, we will see another muscle. This one is known as the semitendinosus due to its long, slender and tendinous portion formed as it descends towards the knee. Remember that as a muscle of the posterior group, this muscle acts as a knee flexor.
Now let's continue on medially now, we will next encounter the semimembranosus muscle. You can see it lies posterior to the adductor magnus muscle. This muscle starts as a thin, flat membranous tendon, however, expands into its large muscular belly as it descends distally as you can see in our cross-section.
And with that, we have now identified all the musculature at this level. We are now ready to move on to the next topic and this time, we're going to talk about the last group of structures we are going to look at today which are the neurovascular structures seen at this level of the thigh.
At first is the femoral artery which you can now see highlighted in green. This artery which provides the main arterial supply to the lower limb is found within an anatomical space known as the adductor canal or Hunter's canal which is bounded posteriorly by the adductor longus and adductor magnus muscles and anteriorly by the vastus medialis and anteromedially by the sartorius muscle. Also contained within the adductor canal is the femoral vein which runs alongside the femoral artery.
You can also identify another of the major veins of the thigh which is the great saphenous vein. As you can see, this vein is located along the medial aspect of the thigh in the subcutaneous fat near the surface of the skin. This vein courses along the medial aspect of the lower limb and drains the blood from this wide region. Now, due to accessibility and the fact that blood from the lower limb can return to the inferior vena cava via the deep circulation, the great saphenous vein is an attractive option for an autologous coronary bypass graft.
The final structure which I would like to point out in our cross-section today is the structure seen here which is the sciatic nerve. Located deep within the muscles of the posterior compartment, the sciatic nerve is the largest branch of the lumbosacral plexus and innervates the muscles of the posterior compartment of the thigh as well as those of the leg and foot via its branches.
And with that, we have now identified all of the major structures of this cross-section. I hope you have enjoyed exploring it with me. But don't go anywhere, I still have to recap. I would like to do a quick recap of what we just learned.
We started with the anterior compartment specifically with the members of the quadriceps muscle group. As we had discussed, these are the rectus femoris muscle, the vastus lateralis muscle, vastus medialis muscle, and the vastus intermedius muscle. In the anterior compartment, we also saw then another muscle – the sartorius muscle. We followed this by identifying some of the muscles of the adductor muscle group. These were the gracilis muscle and the adductor magnus muscle. Then we moved to the posterior compartment of the thigh. There we looked at the biceps femoris muscle, the semimembranosus muscle, and the last muscle for today was then the semitendinosus muscle.
Finally, we briefly looked at some of the neurovasculature visible in our cross-sectional level. We first discussed the femoral artery and its neighbor, then the femoral vein. We also briefly discussed the great saphenous vein and then finally went and talked about the sciatic nerve.
With the end of this summary, we come to then the end of our brief tutorial on the cross-sectional anatomy of the thigh at the level of the gracilis muscle. I hope you have enjoyed it and I will see you on the next one.