Cross sections of the upper extremity: Shoulder level

Without a doubt, the upper limbs are one of the most aesthetically and anatomically impressive parts of the human body. Each one is composed of a highly adapted and specialized collection of bones, muscles, arteries, veins, nerves, and connective tissues which, on one hand, provide us with great strength and power to lift heavy objects like these while, on the other, allow for fine dexterity and precision to do things like this. If you’re an anatomist, is itching to find up more, why don’t we cut this upper limb into cross-section to explore some more? What would all these structures look like and how would we identify them? If you’re curious to explore further, then I think this is the video series for you.

Today, we’re going to be working through the first of four video tutorials which will explore the entire upper extremity in cross-section. Our tutorial today will be focusing on the most proximal region of the upper limb which is, of course, the shoulder region. But before we begin with our cross-sections, let’s first introduce you to this video series as a whole.

Over the course of four tutorials, we’re going to be examining a complete series of cross-sections taken at various levels along the upper limb. We’re going to learn how to identify the bones, the muscles, and the neurovasculature present, and, of course, examine how they’re arranged in relation to one another. Knowledge of cross-sections and being able to identify what can be seen in them is important when looking at CT or MRI scans. Remember that when we look at any given cross-section, we’re looking from the feet upwards towards the head. This means that this is the anterior making this the posterior. And this side is the lateral aspect of the body while this is the medial aspect of the body.

Note that since we’re looking at the upper limb, the lateral aspect is often referred to as radial and the medial aspect as ulnar. And it’s always good to remember this any time when studying the upper limb especially from the forearm down. I’d also like to mention that we’ll mostly be focusing on the identification of structures of the upper limb during this tutorial, however, if you want to learn more about the specific anatomy of any structure we meet today, you’ll be able to explore this in greater detail at kenhub.com, where you’ll find all the information you could ever need.

So, let’s get started with this tutorial. As I mentioned earlier, we’re going to begin with the region of the shoulder joint first, specifically the bones as these make up the framework for the rest of the anatomy.

Let’s first get our bearings of where and what we’re looking at. Starting medially, we can identify the first and second ribs just here seen in cross-section. And these bones mark the outer wall of the thorax. If we look medially, we can confirm this by the structure just here which is the apex or upper portion of the right lung. Anterior to the ribs, we can see another bone here in cross-section. This is the clavicle or the collarbone which we know runs from the manubrium of the sternum across to the acromion of the scapula.

And speaking of the scapula, let’s take a closer look at this particular bone. At this level, we’re looking at a cross-section which cuts right through the shoulder joint and we can see there are a number of structures here which are part of this bone.

Starting posteriorly, these two sections of bone here are the spine of the scapula which runs across the superior posterior surface of the scapula. The lateral end of the spine of the scapula, seen here, will continue on to project as the acromion of the scapula. Moving onto the anterior aspect of the scapula, the part seen here is the base of the coracoid process and lateral to this, we can see the glenoid cavity which articulates with the head of the humerus which we can see clearly here.

Note the relatively small articulation surface of the glenoid cavity with the head of the humerus. This allows for the extensive range of movement capable at the shoulder joint and also explains why it’s so easy to dislocate your shoulder.

We’re going to move down now through some sections of the scapula so we can see how its shape changes as we move inferiorly. Notice especially the delicate, almost wafer-thin body of the scapula bordered by its thickened medial and lateral borders. As we continue inferiorly, we can see the medial and lateral edges converging as they approach the inferior angle of the scapula.

So now that we’ve identified the major bones in this cross-section, let’s now turn our attention to the muscles which are present here. And as you can see here, there are quite a few for us to mention, so let’s waste no time and get straight them. We’re going to work through these by the order of their functions, beginning medially along the posterior aspect, the first muscle that we see here is the trapezius muscle – the largest superficial muscle of the back and of the neck. And deep to that, we have the rhomboid major muscle, followed by the serratus anterior muscle which is the saw-shaped muscle which wraps around the ribcage up onto the scapula.

Just anterior to the serratus anterior, we have the subclavius muscle. And all of these muscles which we’ve just met are involved with the suspension and movement of the scapula as well as the clavicle. But, remember, although the scapula lies very close to the axial skeleton, it does not articulate with it. The scapula, therefore, relies on these muscles as well as the clavicle to keep itself suspended in position.

So continuing on with the muscles of the shoulder joint, let’s now look at this muscle anterior to the clavicle, now highlighted in green. And this muscle is the pectoralis major muscle, specifically, its clavicular head. Let’s move a little more lateral towards the scapula now. And here, we can see the subscapularis muscle, a large muscle which originates from the subscapular fossa on the anterior aspect of the scapula.

Moving around the posterior aspect, we have the supraspinatus and the infraspinatus muscles which as their name suggests are located superior and inferior to the spine of the scapula. And something that’s important to note is that these three muscles all belong to a muscle group known as the rotator cuff, which are essential in stabilizing the shoulder joint by keeping the head of the humerus in position.

So if we move down to a more inferior cross-section, we’ll see the fourth muscle of the rotator cuff which is the teres minor muscle. In this section, we can again see the pectoralis major muscle looking much larger than when we last saw it, and located deep to the pectoralis major muscle, we can see the smaller pectoralis minor muscle.

Alright, if you’re wondering how many more muscles there can possibly be in the shoulder region, you can breathe a sigh of relief because we’re at the last one.

Focusing on the lateral over the shoulder region, we can see the large muscle here which is, of course, the deltoid muscle, and we can see that it wraps right around the shoulder or deltoid region, and it has three main parts which we can also identify here – the clavicular part, the acromial part, and the scapulospinal part – all of which are so-called due to their bony attachment sites. As the name suggests, the deltoid muscle is roughly triangular in shape from most angles. So, let’s continue through our cross-sections moving inferiorly and here we can see the deltoid muscle tapering off along the lateral aspect of the arm.

So to finish off with the shoulder, let’s identify the neurovasculature present in these sections.

There are three main structures which I want to identify a few here – one arterial structure and accompanying venous structure and a group of nerve fibers for good measure. Let’s begin with this structure here highlighted in green, which is the axillary artery. This artery provides the primary arterial supply of blood to the upper limb and is located anterior to the subscapularis muscle. And running alongside it, we have the axillary vein, which similarly provides the bulk of venous drainage of the upper limb. And, finally, the brachial plexus, which, of course, provides the bulk of the nerve fibers which innervates the upper limb.

And that completes our investigation of the shoulder region in cross-section. Remember this video tutorial is just the first in a four-part series on cross-sections of the upper limb and in the next one, we’re going to continue distally into the arm region identifying the bones, the muscles, and the neurovasculature present there.

And that completes our investigation of the shoulder region in cross-section.

I hope you’ve enjoyed this tutorial so far, and I hope to see you in part two of this series!