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Overview of structures found on different cross-section images at different levels of the upper arm.
Welcome back to the second installment of our cross-sections of the upper limb series where this time we’ll be exploring the cross-sections of the arm. If you haven’t already watched the first part of this video series which looked at the shoulder region, please be sure to take a few moments to check that one out first. Now, let’s continue where we left off.
Once again, we’re beginning with the skeletal framework and as I’m sure you already know, the arm contains just a single bone which is, of course, the humerus. The humerus runs from the shoulder joint right down to the elbow joint where it meets the bones of the forearm and we’re going to begin with the proximal end of the humerus which is the expanded upper part of the bone.
Here we can see the head with its articular cartilage which faces superomedially as well as the lesser and greater tubercles just here which are located in the anterior and anterolateral aspects respectively.
If we continue to focus on the humerus as we move down through this series of cross-sections, we’ll see that the proximal end of the humerus tapers off down into the body or the shaft of the humerus. And as we continue along the body of the humerus, you’ll notice its shape changing from a more cylindrical profile in the proximal half to a more triangular one seen here as we reach the distal half. The humerus now presents three well-defined structures – the posterior surface, the anteromedial surface and the anterolateral surface – which are separated by the medial supracondylar ridge, the anterior border and the lateral supracondylar ridge respectively.
At the distal end of the humerus, the bone expands into medial and lateral epicondyles separated posteriorly by the olecranon fossa. And in this cross-section, we can also identify the articular surfaces of the capitulum and the trochlea, and this articulate with the radius and ulna respectively, forming the humeroradial and humeroulnar joints, which, of course, together make up the elbow joint. We’re going to continue with our cross-sections of the arm, this time focusing on the muscles of the arm.
And as you can see, this can be primarily divided into two groups, which are the anterior and posterior compartments. The anterior compartment contains three muscles which are the coracobrachialis muscle, the biceps brachii and the brachialis muscles. Both the coracobrachialis and the biceps brachii muscles originate from the scapula from the inferior aspect of the coracoid process. With that in mind, let’s now look at our cross-section from the proximal portion of the arm, where we see the coracobrachialis muscle just here deep to the pectoralis major and deltoid muscles.
So, if you’ve studied the biceps brachii muscle before, you’ll know that it has two tendons or heads, and in this cross-section, we can identify both of these tendons. So, over here, we have the tendon of the short head superficial to the coracobrachialis muscle with the tendon of the long head located here between the greater and the lesser tubercles of the humerus.
So, we’re going to move distally along the arm now following these muscles as we go along, and as we work through, watch how both heads of the biceps brachii expand into the belly of the muscle with the coracobrachialis muscle tapering off at the same time before inserting into the medial aspect of the humeral shaft.
In this cross-section, we can now identify the third muscle of the anterior compartment of the arm which is located here on the anterior surface of the humerus – and this is the brachialis muscle – and as we continue on distally, we can see both bellies of the biceps brachii and the brachialis increasing in size. On the other hand, as we continue distally towards the elbow region, the biceps brachii begins to decrease in size tapering off into its tendinous parts, and from here, the tendon of the biceps will go to insert into the radius while the brachialis muscle will continue on to insert into the proximal part of the ulna.
Turning our attention now to the posterior compartment of the arm, we have two muscles to consider and they are the triceps brachii and the anconeus muscles. As the name suggests, the triceps brachii muscle has three heads which are the long head, the medial head and the lateral head. Let’s talk first about the long head.
The long head is the only head to originate from the scapula just inferior to the glenoid cavity. The other two heads both originate from the posterior aspect of the body of the humerus. Looking at our cross-sections beginning at the proximal shaft, we can see the belly of the long head of the triceps brachii here which is well-developed by this level which is we can see is pretty well-developed at this level. And if we look closely at the shaft of the humerus, we can identify the lateral and medial heads which are close to their origin points at this level.
As we continue distally along the arm, notice how the triceps brachii muscle is larger than the muscles of the anterior compartment of the arm. However, in the distal half of the arm, we can see the reverse with the triceps brachii now being smaller in comparison to the anterior muscles. When we reach the distal end of the arm, we can see the medial head of the triceps brachii as well as the tendon of the long and lateral heads more superficially. And it’s important to note at this point that the triceps brachii will continue on from here to insert onto the olecranon of the ulna.
At the distal end of the humerus, we can’t forget to mention the second muscle of the posterior compartment of the arm which is the anconeus muscle, which is a small muscle but is nevertheless important. The anconeus muscle originates from the lateral epicondyle of the humerus and inserts into the posterior surface of the ulna.
So that covers the musculature of the arm, but before I finish this video tutorial, I’d like to now look at the major nerves and blood vessels which are visible in the cross-sections that we’ve just looked at.
So, when we look at the neurovasculature of the shoulder region, we were able to identify the axillary artery and the vein as well as some branches of the brachial plexus. If we move distally through the arm, however, to about the midpoint of the shaft, we can now see some of the branches of these structures beginning with the brachial artery which is derived from the axillary artery and is the primary artery of the arm and the brachial artery runs along the medial aspect of the arm close to the humerus.
Beside the brachial artery, we can see the brachial veins just here which is similarly derived from the axillary vein. And these veins usually do a number follow the same pathway as the brachial artery which makes them pretty easy to spot. Also located on the medial aspect of the arm is the basilic vein as well as this final vein which is the cephalic vein, which is the large superficial vein of the anterolateral arm.
The nerves can be a little bit more difficult to spot, however, in this section, we can identify the radial nerve located posterior to the humerus and the median nerve seen here on the medial aspect of the arm, and close to that, we have the ulnar nerve. Barely visible in this section is the musculocutaneous nerve which we can see here deep to the biceps brachii muscle.
The radial nerve innervates the muscles of the posterior compartment as well as a portion of the brachialis muscle, while the median and ulnar nerves innervate the structures of the forearm and the hand. And, finally, the musculocutaneous nerve innervates most of the anterior compartment of the arm.
And that completes our investigation of the arm region in cross-section. Remember, this video tutorial is the second in a four-part series on cross-sections of the upper limb. So, in the next one, we’ll be continuing distally into the region of the forearm once again identifying the bones, the muscles, and the neurovasculature present there.
I hope you’ve enjoyed this tutorial so far, and I hope to see you in part three of this series.