Hello everyone! This is Joao from Kenhub, and on today's video, we're going to continue our cross-section tutorial series, this time focusing on the anatomy at the level of the coracobrachialis muscle. Our tutorial today will familiarize us with the bones, muscles, and vessels that we can see at the cross-sectional level of the coracobrachialis muscle. We will be using the image we see here and instead of going into depth about every single structure that is visible, we will learn the relations that each structure has to one another and also get an appreciation for how these structures are spatially related to each other.
So, in this tutorial, we will review some anatomical terminology most relevant for studying cross-sections. After that, we will look at some of the viscera or organs, bone structures, musculature, and neurovasculature at the coracobrachialis level. We will conclude the tutorial with a clinical note and then review what we have learned. But, first, let us quickly review what a cross-section is before we dive into detail in this tutorial.
Remember that there are three planes in which the body can be divided. We have the coronal plane, the sagittal plane and the axial or transverse or cross-sectional plane. The latter of these planes splits the body into superior and inferior parts and is very important when we study MRIs, CT scans, and cross-sectional anatomy. In clinical anatomy when we look at CT or MRI scans and cross-sections of the body, we are viewing the image as if we are staring up towards the head from the patient's feet. So we have the front or anterior side of the body here and the back or posterior side of the body here with the right side of the body here and the left side here.
We will be focusing on the right-hand side during this tutorial so it's also good to remind ourselves that these will be the lateral and medial aspects of the body here.
Now that we are familiar with how we should orient ourselves, let us dive into the cross-sectional anatomy at the coracobrachialis muscle level. Let's start by looking at the viscera or organs that are visible at this level and if we take a look at the medial aspect of the image, we can easily spot this main organ here which is the superior lobe of the right lung. Since we are at a relatively higher level within the body, the lungs do not appear as large as they would if we were to look at a more inferior cross-section of the thorax.
Surrounding the lungs and protecting the thorax, we have the thoracic cage commonly known as the rib cage. Specifically, in this image, we can see a section of the second, third, fourth and fifth ribs as well as the sternum here. Around the thoracic cage, there are a number of muscles that we will look at shortly so let this skeletal structure be one of the three skeletal landmarks to help us orient ourselves.
Now that we know where the rib cage can be found, there are two other skeletal structures you should use when orienting yourselves. The first one is a scapula or shoulder blade now highlighted in green and is found on the posterolateral aspect of the thoracic cage but we will see that it further acts as a boundary to compartmentalize different muscles. Primary function of the scapula, however, is to attach the upper limb to the trunk of your body. The second bone of interest here is the humerus which we can see now highlighted in green. The humerus is the relatively large single bone that makes up your arm and it articulates with the scapula at the glenoid cavity.
Now it is time for us to look at the musculature. We will identify quite a few structures beginning first with the deltoid muscle which is the most lateral and one of the largest muscles we will see in this cross-section. The deltoid as seen here in the illustration acts as a cap on the shoulder. It is made up of three parts – the anterior or clavicular part, the middle or acromial part and the posterior or spinal part. They all act together to abduct the arm at the shoulder joint while the anterior part and posterior part aid in flexion or extension of the arm at the shoulder respectively.
Moving medially from the deltoid, the most anterior structure we can see is the pectoralis major highlighted here. The pectoralis major is a large fan-shaped muscle that also aids in the flexion of the arm but also adduction as well as internal rotation of the arm at the shoulder joint.
As we know, there is also a pectoralis minor muscle which can be seen highlighted here. This found just deep to the pectoralis major and its function is to contribute to the attachment of the scapula to the rib cage as well as aid in scapular depression. Adjacent to the pectoralis minor is the coracobrachialis muscle. In the cross-section, we can see that the coracobrachialis muscle is medial to the humerus and deep to some parts of the deltoid and the pectoralis major.
What we can see sandwiched between the coracobrachialis and the anteromedial portion of the deltoid here is the tendon belonging to the long head of the biceps brachii muscle. As you can see in the illustration, the long head of the biceps brachii muscle has a long tendinous portion which originates from the coracoid process of the scapula. It crosses the head of the humerus before proceeding along the humeral shaft.
If we focus a little more medial to the long head of the biceps brachii on our cross-section now, just anterior to the coracobrachialis, we can similarly see the tendinous portion of the short head of the biceps brachii muscle. In the illustration here, we can also see that the short head is located more medial than that of the long head and proceeds directly from the coracoid process rather than around the head of the scapula. If I add in the coracobrachialis muscle to this illustration, we can see the origin of the short head of the biceps brachii lies just anterior to the origin of the coracobrachialis which also correlates with our cross-section.
Opposite or antagonistic to the biceps brachii, we have the triceps brachii. The triceps brachii has three heads associated with it all of which are just about visible in this particular cross-section. Let's see if we can identify them at this level of the cross-section. The long head of the triceps brachii is cut through its muscular belly which makes it easily identifiable just medial to the deltoid muscle and posterior to the humerus.
Deep to the long head in our cross-section, we can see the most proximal portions of the lateral head of the triceps brachii which at this level has just left its origin on the posterior surface of the humerus. This head will continue distally in the large lateral belly of the triceps brachii and just medial to the lateral head of the triceps brachii, we can just about see the medial head of the triceps brachii at this origin point here. We can see in this illustration the medial head lies deep to the lateral and long heads. Collectively, the medial, lateral and long heads of the triceps brachii will continue distally to insert into the olecranon of the ulna and work together to extend the forearm at the elbow joint.
As we move more medially along the posterior aspect of our cross-section, we have the teres major muscle. As you can see in the illustration which is a posterior view of the shoulder joint, the teres major originates from the inferior angle of the scapula and crosses anterior to the long head of the triceps brachii as you can see here. Superior and slightly posterior to the teres major is then the teres minor muscle also now highlighted in green. The teres minor muscle originates from the lateral margin of the scapula as we can see in our cross-section here.
Continuing medially, the next muscle we will meet is the infraspinatus muscle highlighted in green in the images. The infraspinatus is located posterior to the wafer-thin body of the scapula and rests specifically upon the infraspinous fossa of the scapula. On the other side of the scapula or anterior surface of the scapula, we find the much larger subscapularis highlighted here that rests between the scapula and the rib cage. The primary role of this muscle is to aid in internal rotation of the humerus with a minor role in adduction of the arm.
Now sandwiched right between the subscapularis and the rib cage, we have the serratus anterior. We can see from the illustration that the serratus anterior lies in direct contact with the thoracic cage. Using this bit of information, it is easy to then locate the serratus anterior in our cross-section. Not only does the serratus anterior act to attach the scapula to the rib cage, it also functions to protract the scapula or pulling the shoulder blade forward.
The last two muscles we will be looking at at in this cross-section are located at the posterior portion of this cross-section. The trapezius muscle which is a large muscle that traverses the superior half of the back and we can see it highlighted here as the most posterior muscle in the cross-section. Its primary function is to elevate the scapula upwards which is accomplished by the superior or descending fibers of this muscle. The middle fibers and most inferior or ascending fibers of the trapezius, however, aid in retraction and depression of the scapula respectively.
Located deep to the trapezius or anterior in the case of this cross-section, we have the rhomboid major muscle highlighted right here. This particular muscle pulls up on the medial border of the scapula. This causes the shoulder blade to rotate downward ultimately causing depression of the shoulder joint.
Now that we have gone through the musculature within the cross-section at the level of the coracobrachialis muscle, let us quickly point out important vasculature and nervous structures within this cross-section. We're going to be focusing around three main structures in this section of our tutorial which are the axillary artery, brachial plexus and axillary vein. Before we identify these structures in cross-section, it is important to note how the three of these structures relate to one another.
As you can see in the illustration on the left, the neurovasculature at this level is focused around the area of the axilla or armpit anterior to the subscapularis muscle and posterior to the coracobrachialis. So in our cross-section, we should expect to find our structure in this region here. The structure that we see in red is the axillary artery with its venous counterpart, the axillary vein, seen in blue. The brachial plexus can be seen in yellow. Notice how it wraps around the axillary artery in this region.
Now let's look at the structures in cross-section. We can see the axillary artery here highlighted in green on these images along the posteromedial aspect of the coracobrachialis. And the axillary artery eventually becomes the brachial artery which serves to supply the entirety of the upper limb. As we move more medial, we can find then this one here, the axillary vein, highlighted in green. The axillary veins drain the upper limb of venous blood eventually returning it to the right side of your heart.
Also, in this area, we can see then the brachial plexus within the fatty tissue of the armpit. The brachial plexus branches into numerous different nerves that supply both the musculature and the sensory organs of the upper limb.
Before we get into a brief summary of what we covered today, let's touch upon a clinical feature at this particular level of cross-section. The coracobrachialis muscle is at the same level of the axilla or the armpit. Within the armpit, there are numerous lymph nodes where immune cells can reside. The image here has highlighted the axillary lymph nodes in green. These particular lymph nodes receive lymph fluid from the breast tissues and other areas of the chest and upper limbs. Lymph nodes can also carry cancerous cells that have originated from metastatic tumors and in some cases of breast cancer, the axillary lymph nodes can receive lymph from the breast that contain cancer cells which can then cause enlargement or immune response in these nodes.
To examine the axillary lymph nodes for this, the doctor may palpate or lightly massage the lymph nodes under the armpit which if enlarged may indicate metastasis from the primary site within the breast. Clinicians also examine the axillary nodes by means of surgical excision or dissection and more commonly by medical imaging such as CT scans. In our cross-section, there are, unfortunately, no readily identifiable lymph node here, however, if we were to look for them, one area we could expect to see them is close to the major vessels of the axilla, in particular, the axillary vein.
So this brings us to the end of our tutorial on the cross-section at the level of the coracobrachialis muscle. But before we end this, let us quickly review and highlight the structures that we covered here on this video.
We started with the two bony structures – the scapula and then the humerus. From there, we covered musculature anteriorly and moved posteriorly. We started with the deltoid then the pectoralis major and pectoralis minor. We then looked at the coracobrachialis, the long head of the biceps brachii, and the short head of the biceps brachii. We continued with other muscles and looked at the long head of the triceps brachii, the lateral head of the triceps brachii, and also the medial head for this muscle. Just adjacent to these structures, we have the teres major with the teres minor directly behind it.
To complete our musculature portion of this cross-section, we looked at the serratus anterior with the subscapularis beside it. We saw the infraspinatus behind the scapula in this image and the rhomboid major and trapezius as the most posterior muscles. Lastly, we looked at the vasculature and nervous supply running through this region. We saw the axillary artery and the venous counterpart – the axillary vein – here with then the brachial plexus in between the vessels on this image.
And that wraps up our tutorial on the cross-section at the level of the coracobrachialis. Thank you for watching and I hope to see you on the next video.
And this, my friends, concludes the video tutorial on the functions of the adductor brevis muscle. Thanks for watching and I will see you on the next one.