Hey everyone! This is Nicole from Kenhub, and welcome to our tutorial about the cross-section at the level of the third thoracic vertebra. And in this short tutorial, we're going to be looking at some of the muscles, the bones, the vessels and the organs that are visible at this level of the body using the image that you can see just here. And just to note, we're not going to analyze every single structure that we can see. Rather, we're going to focus on describing what we can see in this particular section. But if you'd like to learn more about the anatomical features of these structures in more detail, then you can go along to Kenhub.com and enter the structure that you're looking for in the search function and there you'll be able to find the corresponding video for your needs.
But before we begin, let's discuss briefly what a cross-section is and how we study a cross-section. And after that, we'll go through the different structures that we can see at the level of T3.
So, a cross-section is a section of the human body at the axial plane. And as you probably remember from our tutorials in the basic section, there are three planes that are used in order to describe the human body. And these planes are the coronal plane, the sagittal plane and the axial or transverse or cross-sectional plane. And as you can see here, this last plane passes through the body at a right angle to the long axis of the body. And the axial plane divides the body into superior and inferior parts.
In clinical medicine, the cross-sectional plane is commonly referred to as an axial plane. The cross-sectional plane is particularly relevant in radiology where the CT scans and the MRIs are primary generated in this axial plane. So, as you can see in the CT image above, the plane of this section is the axial or cross-sectional plane, and below, we have the corresponding cross-section. So, as you can see, knowing your structures in cross-section will really help with recognizing structures in radiological images as well.
Another key point that it's important to remember when you see a cross-section or a CT or an MRI image is that these images are positioned for viewing as if the patient is lying on his or her back with the viewer positioned at the patient's feet looking superiorly towards the patient's head. So, if we're looking at the CT section over here, then our corresponding anterior section is over here, the posterior surface of the patient is down here and the left side of the patient's body is to the viewer's right, and the right side of the patient's body is to the viewer's left. And this is the same for the cross-section below. So, anterior, posterior, right and left. So, always keep in mind, the right side of the screen corresponds to the left of the body and the left side of the screen corresponds to the right of the body.
So, now, let's begin looking at our structures at the level of T3 beginning with our image of a very handsome male just here, and the first thing we're going to do before going onto our cross-section is working out what level we're on. So, our section here is going to be part of the upper part of the thorax at the level of the third thoracic vertebra. So, let's just splice our male along this line here and here we have our cross section of the body at the level of the third thoracic vertebra. And, of course, highlighted in green here, we have our third thoracic vertebra also split in half.
So, now here, we're looking at all the structures on this cross section, and the first structure that I want to point out are the two lungs – the right upper lobe of the lung on the left here and the left upper lobe on the right. And highlighted, we have the upper lobe of the right lung as we mentioned as we're in the upper part of the thorax. An important thing to remember as we move down the body in cross-section is that the lungs will appear bigger as we go further down the thorax.
In close proximity to the lungs, we have, of course, the hollow tube known as the trachea. And the trachea is visible up to the level of the intervertebral disc between the fourth and the fifth thoracic vertebrae where it terminates by bifurcating into the two main bronchi. And as we can see in this image of the trachea, the trachea splits into the bronchi at the level of the fourth and fifth vertebrae as we just mentioned. And immediately behind the trachea, we have another tube, which this time belongs to the digestive system and this tube is the esophagus. And as you can see in the illustration, the esophagus lies behind the trachea and, of course, when we say the esophagus lies behind the trachea, we're seeing it below or posterior to the trachea because we're seeing the cross-section as viewed from the legs looking up toward the head.
And zooming in to this section to the left and lateral of the trachea, we can see the left subclavian artery. And as you can see on the illustration on the right side, the left subclavian artery stems directly from the aortic arch. And, if you can't see it, I'll just point it out with my mouse just here. And just to remind you again that this side is the left side of the body.
The other important vessel to the left of the trachea that also stems direct from the aortic arch is the left common carotid artery which you can see here in this zoom and note that it's anterior to the left subclavian artery and posterolateral to the brachiocephalic trunk. And anterior to the trachea is the brachiocephalic trunk which is highlighted in green on this slide and this vessel gets its name from the fact that it supplies the upper extremity or brachium and the head from the Greek word "cephalic" meaning head. So, as you can see in this illustration on the right, the brachiocephalic trunk stems from the aortic arch and gives off two major vessels – the subclavian which supplies the upper extremity and the carotid artery which supplies the head.
So, now, let's take a look at a couple of veins that are located in proximity to the previous vessels. And the first vein that we're going to talk about is the right brachiocephalic vein, which as you can see in the illustration on the right, is the continuation of the right subclavian and right jugular veins. And I'll just point them out so our right subclavian over here and our right jugular over here. And the corresponding vessel to the right subclavian on the left side is called as you can imagine the left brachiocephalic vein, and this vein has more of an oblique course which is why it appears wider than the right vein in this section. And on this image on the right, we can see the right and left brachiocephalic veins joining together to form the superior vena cava.
So now that we've looked at the structures of the thoracic cavity, let's take a look at the bones and muscles of the thoracic wall. And of course we'll begin with the bony structures and the first one that we have highlighted is, of course, the manubrium of the sternum. And as we can see in the cross-section, the manubrium is located anteriorly as it's an anteriorly located structure. And the manubrium articulates with the two clavicles in these joints just here which are known as the sternoclavicular joints, and sternum means chest or breast while clavicula means little key which pretty much describes the action of the sternoclavicular joint which is to move the clavicle in several planes in relation to the chest.
And just external to the sternoclavicular joint as you can clearly see here in this illustration is the first rib, and the first rib is the smallest and strongest of all the ribs. And on the opposite side of the body in the posterior region of the cross-section is another important bone, the scapula. And in the cross-section, we see both the left and the right scapula highlighted – our right scapula on the left here and the left scapula on the right. And, of course, the scapula is a major bone of the shoulder girdle providing attachment sites for many major muscles of the upper body.
And speaking of the major muscles of the upper body, let's now point out some of the muscles that are visible in this section. And in this cross-section and image, we can of course see the deltoid muscle. And the deltoid muscle is a strong and wide muscle lying directly below the skin. In the anterior surface of the body, the most superficial muscle is the pectoralis major muscle which we can see directly below the skin just here. And immediately behind the pectoralis major muscle is the other pectoralis – the pectoralis minor muscle. And in this cross-section, you can see it lying posterior to the pectoralis major muscle just here.
Moving to the posterior surface of the body, we can see the trapezius muscle down here at the bottom of the image and the trapezius muscle is, of course, the most superficial muscle of the back muscles, and just anterior to this is the rhomboid major muscle which we can see highlighted just here. And, finally, let's just take a look now at the muscles that are located close to the scapula starting with this one that is now highlighted – the subscapularis muscle. And this muscle fills the subscapular fossa lying anterior to the scapula which we can see on our image just here. And, again, let's not forget that this side of the image is our left while this side of our image is our right. And, on the posterior aspect of the scapula, we have the infraspinatus muscle and the name, of course, suggests the infraspinatus muscle lies inferior to the spine of the scapula.
And, finally, also behind the scapula and inferior to the spine is the teres minor muscle which runs obliquely and upwards to reach the humerus. The teres major muscle lies a bit more inferiorly so we won't be able to see it in this section.
And that concludes our tutorial on the cross-section. Thanks for watching.
This video is more fun than reading a textbook, right? If you want more videos, interactive quizzes, articles, and an atlas of human anatomy, click on the “Take me to Kenhub” button. It is time to say goodbye to your old textbooks and say hello to your new anatomy learning partner, Kenhub!