 So, we are going to demonstrate the extrinsic back muscles. This is a prone canaver and we are demonstrating on the right side. So, we have completely reflected the skin and removed the fat from the right side. So, let us take a look at extrinsic back muscles. The first muscle that we can see here which I am tracing with my finger right now. This is the trapezius, why is it called the trapezius? Because when you combine both the sides together and if you disconnect right from up to its origin from the neck, it forms a diamond shaped structure that is why it is called the trapezius. But here we are seeing only one half of it. The next extrinsic back muscle also belonging to the superficial group is this muscle that we see here. This is the latissimus dorsi. The latissimus dorsi is again a very big muscle just like the trapezius. It does not act on the back. It is an extrinsic back muscle. It acts on the pectoral girdle and it is also supplied by branches of the brachial plexus. In contrast, the trapezius is supplied by the spinal accessory nerve which is cranial nerve number 11. This latissimus dorsi has got an extensive origin and it takes also origin from the outermost layer of the thoracolumbar fascia and we can see the outermost layer of the thoracolumbar fascia here but I will show it just now in a little more detail when I reflect it. So the latissimus dorsi takes partial origin from the outermost layer of the thoracolumbar fascia also. The rest of the muscle has been cut earlier. The muscle continues all the way up like this and it gets inserted onto the arm. So it is a muscle of the pectoral girdle. So these are the two large muscles on the back which are extrinsic muscles. What we are going to do now, we are going to reflect the trapezius and that is what I am doing now. So I have cut it from its origin to the spinous process and I am reflecting it now. And when I reflect the trapezius, we can see some other muscles deep inside. What are the muscles that we see here? First that we see here, this is the rhomboidus major. The next one just about that that we see here, this is the rhomboidus minor and further up is this muscle which I have lifted up with my instrument. This is the libator scapulae. So this is the next layer, the intermediate layer of the extrinsic back muscles. The rhomboidus major, rhomboidus minor and the libator scapulae. These are again muscles of the pectoral girdle. They act on the pectoral girdle and they are supplied by branches of the brachial flexors. So therefore they are not true back muscles. This is why they are clumped under extrinsic back muscles. Just like we reflected the trapezius, we are going to do the same thing with this muscle also. This was the latissimus dorsi. And you can clearly see it is taking partial origin from the posterior layer of the thoracolumbar fascia. And you can see the muscle fibers are taking origin. So we have reflected it here and as I mentioned, this side was already cut. So we will reflect it this way. And once we reflect the latissimus dorsi, we see yet another muscle under that. We can see that the medial portion is aponeurotic and shiny, silvery. The lateral portion is more fleshy. And this muscle is the serratus posterior inferior. This is also an extrinsic back muscle of the intermediate group. This is not a true back muscle. It is an accessory muscle of respiration and it is also supplied by the intercostal nerves. Just like we had serratus posterior inferior here, what we are going to do now is we are going to reflect and I have cut the rhomboidus major here and I am reflecting the rhomboidus major. And once I reflect the rhomboidus major, my assistant is now reflecting the trapezius and the scapula. And we have already cut and reflected the rhomboidus major and once we did that, we can see yet another muscle here which I have lifted up here and I am putting my instrument all the way inside and it is going up and lifting up the muscle. This is the serratus posterior superior. You can notice that the medial fibres are more aponeurotic. So these are the seven back muscles that we, extrinsic back muscles that we saw. So therefore to quickly summarize, we will show again, this is the latsomus dorsi. This was the trapezius which we have reflected and on reflecting the trapezius, we saw the rhomboidus major, rhomboidus minor and the libator scapulae. We will see these muscles again when we come to the pectoral girdle and once we reflect the rhomboidus major, we see this muscle here, serratus posterior superior and when we reflected the latsomus dorsi, we saw this muscle here. This is the serratus posterior inferior. And before I conclude, I am going to show the thoracolumbar fascia. This is the thoracolumbar fascia that we have got here and as you can see lower down, it is so thick and as it goes higher up, it becomes thinner and thinner. The thoracolumbar fascia is an extensive, thick, tough fascia in the back which is formed by the fused posterior aponeurosis of the transverse abdominis and internal oblique muscle. Once we reflect it, we can see some muscles here which I am going to describe in a subsequent video. These muscles are called the intrinsic muscles or the true back muscles. These intrinsic back muscles are the erector spinae or the sacrospinalis muscles which are the true posture muscles and these are the ones which we can feel as a thick ridge on either side of the vertebral column in any normal person and these are the ones which maintain the erect posture. So that is all for now about the extrinsic back muscles. Thank you very much for watching. Dr. Sanjay Sanyal signing out. If you have any questions or comments, please put them in the comment section below. Have a nice day. Thank you to all my assistants.