 Welcome to our dissection of the back. My name is Dr. Sanjay Sanyal, Department Chair of Anatomical Sciences in the Caribbean. Today our dissection will be focused on the extrinsic muscles of the back. So this is the prone cadaver here and we have reflected the skin from the right side of the back, from the left side of the back. So let's start off. Straight away we can see two muscles. We can see one muscle here and we can see another muscle, a big muscle here. These two are the extrinsic muscles of the back. Now why do we call them the extrinsic muscles of the back? They are located in the back but they do not control the vertebral column. Instead they act on the pectoral gird. That is reason number one. Reason number two is they are not supplied by the dorsal ramia of spinal nerves. Instead they are supplied by the brachial plexus which is the ventral ramia spinal nerves. So therefore they are not the true back muscles. Having said that, the first part of today's video will focus on this muscle here and the muscle under that which I am going to reflect and show you. And the second part of today's video will focus on this muscle. Okay, so this muscle that we see here, this is the trapezius. We can see only the left side. It is called the trapezius because when you combine both the sides together they form a trapezium. The trapezius has got descending fibres, horizontal fibres, ascending fibres. They take attachment from the superior nuclei, the spinous processes and they get inserted onto the spinous scapula, the chromium process and the clavicle. One of the functions of the trapezius is to shrug the shoulder and the other function of the trapezius is to act on the pectoral girdle as I mentioned earlier. And the trapezius is unique insofar that it is not supplied by the brachial plexus. It is supplied by the spinal accessory nerve that is cranial nerve number CN11 along with the sternocleidomastoid. Now what we shall do is I have already incised it here. I'm going to gently reflect the trapezius up to show you what is the structure under the trapezius. So I have already incised it and I have reflected the trapezius. Once I reflect the trapezius we immediately see another series of muscles here. So let's take a quick look at them. These are also muscles acting on the pectoral girdle. The three muscles from above downloads are the levator scapulae. My probe is now pointing at the rhomboids minor and just below that, the muscle which you see is the rhomboids major. We cannot see the levator scapulae because it's situated high up in the neck and it is inserted onto the medial border of the scapula near the superior angle and is responsible for elevation of the scapula as the term implies. The rhomboids minor and the rhomboids major are both inserted onto the medial border of the scapula from above downwards and their collective function is to retract the scapula towards the midline and when these two muscles are paralyzed the scapula on that side is debated more laterally compared to the normal side. And these three muscles are supplied by the dorsal scapular nerve which is the first branch from the root of the brachial plexus that is C5 and I shall show you the dorsal scapular nerve just now. In order to see the dorsal scapular nerve I shall separate the levator scapulae and the rhomboid and when I do that you will see the neurovascular structures coming here they supply from the deep aspect these are the dorsal scapular vessels. The dorsal scapular artery arises from this second part of the subclavian artery and the dorsal scapular nerve arises from the root of the brachial plexus. So this is the levator scapulae and the rhomboids. Before I go out of this when I put this back here you will notice a small triangular space between the lateral border of the trapezius the superior border of the leximus dorsi and the medial border of the scapular. This is known as the so-called triangle of oscultation and the floor of the triangle of oscultation is partly composed of the rhomboids major. One more thing I would like to show you this is a branch from the supra scapular artery and we can also see the branch from the accessory nerve both of these supply plexus muscle from the deep surface. So this is about the muscles, extrinsic muscles of the upper part of the back. Before I mention about the leximus dorsi I need to tell you something about and a very important facial structure which is located in the back which is called the thoracolumbar fascia. Now quite a bit of the thoracolumbar fascia has been removed here but I need to tell you that because quite a bit of it is relevant to our presenter section. The thoracolumbar fascia is a tough fascia which starts from the eponeurosis of the flat muscles of the abdomen which a little bit of can be seen here. And as it moves medially on the posterior aspect it splits into three layers. The anterior layer and the middle layer encloses this muscle here which is not relevant to our present discussion but I'm just mentioning it because it is visible here. The anterior and the middle layer of the thoracolumbar fascia encloses this muscle which is the quadratus lumbarum muscle which is the muscle of the posterior abdominal wall. The middle layer and the posterior layer of the thoracolumbar fascia fuses to form this facial structure here which encloses the intrinsic of the true muscles of the back which I shall demonstrate in a subsequent dissection tomorrow. Relevant to our present dissection is that this muscle which I'm going to describe now from the latissimus dorsi takes origin apart from other structures it takes origin from the posterior most or the third layer of the thoracolumbar fascia. Of course it's a very big origin it takes origin from the spinous processes it takes origin from the iliac crest it also takes origin from the posterior layer of the thoracolumbar fascia. This latissimus dorsi as it moves up the fibers converge. It converges laterally and it also rotates a little bit and they all get inserted onto the inter tubercular sulcus of the humerus. Latissimus dorsi is also supplied by a branch of the brachial plexus known as the thoracodorsal nerve root value C678 and it is responsible for adduction of the arm. When we split open this fascia here we notice that the latissimus dorsi also comes away with it because the latissimus dorsi is taking origin from the outermost layer of the thoracolumbar fascia here. That's all for today's dissection. If there are any questions or comments please mention the comments section below. Thank you very much for watching. Dr. Sanjay Sanyal signing out. Don't forget to like and subscribe.