 Now, we are going to demonstrate another very big muscle, which acts on the pectoral girdle and which is seen from behind and that is this muscle in front of us. This muscle that we see, this is the latissimus dorsi. Let us take a look at the origin insertion and lousa by action and a few other features about the latissimus dorsi. The latissimus dorsi takes origin from the spinous process of T7 to T12. It takes origin from this shiny structure that we see here. This is the posterior layer of the thoracolumbar fascia and we can see that fibers are taking origin from that. Then it takes origin indirectly through the thoracolumbar fascia. It takes origin from the iliac crest and also takes origin from the lower few ribs and we can feel the ribs here and I am going to show the ribs when I retract this muscle and a little bit from the scapula here. So therefore it is a very big muscle. It covers almost three-fourths of the back of the patient and this is the iliac crest where we can feel. The muscle goes up like this and then the fibers converge and they get inserted onto the floor of the inter tubercular crest of the humerus where my finger has gone in but we cannot see it in this dissection at this position. So this is the insertion of the latissimus dorsi. The nerve supply of the latissimus dorsi is the thoracodorsal nerve which I am going to show you six, seven, eight from the brachial plexus and it is also supplied by the thoracodorsal branch of the scapular artery which also I shall show you just now. Let me come to the action of the latissimus dorsi. The latissimus dorsi is known as the climber's muscle. When a person is holding onto an object or something above his head and he pulls his body up using his arms he is using the latissimus dorsi. So therefore rock climbers and people who are doing lifts they use the latissimus dorsi. The latissimus dorsi because it is inserted onto the floor of the inter tubercular sulcus it is also responsible for extension of the arm, adduction of the arm and medial rotation of the arm. So these are some of the other actions of the latissimus dorsi. The latissimus dorsi as we can see here the lateral border of the latissimus dorsi it forms the posterior wall and the posterior fold of the axilla and this is where we can feel this. How do we test for latissimus dorsi? We can test for latissimus dorsi by asking the person to press against a table with his elbow and we can feel the lateral border of the latissimus dorsi on the posterior fold of axilla and that is how we test for the latissimus dorsi. Now what I will do is I will I have slit open the thoracolumbar fissure because I told you the latissimus dorsi takes origin from that and we have cut the latissimus dorsi here and I am going to reflect it and this is what I have done. And we can see that this whole thing is the latissimus dorsi and we can now see the posterior parts of the ribs and we can see some other muscles but they are not within the purview of our present demonstration. When we do that we can see the neurovascular structures running on the deep surface of the latissimus dorsi. Therefore to show the neurovascular structures I am putting back the muscle again and I will turn it around and I will show you the neurovascular structures. These are the neurovascular structures which are running. This is the thoracodorsal nerve which is coming from the brachial plexus C678 and this is the thoracodorsal vessels, thoracodorsal artery which is the continuation of the sub scapular artery after it is given of the circumflex scapular branch. We can also see the same neurovascular structure between the latissimus dorsi and the t-rease major and this is what we can see. We can see the thoracodorsal nerve here. We can see the thoracodorsal nerve here and we can see the thoracodorsal arteries and the other accompanying vein running here. So these are the neurovascular structures. The injury to the thoracodorsal nerve can occur during axillary dissection for breast cancer. When there is a paralysis of the latissimus dorsi, apart from loss of all the actions that you mentioned, the person will not be able to use a crutch. So when he tries to use a crutch and he presses down with his shoulder on the crutch, the shoulder instead of pressing down on the crutch, the shoulder will move up. So the person will not be able to use a crutch in paralysis of the latissimus dorsi. Apart from forming the posterior fold of the axilla which I mentioned and the posterior fold of the latissimus dorsi also forms boundary of two triangles. One of the triangle is this triangle that we see here in front of us. This is the triangle of oscultation which is bounded by the trapezius, the scapula and the superior fold of the latissimus dorsi. So therefore this is the triangle of oscultation and latissimus dorsi. Upper margin is forming a boundary. Likewise if you go straight all the way down to the iliac crest which I can feel here, the latissimus dorsi forms a boundary of a small triangle called the inferior number triangle of pethi. That triangle is bounded by posteriorly it is bounded by the latissimus dorsi, anteriorly it is bounded by the posterior free margin of the external oblique muscle of the abdomen and inferiorly it is bounded by the upper border of the iliac crest. And that is a potential site of lumbar hernia. So that is another triangle which is bounded by the latissimus dorsi. The latissimus dorsi has also been used by plastic and reconstructive surgeons for various microteinase reconstructive flaps and procedures. Thank you very much for watching. If you have any questions or comments please put them in the comment section below. Dr. Sanjay Sanyal, Sany out. Have a nice day.