 So this is a demonstration of the muscle acting on the pectoral girdle from behind. So this big muscle that we see in front of us, this muscle going all the way, this is the trapezius muscle. Why is it called the trapezius? Because when we combine both the sides, it looks like a rhomboid, like a trapezius, that's why it's called a trapezius. This is the chrome cadaver, I am standing on the left side of the cadaver and the camera person is standing on the right side of the cadaver. So let's take a look at some of the features of the trapezius in how it acts on the pectoral girdle. The trapezius as you can see is a considerably big muscle. The trapezius has got three sets of fibres. The descending fibres which come from above, the horizontal fibres and the ascending fibres. The fibres take origin from this place, this region here, from the external occipital protuberance and from the medial part of the superior nucleus line and they take origin from the nucleus ligament. Then the spinous process of C7, all the way to T12, then the fibres converge and then they get inserted on to the inner surface of the spinal scapula, the chromium process and the clavicle. So this is the origin and the insertion of the trapezius. The nerve supply of the trapezius is spinal axillary nerve CL11. So this is a unique nerve supply in so far that trapezius is the only muscle of the pectoral girdle which is not supplied by branches of the brachial plexus. So therefore in brachial plexus, complete brachial plexus injury, the trapezius is spared. More about the clinical correlations later on. Let's take a look at the action of the trapezius. The descending fibres which come from the skull bone and from the back of the neck, they are responsible for elevation of the shoulder or shrugging the shoulder and that is used as one of the tests for the trapezius. We ask the patient to shrug his shoulder against resistance. The middle fibres, the horizontal fibres, they are responsible for retracting the shoulder that is bringing the scapula closer to the midline and the ascending fibres, they are responsible for depressing the shoulder assisted by gravity. So when there is a paralysis of the trapezius muscle, the shoulder droops down. In fact, this sloping appearance of the shoulder that we see in a normal person is because of the descending fibres of the trapezius and in the paralysis of the trapezius, the shoulder will be drooping down and the person will not be able to shrug his shoulder against resistance. However, it will be partially compensated by the libator's scapulae muscle which is also by the superior fibres of the cirrhosis anterior. There are few other actions of the trapezius. The superior fibres, the descending fibres and the ascending fibres, both of them together, they are responsible for rotating the glenoid cavity of the scapula upwards. Now we shall reflect the trapezius. We have inside the trapezius here and I am reflecting one part of the trapezius, the medial portion. Up like this, I have reflected it up and I am going to reflect the other part of the trapezius here to see some of the neurovascular structures which are present on the deep surface of the trapezius. Straight away, we can see this vessel here. This is a branch of the dorsal scapular artery which comes from the second part of the subclavian artery and this is the one which runs and we supply these muscles just subsequently. The next structure that we can see here is this nerve and we can see that it is supplying the trapezius from its deep surface. This is the spinal accessory nerve which I talked about just now. This comes from the neck. It runs in the lateral cervical region and it enters the trapezius on its deep surface at the junction between the upper two thirds and lower one third of its anterior border of the descending fibres and it supplies the trapezius from its deep surface. We can also see a remnant of the nerve on this side, on this part of the muscle also. This is the spinal accessory nerve. So these are the two structures that we can see here in the trapezius. Some other relations of the trapezius. The descending fibres of the trapezius which come from the nuclear line and from the occipital protuberance, they constitute the posterior margin of the lateral cervical region. That is another important relation of the trapezius. When I put the fibres of the trapezius back in place, the trapezius also forms a boundary of another triangle and that triangle we can see here. This is known as the triangle of oscurtation. The triangle of oscurtation is on the back and it is bounded by the lateral border of the trapezius, the superior fibres of the leximus torsae and it is bounded by the medial border of the scapula. So this is the triangle of oscurtation and this area is relatively bereft of the muscle except for a little bit of the rhomboid is major and therefore this is called the triangle of oscurtation. So therefore the trapezius forms boundaries of two triangles, anatomical triangles in the human body. The trapezius muscle has been used and is being used by plastic and reconstructive surgeons and cosmetic surgeons for various myocardial flaps. So this is one clinical application of the trapezius muscle.