 Good day everybody. This is Dr. Sanjay Sanyal, Professor of Department Chair. So this is going to be a demonstration of the muscles on the posterior compartment of the arm and the pectoral region. So this is the prone cadaver. We are standing on the left side. Let's start with the muscles which act on the pectoral girdle and on the shoulder of the posterior aspect. This is the trapezius muscle that we have reflected here. And once we reflect the trapezius, we can see the outline of the scapula here. This is the medial border of the scapula. This is the spinous process and this is the root of the spinous process of the scapula. This muscle that you see here. This is the supraspinatus muscle. This is the first of the rotator cuff muscles. Then we have this muscle which is below the spinous process. This is the infraspinatus muscle. This is the second muscle of the rotator cuff. And this is the third muscle. This is the t-dice minor. There's a fourth muscle of the rotator cuff, which you cannot see in this picture, because it is located in front of the scapula and where my hand is located. This is the third fourth muscle of the rotator cuff and that is called the subscapularis. Why are these muscles called the rotator cuff? Because they all form a cuff around the shoulder joint. That's why they're called cuff. But not all of them are rotators of the shoulder joint. For example, the supraspinatus does not rotate the shoulder. It is just responsible for initiating the first 10 degrees of abduction. But this is the most important muscle of the rotator cuff because if this gets stoned, then it produces the condition known as rotator cuff syndrome. The person will not be able to initiate his abduction. The infraspinatus and t-dice minor, these are lateral rotators of the shoulder. And the subscapularis is a medial rotator of the shoulder. That's why these muscles are called rotator cuff muscles. They help to stabilize and strengthen the capsule of the shoulder joint because the tendon of insertion, it merges with the capsule of the shoulder. The supraspinatus, infraspinatus and t-dice minor are inserted onto the greater tubercle of the humerus. And the subscapularis is inserted onto the lesser tubercle of the humerus. So that's about these muscles. The next muscle, which I'll draw your attention to is this one here. We can see the full length, we can see it is taking origin from the lateral border of the scapula lower down. And the fibers are going up and laterally and here they are becoming tendinous. And they are getting inserted onto the medial lip of the intertubular sulcus, where my finger is rotated right now. This is the t-dice major muscle. So t-dice major, t-dice minor. T-dice major is not a part of the rotator cuff muscle. The t-dice major is supplied by the lower subscapular nerve. In contrast, the supraspinatus and intraspinatus are supplied by the suprascapular nerve, which is the 5-6. The t-dice minor is supplied by the axillary nerve, which I should show you just a little later. So this t-dice major is not part of the rotator cuff. And it is inserted onto the medial lip. So what is the action of the t-dice major? It's a medial rotator extensor and adductor of the arm. It medially rotates, extends and adducts the arm. So that is the action of the t-dice major. Now let's come further lower down. Let's take a look at this muscle, which I have lifted up here. What is this muscle here? This is the deltoid muscle. Why is it called the deltoid? Because it is shaped like a delta. That's why it is called a deltoid. Oid means delta-like. It has got three sets of fibers. The first set of fibers are these fibers, which are taking origin from the spine of the scapula. So therefore, this is the spinal part of the deltoid. Then we have the middle set of fibers, which are taking origin from the acromion process of the scapula. Therefore, they are called the acromion part. And further anteriorly, we have these fibers, which take origin from the clavicle. Therefore, they are called the clavicular part. The three sets of fibers, they converge into a tough tendon, which gets inserted onto this projection on the lateral aspect of the humerus, which is called the deltoid tuberosity. So this is the deltoid muscle. And this is the one which is supplied by the axillary nerve, which I'll show you just now. This is the axillary nerve. This deltoid is responsible for abduction of the arm from 10 degrees to 90 degrees. The posterior fibers and anterior fibers are responsible for forward and backward swinging of the arm. And they also act as guide ropes to stabilize the arm when the middle set of fibers are abducting the arm. The deltoid muscle is also a shunt muscle. It prevents inferior dislocation of the shoulder when you're lifting a heavy object, like for example, a bucket full of water. So this is the deltoid muscle. Now let's come further lower now and take a look at the next muscle here. We can see this muscle here. This is collectively called the triceps muscle. Why is it called triceps? Because it has got three heads. This is the first head. We have highly exaggerated it. This is the long head of the triceps. It takes origin from the infraglinoid tubercle of the scapula. This is the lateral head of the triceps. Why is it called the lateral head? Because it is situated supralateral to the radial groove. This is the lateral head. And further lower down we can see this head of the triceps. This is the medial head of the triceps. Why is it called the medial head? Because it is located infromedial to the radial groove. So therefore the radial or the spiral groove on the back of the humerus is a groove between the lateral head and the medial head. And we have already guessed it. Running through this radial groove is this structure here. This is the radial nerve and this is the profunda brachii artery. So these are the structures which are running through the radial groove. So therefore if there is a fracture of the humerus, the shaft of the humerus, it can injure the radial nerve. And that can lead to paralysis of the triceps muscle as well as muscles on the extensor aspect of the arm, forearm. Which will lead to condition called wrist crumb. Having mentioned this, now let's mention the few triangles and spaces. So first, we have this triangular space here. What are the boundaries of this triangular space? Td's minor, Td's major, long end of the triceps. This triangular space has got this content. This is the circumflexed scapular artery and it's at the vein. What is the significance? The circumflexed scapular artery, it participates in the scapular elastomosis. And that provides a collateral circulation between the subclavian artery and the auxiliary artery in case of subclavian axillary occlusion. So that is the significance of this triangular space. Now let's take a look at the next. We can see this triangular area. This is called the triangular interval. Remember I have exaggerated these spaces. The superior boundary is formed by the Td's major. The middle boundary is formed by the long end of the triceps and the lateral boundary is formed by the lateral head of the triceps as well as the shaft of the humerus. Why is it called the triangular interval? Because through the triangular interval, we can see the radial nerve and the profunda bricaea artery. And it is these two structures which then continue behind the humerus in this radial or spiral groove which I alluded to just now. The significance of this is if these muscles get hypertrophied, they can entrap this nerve to produce what is known as the entrapment syndrome. And of course, as I have already mentioned, fracture of the shaft of the humerus can also injure this nerve. Now let's take a look at the next space. This is the Td's minor as I have already told you. This is the Td's major. This is the long end of the triceps. And this is the surgical neck of the humerus. So therefore this is called the quadrangular space. The quadrangular space is a passage where the axillary structures come on the posterior aspect. And what is the structure which is coming on the posterior aspect? This bundle of nerves that we see here. This is the axillary nerve. This is the axillary nerve. The axillary nerve goes through the quadrangular space and it's accompanied by the posterior circumfix humeral artery. And then it goes around like this behind around the surgical neck. And as it goes, before that it gives a branch to the Td's minor. And then it ramifies and gives numerous branches to the Td muscle from inside. And after that it pierces the Td and it supplies a little bit of the skin on the lateral aspect of the upper arm. What is the significance of this quadrangular space? Two significance. If there's an inferior dislocation of the shoulder or if there's a fracture of the surgical neck of humerus, either of them can injure the axillary nerve and can lead to paralysis of the Td. And there will be a patch of anesthesia on the upper lateral aspect of the arm. Accompanied by atrophy of the Td. So therefore this is a very important clinically important space and a content. Having mentioned that, now let's mention the remaining nerve which is visible in this dissection. We can see this nerve here where my finger is tracing right now. This is the ulnar nerve. The ulnar nerve, it runs initially anterior to the medial intermuscular septum and then it pierces and comes to the posterior aspect. Therefore we can see it on the posterior aspect. This ulnar nerve does not supply any structure on the arm. It just gives a few particular branches to the elbow. And after that it disappears here. It goes through a tunnel called the cubital tunnel which is located where my finger and then it goes to the forearm. This ulnar nerve can be endured when there's a fracture of the medial epicondyle. We had talked about the radial nerve just now. So now let's trace the radial nerve and see what it does. After it goes through the radial groove, it comes on the lateral aspect and we can see it here. I've lifted it up here. And when you see when I exert traction here, we can see it is moving here. So this is the radial nerve. And then it continues down and it comes to lie in front of the lateral epicondyle of the humerus under the brachioradialis where it supplies the forearm. And finally to conclude, we can see one nerve coming here. This is the cutaneous branch of the musculocutaneous nerve where it continues down as the lateral cutaneous nerve of the forearm. So these are the structures which I wanted to show you with the clinical correlations in the posterior dissection of the pectoral girdle and the shoulder. Thank you very much for watching. Dr. Sanjay Sanyal signing out. Vansh Patel is my camera person. MD1 students are my assistants. If you have any questions or comments, please put them in the comment section below. Have a nice day. Please like and subscribe.