 Okay, so this is a demonstration of the quadrangle and the triangles, spaces and intervals which are usually seen on the posterior aspect of the pectoral girdle. So this is the prone cadaver, we are standing on the left side of the cadaver and the camera person is also on the left side towards the head end. First let's identify some essential landmarks. This muscle that we see here, this is the TD's major muscle. The TD's major muscle takes origin from the lateral border of the scapula near the angle of the scapula and the fibers then go up and they get inserted onto the medial lip of the inter tubercular sulcus of the humerus. This muscle that we see here, this is the TD's minor. This takes origin also from the lateral border of the scapula just above the TD's major and it gets inserted onto the greater tuberosity, the lower part of the greater tuberosity. This muscle that we see here, this is the long head of the triceps which takes origin from the intraglina tubercular of the scapula and then continues down and merges with the other fibers. Having mentioned these, now let's come to the quadrangles and the spaces where my finger has gone in. This is the quadrangular space. The quadrangular space is a space which allows structures to come out from the axilla posteriorly. What are the boundaries of the quadrangular space? Superiorly, the quadrangular space is bounded by the TD's minor head of the humerus and further anteriorly will be the tendon of the subscapularis where my finger has gone in. That is the superior boundary. The inferior boundary of the quadrangular space is bounded by this structure here, this structure. This is the TD's major. So in simple terms, superior boundary is TD's minor, inferior boundary is TD's major. The medial boundary is by the long head of the triceps and the lateral boundary is by the where my finger is touching the surgical neck of humerus and the coraco brachialis muscle. So these are the four boundaries of the quadrangular space. The quadrangular space gives passage to, first of all, we can see this structure here. This is very important. This is the axillary nerve. It comes out from the brachial plexus, it passes through the quadrangular space and it enters into the deep surface of the deltoid muscle which I'm holding up here and it supplies the deltoid muscle through its numerous branches and we can see all the branches of the axillary nerve and after that it pierces through the deltoid and it supplies the skin of the upper lateral aspect of the arm here. So that is one important content. The next important content of the quadrangular space is these vascular structures that we can see here which I have lifted up. These vascular structures are the branches of the posterior circumflix humeral artery which is a branch from the third part of the axillary artery. So they also pass through the quadrangular space. So having mentioned the quadrangular space, what is the important clinical correlation pertaining to the quadrangular space? This axillary nerve which passes through the quadrangular space can be injured in inferior dislocation of the humerus because the head of the humerus is located just above its superior boundary of the quadrangular space and when the axillary nerve is injured or compromised, then it can lead to paralysis of the deltoid muscle. So that is an important clinical correlation pertaining to the quadrangular space. Now let's come a little down and medially. This space where my finger has gone in, this is the triangular space. The triangular space is bounded by the dearest minor above the dearest major below and laterally by the long head of the triceps. So this is the triangular space. Of course the space is not so obvious. We have made the space big and this gives passage to these vascular structures that we can see here. This is the circumflex scapular artery. The circumflex scapular artery is a branch from the sub scapular artery which is the last branch of the axillary artery and after it has given out the circumflex scapular artery, the rest of the sub scapular artery continues as the thoracodorsal artery which I have picked up here and we can see the thoracodorsal artery here also which supplies the latsymus dorsi. So the circumflex scapular vessels are the main contents of the triangular space. In fact, circumflex scapular artery participates in the scapular anastomosis which is a very important anastomosis between the first part, branches of the first part of the subclavian artery and the third part of the axillary artery. And as the circumflex scapular artery passes medially, it forms a groove between the origin of the T.D.'s minor and the origin of the T.D.'s major. So that is the important content of the triangular space. Now I am coming to the triangular interval. So we have moved further laterally and below. So it is lateral to the triangular space and below the quadrangular space. This is the triangular interval. This triangular interval is bounded by medially by the long head of the triceps. Superially it is bounded by the T.D.'s major and laterally it is bounded by the lateral head of the triceps and the humors. So this is the triangular interval and what are the contents of the triangular interval and we can see the content. One content is the branches of the profunda brachii artery which pass through the triangular interval and we can see some of the branches down there and it also gives passage to the radial nerve. The radial nerve passes down and we can see it is supplying the triceps muscle. Here there is a very important clinical correlation. The radial nerve can be injured in this region in fracture of the humors in which case there will be paralysis of the lateral head of the triceps and the structures of the extensor muscles of the arm. Similarly the structures in the triangular interval may be compromised by wide or thickened the T.D.'s major and the long head of the triceps. So that is the triangular interval. So these are the quadrangular space, triangular space and triangular interval with the respective vestular structures and the clinical correlations. Thank you very much for watching. If you have any questions or comments, please put them in the comment section below. Dr. Sanjay Sanyal signing out.