 The best study point for the brachial plexus is to demonstrate the M landmark. This is the M of the brachial plexus. If you see clearly, I have lifted up with my instrument the following structures. One limb of the M going up, the short limb of the M, the other short limb of the M making the middle limb of the M and the other limb of the M. So we can see an M shaped structure here. This is referred to as the M landmark. What is it composed of? This is the ulnar nerve. This is the median nerve. This is the medial root of the median nerve. This is the lateral root of the median nerve. The two unite to form the median nerve itself. And this is the musculocutaneous nerve. So we can see the M here. This is the useful study point for understanding the brachial plexus. Now we shall continue further. Look at this structure here. This is the medial cord of the brachial plexus. Why is it called the medial cord of the brachial plexus? Because it is situated medial to the axillary artery. And by the same argument, this is the lateral cord of the brachial plexus. Because it is lateral to the axillary artery. And this is the posterior cord of the brachial plexus because it is posterior to the axillary artery. So we have seen the three cords, medial cord, lateral cord and the posterior cord. At this juncture, let's take a step back. The brachial plexus is formed from roots, trunks, divisions, cords, branches. The roots and trunks are in the neck. So therefore, in this dissection, we cannot see them. The divisions are located in the cervical axillary canal which I showed just now with my finger. So therefore, that also we cannot see. Most of the structures that we can see in the axillary artery are the cords and their terminal branches. So that's why we mentioned this is the medial cord, this is the lateral cord and this is the posterior cord. Arrosing from the medial cord, we have the following branches. One I mentioned just now, this is the ulnar nerve. Ulnar nerve, it runs medially and it goes behind the medial intramuscular septum and it runs behind the medial epiconline. The next structure rising from the medial cord is this nerve and we can see it's a long thin nerve and it divides into two here. This is the medial cutaneous nerve of the arm and it continues as the medial cutaneous nerve of the forearm. This is the only cutaneous branch from the brachial plexus which supplies the skin on the medial side of the arm and the forearm and it is accompanied by the basalic vein and we can see the part of the basalic vein here. Basalic vein is the one which ultimately continues up and forms the axillary vein which we can see here, which has been lifted up by my assistant here. The medial nerve, as I mentioned, is formed by the union of the medial root and the lateral root and these two roots they unite to form the medial nerve. The medial nerve runs in close proximity with the axillary artery and it continues in close proximity with the brachial artery. The next nerve that we can see here is this one. This is the musculocutaneous which arises from the lateral cord of the brachial plexus. Musculocutaneous nerve enters into the arm at the point of insertion of the coracobrachialis muscle and this is the coracobrachialis muscle. In fact, the coracobrachialis muscle is considered as the landmark muscle because it indicates the point where the musculocutaneous nerve enters into the arm and that is the place where there is also a humoral nutrient artery. If I retract it, we can see the rest of the musculocutaneous nerve coming up here. It supplies the coracobrachialis, it supplies the biceps and we can see it's supplying multiple branches to the biceps. It runs between the biceps and the brachialis and we can see it's supplying multiple branches to the biceps and brachialis. It continues between the two and here it comes out. Between the biceps and the brachialis it comes out laterally and here it is running in accompaniment with the cephalic vein and here it becomes cutaneous and here it is known as the lateral cutaneous nerve of the forearm. That is why this is called the musculocutaneous nerve. Now let's come to the posterior cord. Now I have lifted up the posterior cord and as you can see it is behind the axillary artery. The posterior cord gives two main terminal branches. The smaller proximal terminal branch is this one which I have lifted up here. This is the axillary nerve and we can see the axillary nerve is disappearing through a space here. This is the quadrangular space. The quadrangular space is between the t-rease minor and the t-rease major and it is just under the head of the humerus. From here it runs in accompaniment with this artery here. This is the posterior circumflix humeral artery. If there is an inferior dislocation of the head of the humerus it can impinge on the axillary nerve and it can injure the axillary nerve and it can reduce paralysis of the deltoid muscle. It also gives a branch to the t-rease minor and it supplies the skin on the lateral side of the arm. So that is one branch of the posterior cord. The main posterior cord then continues and we can see it continuing here and we can see it is disappearing again in another space here. This is the triangular interval. The triangular interval is below the t-rease major and we can see this is the t-rease major here. From the triangular interval it runs behind the humerus in the radial groove and here it is accompanied by this artery here that we can see. This is the profunda brachii artery and here it supplies the triceps. So these are the two branches from the posterior cord of the brachial plexus. Now my assistant has detected the axillary artery laterally and I have picked up one nerve here. This is the medial pectoral nerve. If we trace the medial pectoral nerve we can see it is coming like this and it is piercing this muscle here. This is the pectoralis minor. And when we turn the pectoralis minor we can see it disappears the pectoralis minor and it has come here. So it supplies the pectoralis minor, it pierces the pectoralis minor and it supplies the pectoralis major. So this is the medial pectoral nerve which also comes from the medial cord of the brachial plexus. Since we are talking about the medial pectoral nerve it is natural to show the lateral pectoral nerve. This nerve that I have picked up here from the lateral cord of the brachial plexus this is the lateral pectoral nerve. And if we trace the lateral pectoral nerve we can see that it has got multiple branches and it is supplying the pectoralis major muscle directly. This lateral pectoral nerve it does not pierce the pectoralis minor it does not supply the pectoralis minor. It pierces through the membrane here which is referred to as the costoporobite membrane. So this is the lateral pectoral nerve. It supplies the pectoralis major muscle directly and we can see that here. And when it is supplying the pectoralis major this gives a communication and we can see that communication here. And this communication communicates with the medial pectoral nerve and we can see that here. The next structure which I want to demonstrate is this nerve which I have picked up here. This is the long thoracic nerve of Bell. And we can see it is running on the surface of the serratus anterior muscle. This is a nerve which arises from the roots of the brachial plexus. It rises from roots C567. So therefore the origin cannot be seen here because the origin is in the neck. And it runs close proximity to the surface of the chest wall as we can see here. It runs on the surface of the serratus anterior muscle and here it is accompanied by an artery which is the lateral thoracic artery and it supplies the serratus anterior. Because of the fact that it is relatively superficial on the chest wall this is liable to injury in which case it will produce paralysis of the serratus anterior and winging of the scapula. This is a clinical picture from the posterior aspect to show winging of the right scapula on attempted pushing against a wall. The next nerve that we can see here is this one which I have lifted up. This is the thoracodorsal nerve and we can see it is supplying the leximus dorsal. The thoracodorsal nerve arises from the posterior cord of the brachial plexus and it is accompanied by this artery here. This is the thoracodorsal artery which is the terminal branch of the subscapular artery arising from the third part of the axillary artery. If we trace the thoracodorsal nerve which I am doing right now we see that it is arising from the posterior cord of the brachial plexus and this is the posterior cord of the brachial plexus here. And we can see the thoracodorsal nerve is arising from there. This above the thoracodorsal nerve is this nerve here which I have lifted up here. This is the upper subscapular nerve and we can see it is supplying the subscapularis muscle. And below that is yet another nerve here which I have lifted up. This is the lower subscapular nerve which supplies the subscapularis and the teres major. So we have seen the three nerves which are arising from the collateral branches arising from the posterior cord of the brachial plexus the upper subscapular, the thoracodorsal and the lower subscapular. With this juncture I would like to remind you that the terminal branches of the brachial plexus are not the true parts of the brachial plexus and the collateral branches are the actual parts of the brachial plexus.