How to Identify the Cords of Brachial Plexus in a dissected Specimen

brachial plexus supplies nerves to the upper limb. The books describe the cord of the brachial plexus nicely in relation to the axilllary artery. The cord lateral to the artery is the lateral cord. The cord medial to the artery is the medial cord. The cord posterior to the artery is the posterior cord. But, when it comes to dissected specimen of brachial plexus, at ones it might look like a total mess. Therefore, the students usually find it confusing to identify each cord and its branches. 

First start with simple things. Then you can read more and go in to details. First the way to identify the main 3 cords, their terminal branches and their main branches will be described. When you are given a dissected specimen of brachial plexus, first try to find out the muscularcutaneous nerve. Muscularcutaneous nerve passes laterally in to a muscle in the arm. You will see this nerve piercing through the muscle. This muscle is called coracobrachialis. Once you identify this nerve, the rest is easy. 

Now trace the nerve backwards towards the plexus. The you will find the cord which the nerve arise from. This is the lateral cord. Now you will see that there is another branch arising from the medial aspect of the lateral cord. That is the lateral root of the median nerve. This branch goes downwards and units with another short branch coming from the medial side. That is the medial root of the median nerve. The nerve formed by the union of these two roots is the median nerve.

Now trace the medial root of the median nerve up to find the medial cord. You will see that the medial cord continues downwards along the medial aspect of the arm. That is the ulnar nerve. Now that you have identified the medial and lateral cords, median nerve and the ulnar nerve, the remaining cord posterior to the artery is the posterior cord. Now isolate the posterior cord and look carefully. You will see that the cord goes through the muscles of the upper arm. that is the radial nerve. Higher up in the posterior cord, you will find a branch arising. This branch goes through muscles of the arm. That is the axillary nerve. Now that you have identified the 3 cords and their main and terminal branches, it will be easier to find out the rest. 

Veins Without Valves

Generally veins have valves. This is to prevent the backward flow of blood within veins. Arteries carry blood from the heart to peripheral tissues. At tissue levels, the arteries branch out and finally give rise to capillaries. The capillaries then unit and forms the veins. Veins carry blood from peripheral tissues to the heart. As the vessels branch out in the above manner, the pressure within those vessels gradually decline. When it comes to the capillaries, the pressure is very low compared to that of major vessels such as the aorta. The pressure within the veins is even more less. Therefore, there is less force to pump blood back to the heart. 

Usually, the body takes the help of muscle contraction to pump blood up the veins. So, when the muscles contract, they squeeze blood up in the veins. When this happens, there should be mechanism to prevent blood from going back. This is where the valves come in to play. They prevent backward flow of blood. 

Not all the veins in the body contain valves. There are some veins that do not have veins. They are,

• The pulmonary veins
• Superior vena cave
• Left and right bracheocephalic trunks
• Azygos system of veins