Nerve · Upper arm

Intercostobrachial nerve

Nervus intercostobrachialis

A nerve of the chest wall that escapes into the arm. It is the lateral cutaneous branch of the second intercostal nerve: instead of stopping in the skin of the side of the chest as the others do, it pierces the wall, crosses the floor of the armpit and joins the medial cutaneous nerve of the arm. Every other nerve reaching the upper limb comes through the brachial plexus, so this is the one strip of arm still wearing the trunk's segmental supply, and it is the reason the skin of the armpit and the inner upper arm belong to the chest's nerve map rather than the limb's. It crosses the open axilla rather than lying buried in muscle, running through the fat and lymph nodes there with little covering of its own.