The anterior rami of the thoracic nerves, one through ten, course together with the intercostal vessels in an intercostal space. The anterior rami give rise to lateral cutaneous branches in the midaxillary line. The lateral cutaneous branches divide into anterior and posterior branches. The Intercostobrachial nerve is the posterior branch of the second and the third intercostal lateral cutaneous. This explains the T2 and T3 dermatome of the posteromedial aspect of the arm.