The anterior branches of the spinal thoracic nerves run as intercostal nerves between the ribs, initially on the inner surface of the thorax and later within the internal inter-costal muscles. We distinguish between an upper group and a lower group of inter-costal nerves.
The nerves of the upper group (T1 – T6) run up to the sternum and supply the intercostal muscles (C7), the superior and inferior pos-terior serrate muscles, and the transverse thoracic muscle. They give off sensory branches to the skin of the thorax, namely, the lateral cutaneous branches (AD8) at the anterior margin of the anterior serrate muscle, which further divide into anterior and posterior branches, and the anterior cu-taneous branches (AD9) close to the sternum,which also divide into anterior and poste-rior branches. The lateral and medial cu-taneous branches of anterior branches 4 – 6, which extend to the area of the mammary gland, are referred to as lateral and medialmammary branches.
The nerves of the lower group (T7 – T12), the intercostal segments of which no longer end at the sternum, extend across the costal cartilages up to the white line. They take an increasingly oblique downward path and supply the muscles of the abdominal wall (abdominal transverse muscle [C10], exter-nal [C11] and internal [C12] abdominal ob-lique muscles, rectus abdominis muscle [C13] and pyramidal muscle).
Special features.Intercostal nerve 1 partici-pates in forming the brachial plexus and sends only a thin branch to the intercostal space. Intercostal nerve 2 (and often 3 as well) gives off its lateral cutaneous branch to the upper arm (intercostobrachial nerve) (B14), where it connects with the medial cu-taneous nerve of the arm. The last intercostalnerve running beneath the twelfth rib is re-ferred to as the subcostal nerve; it runs ob-liquely downward across the iliac crest.
The inguinal region and hip region receive their sensory innervation from the upper-most branches of the lumbar plexus, namely, from the iliohypogastric nerve (D15) (lateral branch and anterior branch), from the ilioinguinal nerve (D16), and from the genitofemoral nerve (genital branch [D17,femoral branch [D18]).