Anatomy III -- intercostal nerves and arteries

Dorsal and ventral rootlets become the dorsal and ventral roots, which fuse to form the spinal nerve, which then splits into the dorsal and ventral rami. The intercostal nerves derive from T1-T11 (T12 is the subcostal nerve) anterior rami. The intercostal nerve (page 20, Grant's 12e) runs in the groove between the innermost and internal intercostal muscles. The intercostal nerve innervates the intercostal muscles and overlying skin regions. Laterally, it gives off the lateral pectoral cutaneous branch, which itself has branches, including lateral mammary and posterior branches. Anteriorly it gives off the anterior pectoral cutaneous branch, which has branches, including the medial mammary branch. The posterior ramus innervates the deep muscles of the back (i.e. erector spinae) and skin adjacent to vertebra column.

The blood supply is similar. The posterior intercostal artery derives from the aorta, with the posterior branch supplying the back, and the main artery looping around laterally, with a lateral pectoral cutaneous branch, as with the nerve. The anterior supply, however, is distinct and derives from the internal thoracic artery. The anterior intercostal artery runs in the same intercostal groove, while the anterior perforating branch comes through to the skin in the parasternal region.