These nerves on issuing from the spinal canal divide into two sets of branches, as we saw occur in the case of the cervical nerves. The upper divisions, which are the smaller, ascend to reach the dorsal muscles and skin of the back, to both of which they are distributed. The inferior divisions descend, and having gained the intercostal spaces, pass downwards along the posterior border of the ribs beneath the pleura as the intercostal nerves.
In their course they give branches to the panniculus and skin, and the first seven or eight terminate in the pectoral muscles and skin covering them, and the last ten are distributed to the rectus abdominis, the transversalis abdominis, and the skin of the belly. The first and second intercostal nerves send branches to the brachial plexus from their inferior divisions.