There are thirteen pairs of dorsal spinal nerves and their arrangements are almost similar.
The dorsal branches pass up into the intertransverse space and divide into medial and lateral branches of which the former supply the spinal muscles and skin of the region.
The lateral branches are directed upward, pass through the longissimus dorsi and appear between it and the transversalis costarum. They break off into filaments to supply the skin.
The ventral branches are the intercostal nerves. The first and second dorsal enter into the formation of the brachial plexus.
The ventral branches of the rest are much larger than the dorsal branches and descend into the intercostal spaces in company with the vessels.
At the upper half they run in between the intercostal muscles and supplies it.
About the middle they comes out of the muscle as perforating branch supply the cutaneous muscle and skin of the thorax and abdomen.
The intercostal nerves from the eighth to the thirteenth supply sensory branches to the diaphragm.
