The posterior serrate muscles are two thin sheets –
partly fleshy, partly aponeurotic – that lie on the back of the thorax.
The serratus posterior superior passes downwards and laterally from the seventh cervical spine and the upper two or three thoracic spines to the second, third, fourth, and fifth ribs; and it raises these ribs. It is supplied by the second, third and fourth intercostal nerves.
The serratus posterior inferior arises from the lumbar fascia opposite the upper two lumbar spines and lower two thoracic spines, and passes upwards and laterally to be inserted into the lower four ribs. It is encountered in an operation on the kidney through the loin. It fixes the lower ribs when the diaphragm contracts; and it is supplied by the lower intercostal nerves and the subcostal nerve.