Besides the muscles in the third and fourth layers of the back, there is a series of twelve small muscles found in the deepest part of the back that belong to the thorax and are supplied by the anterior primary rami of thoracic nerves. They are the levatores costarum, which radiate from the tips of the transverse processes of the seventh cervical and upper eleven thoracic vertebrae to the posterior part of the shafts of the twelve ribs. Each levator costae is supplied by the nearest intercostal nerve.