muscles fully contained (origin, belly, and insertion) within and acting on the back; these muscles occur as the deepest of three groups of back muscles, the superficial and intermediate groups consisting of extrinsic muscles innervated by anterior rami of spinal nerves. The intrinsic back muscles, in turn, are organized into three layers: a superficial layer of splenius muscles, an intermediate layer of erector spinae muscles, and a deep layer of transversospinal muscles, all innervated by posterior rami of spinal nerves.
