The spinalis muscle is a subsection of the erector spinae, which is a confluence of tendons and muscles that is located near the spine. The spinalis is the body's most medial, or closest to the midline of the body, deep back muscle and originates at the spinous processes (bony projections at the back of each vertebra) of the lower spinal vertebrae. It inserts there, as well as at the skull's base. It innervates at the posterior (back) branches of spinal nerves C2 to L3.

The spinalis is split into three parts: the spinalis dorsi, the spinalis cervicis, and the spinalis capitis. The spinalis dorsi and spinalis capitis are rarely considered as separable distinct muscles; they are connected with the semispinalis capitis. The spinalis cervicus is an inconstant muscle that inserts into the spinous process of the spine's axis and sometimes into its two lower vertebrae.  The function of the spinalis muscle is to extend and laterally (side to side) bend the neck and trunk.