The spinalis is a muscle included in the erector spinae muscle group in the human back. Previously known as the sacrospinalis, the erector spinae group is made up of three separate muscles and their attaching tendons: the spinalis, the longissimus, and the iliocostalis. These muscles are situated directly alongside the spine and act to extend the vertebral column. The spinalis, which can be subdivided into three sections, also acts to laterally flex, or bend to the side, the head and neck.
All three erector spinae muscles are arranged in vertical columns that run parallel to the spine, with the iliocostalis being the most lateral, or farthest from the spine, the longissimus being the intermediate of the three, and the spinalis being the most medial, or nearest to the spine. The segments of the iliocostalis extend from the sacrum, just above the tailbone, to the ribcage, and include the iliocostalis lumborum, the iliocostalis dorsi, and the iliocostalis cervicis. Staggered slightly higher are those of the longissimus — the longissimus thoracis, longissimus cervicis, and longissimus capitis — which stretches from the lumbar spine to the base of the head, making it the longest of the three muscles.
The spinalis is the innermost and uppermost of the three and consists of three segments stretching from the top two lumbar vertebrae up to the second cervical vertebrae: the spinalis dorsi, the spinalis cervicis, and the spinalis capitus. Like the other erector spinae muscles it is arranged in bundles of muscle fibers and tendons that attach to each of the vertebrae that it spans. The fibers of the spinalis dorsi, which merge with those of the longissimus dorsi alongside it, originate on the top two lumbar vertebrae, L1 and L2, and the bottom two thoracic vertebrae, T11 and T12. Its tendons attach to these vertebrae along their spinous processes, the backward-projecting bony protrusions of the spine that are seen and felt down the middle of the back. From here the spinalis dorsi inserts via multiple tendons along the upper thoracic vertebrae.
Arising just above the spinalis dorsi is the spinalis cervicis, whose fibers originate on the spinous process of C7, the bottommost cervical vertebra. Often it also arises from T1 and T2, the uppermost thoracic vertebrae. It then inserts at the top of the cervical spine on the spinous process of C2, also known as the axis, and at times along C3 and C4 as well. Inserting just above here is the smallest of the spinalis segments, the spinalis capitis, which runs medially to and blends with the fibers of the semispinalis capitis, attaching to the occipital bone at the base of the skull.
As it is so intimately connected with the spine, the spinalis acts to move the vertebrae, and it functions differently depending on whether one or both sides are contracting. When the muscle contracts bilaterally, or on both sides, it extends the entire spinal column, as in straightening oneself upward from a forward-hunched position. Unilateral contractions of the muscle, which occur namely along its upper segments, produce lateral flexion of the head and neck, as in tilting one’s ear toward the shoulder.