any of the deep muscles of the back near the vertebral column that, as part of the erector spinae (sacrospinalis) muscle group, assist in extension (e.g., bending backward), lateral flexion (bending to the side), and rotation of the spine. The spinalis thoracis is the major spinalis muscle, arising from the bones of the lower thoracic and upper lumbar vertebral spine and inserted into the bones of the upper thoracic vertebral spine. It may be prolonged into the neck (spinalis cervicis) and head (spinalis capitis).