The structure indicated is the interspinales muscle.
The interspinales muscles belong to the deep group of back muscles. The muscles of the back can be considered in terms of a superficial group, an intermediate group and a deep group of muscles. The deep group of back muscles consists of:
- Spinotransversales muscles
- Erector spinae muscles and transversospinales muscles
- Segmental muscles
- Suboccipital muscles
The interspinales muscles belong to the segmental muscle group of the deep muscles. As the name suggests, the interspinales run between adjacent spinous processes. There is one interspinales muscle on either side, lateral to the interspinous ligament. The interspinales muscles are found in the cervical, thoracic and lumbar region.
Learn about the anatomy of the deep back muscles in this tutorial.