The term "longissimus" can refer to several muscles that are located near the spine.
The longissimus muscle is the longest subdivision of the sacrospinalis muscle. It is located laterally to the semispinalis, and extends forward into the back side of the cervical vertebrae.
The longissimus thoracis is the longest continuation of the sacrospinalis and lies intermediate to it. It begins at the sacrum and the spinous process (bony projections at the back of each vertebra) of the lumbar spine, where a part of its fibers connect with the entire length of the back side of the lumbar vertebrae and the front surface of the lumbodorsal fascia, a sheath of connective fibers that covers the sacrospinalis muscle group.
The longissimus cervicis originates at the transverse processes (bony projections on the right and left sides of vertebrae) of the upper thoracic vertebrae and inserts at the transverse processes of the lower lower cervical spine.
The longissimus capitis originates along the articular processes (projections of bone that fit with adjacent vertebrae) of the cervical vertebrae and the transverse processes of the thoracic vertebrae. It inserts into the back side of the mastoid process, a cone-shaped portion of the temporal bone that is located just behind the ear.