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Longissimus is one of the erector spinae group of spinal muscles. It is the longest and the intermediate of the three groups. Classically, it is divided into three parts according to the region of insertion of its fibres:
A longitudinal column, it runs between the transverse processes of the vertebrae.
In concert with the other erector spinae muscles, its weak unilateral action is to laterally flex the spine; bilaterally, it extends the spine.
It is innervated by the dorsal primary rami of the segmental spinal nerves.