The sacrospinalis muscle begins on the sacrum and passes up into the loin, where it thickens to form the mass of muscle alongside the lumbar spines; it then divides into three columns which extend upwards over the back of the chest, where they are inserted into ribs and vertebrae but maintain their existence by means of fresh slips that arise from these bones and are continued up into the neck, and one of them (the longissimus capitis) reaches the skull.

 

The semispinalis is situated more deeply and it is made up of a succession of slips that arise from the thoracic transverse processes, join together and, pass obliquely upwards. It is divided into three parts according to the insertion – semispinalis thoracis into upper thoracic spines, s. cervicis into cervical spines and s. capitis into the skull.

 

The members of the fourth layer are deeply-situated muscles connected with the back of the sacrum, with transverse processes, with laminae and with spines. They are named multifidus, rotatores, interspinales, intertransversarii, and the suboccipital muscles.

 

The muscles of both the third and fourth layers are all supplied by the posterior primary rami of the spinal nerves, and their actions as prime movers, synergists, antagonists, and fixation muscles are of importance in all movements of the body.