Next, study the deep layer of the back muscles in the lumbar and thoracic regions. The erector spinae muscle is formed by three columns of muscles (Figure 1.6). The most lateral column is the iliocostalis thoracis muscle, the intermediate column is the longissimus thoracis muscle, and the most medial column is the spinalis muscle. There are three divisions of iliocostalis muscle that are named for their regional location: iliocostalis lumborum, iliocostalis thoracis, and iliocostalis cervicis. There are also three divisions of the longissimus muscle: longissimus thoracis, longissimus cervicis, and longissimus capitis and three divisions of the spinalis muscle: spinalis thoracis, spinalis cervicis, and spinalis capitis.
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