Complex sentence: The medial branches of the posterior divisions of the "upper six" thoracic nerves run between the Semispinalis dorsi and Multifidus, which they supply; they then pierce the Rhomboidei and Trapezius, and reach the skin by the sides of the spinous processes.
Split into simpler sentences: The medial branches of the posterior divisions of the "upper six" thoracic nerves run between the Semispinalis dorsi and Multifidus, which they supply. They then pierce the Rhomboidei and Trapezius, and reach the skin by the sides of the spinous processes.