A. Lumbar and Thoracic Regions.
—The iliopsoas ([Fig. 162], 8) belongs in this group, but since it moves the leg it is [described] with the muscles of the pelvic limbs.
M. psoas minor ([Fig. 162], 9, [page 398]).—This muscle lies along the ventral surface of the vertebral column, next to the peritoneum, and extends from the thoracic vertebræ to the ilium.
Origin usually by five heads, from the caudal border of the centra of the last two (or one) thoracic and first three (or four) lumbar vertebræ. These heads also serve as origin for a part of the iliopsoas, and are closely united at their origin with the quadratus lumborum. They unite to form an, at first, rather large flat muscle, which rapidly becomes smaller caudad, finally forming a slender tendon which is inserted on the iliopectineal line, just craniad of the acetabulum.
Relations.—Dorsolateral surface with the quadratus lumborum and iliopsoas ([Fig. 162], 8). Ventral surface with the pleura and diaphragm near the origin; with the peritoneum caudad.
Action.—Flexes the back in the lumbar region.
M. quadratus lumborum.—A flat muscle lying on the ventral surface of the transverse processes of the lumbar vertebræ. Craniad it has origin on the ventral surface of the last two thoracic vertebræ, and by a few fibres from the last rib. The muscle passes caudad, lying on the lateral side of the psoas minor ([Fig. 162], 9), and becomes attached to each of the transverse processes of the lumbar vertebræ. Caudad the muscle passes into a strong flat tendon which is inserted into the anterior inferior spine of the ilium.
Relations.—Dorsal surface with the transverse processes and the intertransverse muscles. Ventral and medial surfaces with the psoas minor ([Fig. 162], 9) and iliopsoas ([Fig. 162], 8).
Action.—Bends the vertebral column sideways.