Situated underneath the loins, and covered over below by the great psoas muscle. The quadratus lumborum is composed of several flat, narrow slips of muscle, whose fibres pass in a direction forward and inward, extending from the sacrum to the last rib but two.

Origin. - From the sacro-iliac ligament behind.

Insertion. - By its outermost slip to the posterior border of the last rib, by the others to the ends of the transverse processes of the lumbar vertebrae and the under surfaces of the three last ribs at their junction with the spine.

Action. - To draw the last rib backward and incline the lumbar vertebrae to one side.