This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The quadratus lumborum muscle arises from the transverse processes of the lower four lumbar vertebrae, the iliolumbar ligament, and 5 cm. (2 in.) of the iliac crest. It inserts into the posterior half of the last rib and transverse . processes of the upper four lumbar vertebras. The erector spinae is the muscular mass which fills the groove to the outer side of the spinous processes. It arises from the spines of the lumbar vertebrae, the back of the sacrum, the sacrosciatic and sacroiliac ligaments, and about the posterior fourth of the crest of the ilium. It inserts into the posterior portion of the vertebrae and ribs above. The latissimus dorsi arises from the spinous processes of the lower six thoracic vertebrae and the vertebral aponeurosis, which is attached to the spinous processes of the lumbar vertebrae, the posterior surface of the sacrum, and the posterior third of the crest of the ilium. It passes upward and forward to insert into the inner lip of the bicipital groove of the humerus.
It will thus be seen that while the direction of the outer fibres of the latissimus dorsi is from below upward and forward, the direction of those of the quadratus lumborum is upward and backward. It will also be observed that the attachment of the quadratus lumborum is farther out on the crest of the ilium than is that of the latissimus dorsi, reaching about its middle (Figs. 408, 409 and 410).
The lumbar fascia (fascia lumbodorsalis), so called, is the continuation backward of the posterior aponeurosis of the transversalis and internal oblique muscles to the spine. When the aponeurosis, from which these two muscles spring, reaches the outer edge of the quadratus lumborum, it splits; one thin layer goes on its ventral surface to be attached to the roots of the transverse processes of the vertebrae; the other thick posterior layer, on reaching the edge of the erector spinae muscles divides into two, the anterior of which covers the dorsal surface of the quadratus lumborum and the ventral surface of the erector spinae to attach itself to the tips of the transverse processes, while the posterior layer passes over the dorsal surface of the erector spinae to be attached to the spinous processes of the lumbar vertebrae. These three layers are called the anterior, middle, and posterior layers of the lumbar fascia (see Fig. 410).
Fig. 407. - The lumbar region, superficial view.
The anterior layer is attached to the tip of the twelfth rib and arches inward to the transverse process of the first or second lumbar vertebra, to form the external arcuate ligament of the diaphragm. It is practically continuous with the transversalis fascia.
The middle layer is attached above to the last rib, and below to the iliac crest, and is very strong.
The posterior layer is continuous above with the vertebral aponeurosis and give? origin to the latissimus dorsi muscle.