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.
Above the middle of the crest of the ilium is a small triangular space formed by the edge of the external oblique in front, of the latissimus dorsi behind, and the crest of the ilium below. Its floor is formed by the internal oblique muscle, and it is called the triangle of Petit. It forms a weak point in this region through which collections of pus or, more rarely, ventral herniae, may make their appearance (Fig. 407).
Above and a little posterior to Petit's triangle is another triangular space. Its base is the twelfth rib, its anterior side is the posterior edge of the internal oblique, and posterior side is the outer edge of the quadratus lum-borum. It is also called the triangle of Grynfelt and Lesshaft. The lower portion of the kidney lies immediately beneath it and the latissimus dorsi covers it (Figs. 408 and 409).
Pus in the lumbar region usually originates from caries of the vertebrae, from calculus or other renal or perirenal affections, or, if on the right side, sometimes from disease of the appendix. Empyemas may likewise point in this region. Pus starting from the vertebrae may push its way outward under the transversalis aponeurosis (anterior layer of the lumbar fascia) and perforate the trans-versalis muscle where the iliohypogastric, ilioinguinal, and last thoracic nerves enter and thus reach the under surface of the internal oblique and perforate this muscle to find its exit at Petit's triangle. Pus may also perforate the floor of the fascial triangle and follow the anterior surface of the latissimus dorsi down until it points in the angle between the posterior portion of the crest of the ilium and the spine. The quadratus lumborum muscle is thin, and its outer edge, which is not covered by the erector spinae muscle, is readily pierced by pus. The erector spinae is a thick muscle covered both anteriorly and posteriorly by the thick middle and posterior layers of the lumbar fascia, hence pus does not pierce it but always goes around its outer side.
Lumbar hernia usually results from the giving way of a scar from an operation.
Lumbar incisions are made to evacuate pus or to operate on the kidney. Incisions to evacuate pus should be made obliquely from the outer edge of the quadratus lumborum in order to avoid wounding the nerves.
Fig. 409. - Lumbar region, showing the kidney and quadratus lumborum muscle exposed.
Fig. 410. - Transverse section of the lumbar region, showing the lumbar fascias and muscles.