The lumbar region extends from the level of the twelfth dorsal vertebra and twelfth rib to the base of the sacrum, ilio-lumbar ligament, and iliac crest. Externally it is limited on the posterior aspect by the outer border of the erector spinae, indicated by a vertical furrow running between it and the flat abdominal muscles, while on the abdominal surface it is limited by the outer border of the quadratus lumborum, the latter muscle, together with the psoas, forming the floor of the space.

The iliac fossa is bounded above by the ilio-lumbar ligament, and above and externally by the crest of the ilium, internally by the brim of the true pelvis, and in front by Poupart's ligament. It lodges the iliacus muscle, which, blending with the psoas to form the ilio-psoas, runs down to the small trochanter of the femur. The fascial coverings of these muscles, and particularly of the psoas, are of considerable importance surgically.

Three layers of fascia run outwards from the vertebrae, and fuse, enclosing muscles as they do so, to form the lumbar aponeurosis. The most posterior of these three fasciae, called the vertebral aponeurosis, extends outwards from the spines of the vertebrae to meet the middle layer, which arises from the tips of the transverse processes of the lumbar vertebrae, enclosing the erector spinae between them. The anterior layer arises from the junctions of transverse processes and bodies, and extends outwards to meet the middle layer, enclosing the quadratus lumborum, and separating it anteriorly from the psoas (see Fig. 19).

The psoas fascia, or sheath, forms a fourth layer, which, rising from the front of the bodies of the lumbar vertebra (with arches to permit of the passing of the lumbar arteries), runs outwards and fuses with the anterior layer, shortly before it fuses with the middle and posterior layers to form the lumbar aponeurosis. Above, the psoas sheath commences at the internal arcuate ligament of the diaphragm, being derived from the diaphragmatic portion of the transversalis fascia, and thus the psoas muscle only receives its sheath after perforating the diaphragm.

The lumbar aponeurosis is a narrow ligamentous band, extending from the last rib to the iliac crest. Besides giving attachments to the internal oblique and transversalis muscles, it is continuous by its anterior edge with the transversalis fascia, and hence it connects the outer border of the psoas sheath with the inner border of the transversalis fascia. It is pierced near the rib by the last intercostal artery and nerve, and near the ilium by the ilio-hypogastric nerve and accompanying artery.

The fasciae lining the abdominal cavity in the lumbar region are the transversalis, lining the antero-lateral portion, the anterior layer of lumbar fascia, and psoas sheath completing the investment. The three layers forming the lumbar aponeurosis are, like it, inserted below into the crest of the ilium, the lower margin of the anterior layer being thickened to form the Mo-lumbar ligament, which extends from the transverse process of the last lumbar vertebra to the inner lip of the iliac crest (while its upper margin forms the external arcuate ligament). The psoas sheath, however, on reaching the iliac fossa, becomes directly continuous with the iliac fascia, covering the iliacus muscle, and thus it is necessary to consider these two together in that region. This iliac fascia, then, is attached along the whole iliac crest and ilio-lumbar ligament. Then it extends over the psoas, on the inner border of which it is attached to the sacrum and brim of the true pelvis, and ilio-pectineal eminence, and is continuous with the pelvic fascia. Along Poupart's ligament it fuses with the transversalis fascia, save where the external iliac vessels emerge to form the femoral vessels, the transversalis fascia at this point joining in front of, and the iliac fascia behind, the vessels, to form their sheath (femoral sheath). Thus the ilio-psoas muscle and anterior crural nerve enter the thigh through a compartment composed of fascia and bone, which is closed, save for the communication with the psoas above, and with the pelvis below and to the inside. Under the iliac fascia the external iliac, by its circumflex iliac branch, anastomoses with the ilio-lumbar branch of the internal iliac.

The internal surface of the abdominal cavity, then, is lined by a continuous fascial covering, variously named at different parts, the chief portions being the transversalis and iliac fasciae. On the deep surface of the fascia lies a layer of extraperitoneal tissue, which fills in the furrow^ between the muscles, thus presenting a fairly regular abdominal surface, and in which the kidneys, ureters, renal, colic, and spermatic vessels, and iliac vessels and lymphatic glands are embedded. (The anterior crural nerve and lumbar nerves, on the other hand, are under, or external to, the fascia.) On the inner surface of the extraperitoneal tissue, again, the peritoneum lies.

Abscesses in this region may occur either in the extraperitoneal tissue or under the psoas fascia. Extraperitoneal abscesses may arise from appendix, kidney, a parametritis, etc. ; may be of considerable size and widely spread. Such abscesses tend to point above Poupart or to enter the pelvis. Those which occur under, or external to, the transversalis fascia generally point at the iliac crest or above Poupart ; they rarely extend along the inguinal canal into the scrotum. Sometimes, by following the last intercostal or ilio-hypogastric nerves, they may pierce the lumbar fascia, or may pierce the quadratus lumborum, and then, coming through the external oblique, appear at Petit's triangle. Those which occur in the psoas sheath arise generally from tubercular disease of the dorsal or upper lumbar vertebrae (Pott's disease). Where the disease is in the dorsal region, the tubercular debris is first extruded into the posterior mediastinum in which it gravitates downwards, until arrested by the diaphragm, whence, passing under the internal arcuate ligament in company with the psoas muscle, it enters the abdomen within the psoas sheath. This sheath directs it down the posterior abdominal wall, across the blade of the ilium, under Poupart's ligament, through the special iliac compartment already described, in which position it lies to the outside of the femoral vessels. Then the abscess passes under the vessels, reaches the lesser trochanter, and frequently turns up again and overlaps the vessels from the inside. While this is the typical course of a psoas abscess, the pus may sometimes escape from the psoas sheath, as for example by following one of the lumbar arteries between the transverse processes of the lumbar vertebrae, and then, running outwards on the posterior surface of the quadratus lumborum, pierce the origin of the transversalis, and also the internal oblique, and finallv present in the triangle of Petit-the triangular interval whose base is formed by the highest point of the crest of the ilium, while the sides are formed by the free border of external oblique anteriorly, and the latissimus dorsi posteriorly. The floor is formed by the internal oblique. (Above the triangle the latissimus dorsi overlaps the external oblique.) Sometimes also the pus may gravitate into the pelvis through the communication with the pelvic fascia. On the other hand, pus from acetabular disease, or hip disease where the acetabulum is eroded and perforated, may extend upwards, and so simulate a psoas abscess.