1. =Psoas minor= (s. parvus).—This is a fusiform, flattened, pennate muscle, which lies along the ventro-lateral aspect of the bodies of the last three thoracic and the lumbar vertebræ. _Origin._—The bodies of the last three thoracic and first four or five lumbar vertebræ, and the vertebral ends of the sixteenth and seventeenth ribs. _Insertion._—The psoas tubercle on the shaft of the ilium. _Action._—To flex the pelvis on the loins, or to incline it laterally. _Structure._—The muscle arises by a series of digitations which pass backward and outward to be inserted at an acute angle on the tendon. The latter lies along the outer border of the fleshy portion and is flattened. It appears on the surface of the muscle at the third lumbar process and increases gradually in width until it reaches the pelvic inlet, where it becomes narrower. _Relations._—The ventral surface of the thoracic portion of the muscle is related to the pleura, crura of the diaphragm, and sympathetic nerve. In the abdomen, the chief ventral relations are the peritoneum, the vena cava (right side), the aorta and left kidney (left side), the sympathetic nerves, and the ureters. Dorsally, the chief relations are the vertebræ, the psoas major, and lumbar nerves. Near its insertion the tendon is crossed internally by the external iliac artery, and externally by the femoral nerve. _Blood-supply._—Intercostal and lumbar arteries. _Nerve-supply._—Lumbar nerves. 2. =Psoas major= (s. magnus).—This is much larger than the preceding muscle, by which it is partly covered. It is triangular, with the base anterior. _Origin._—The ventral surfaces of the vertebral ends of the last two ribs and the transverse processes of the lumbar vertebræ. _Insertion._—The internal trochanter of the femur, by a common tendon with the iliacus. _Action._—To flex the hip joint and to rotate the thigh outward. _Structure._—The origin of the muscle is fleshy, the belly being in general flattened, thick in its middle, thin at its edges. The thoracic portion is small, the abdominal part much thicker and wider, extending laterally beyond the extremities of the lumbar transverse processes. From the lumbo-sacral articulation it lies in a deep groove formed in the iliacus (with which it is partly united), becomes smaller and rounded, and passes downward and backward to terminate by a strong tendon common to it and the iliacus.[62] _Relations._—Dorsally, the last two ribs and thoracic vertebræ, the lumbar vertebræ, the internal intercostals, quadratus lumborum, longissimus dorsi, and iliacus, and the lumbar vessels and nerves; ventrally, the pleura and peritoneum, the iliac fascia, inguinal ligament, the diaphragm, psoas minor, and sartorius, the kidneys, the spleen, the intestine (duodenum, cæcum, etc.) and the circumflex iliac vessels. _Blood-supply._—Lumbar and circumflex iliac arteries. _Nerve-supply._—Lumbar and femoral nerves.