THE SUBLUMBAR MUSCLES 275 



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 vertebrae. 



Origin. — The bodies of the last three thoracic and first four or five lumbar 

 vertebrae, 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 vertebrae, the psoas major, and lumbar nerves. Near 

 its insertion the tendon is crossed internally by the external iliac artery, and ex- 

 ternally by the femoral nerve. 



Blood-supply. — Intercostal and lumbar arteries. 



Nerve-suppl tj .^Luxnbsir 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 vertebrae. 



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 ex- 

 tremities 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), be- 

 comes smaller and rounded, and passes downward and backward to terminate by a 

 strong tendon common to it and the iliacus.^ 



Relations. — Dorsally, the last two ribs and thoracic vertebrae, the lumbar 

 vertebrae, 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, caecum, etc.) and the circumflex 

 iliac vessels. 



Blood-supply. — Lumbar and circumflex iliac arteries. 



Nerve-supply. — Lumbar and femoral nerves. 



3. Iliacus. — This muscle covers the ventral surface of the ilium external to 

 the sacro-iliac articulation, and extends outward beyond the external border of the 

 bone, underneath the middle gluteus. 



Origin. — The ventral surface of the ilium external to the ilio-pectineal line, 

 the ventral sacro-iliac ligament, the wing of the sacrum, and the tendon of the 

 psoas minor. 



Insertion. — The internal trochanter of the femur, by a common tendon with 

 the psoas major. 



^ On account of the intimate union between the psoas major and iliacus they are frequently 

 considered a single muscle, to which the name ilio-psoas is applied; some anatomists include the 

 psoas minor also under this term. 



