THE MUSCLES THE STJBLUMBAR MUSCLES 319 



The Muscles 1 



I. THE STJBLUMBAR MUSCLES (Figs. 287, 575) 



The muscles of this group are not confined to the sublumbar region, but ex- 

 tend beyond it both before and behind. Their chief function is to flex the hip 

 joint. Two, however, — the psoas minor and the quadratus lumborum, — have not 

 this action, 



1. Psoas minor. — 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 join the tendon at an acute angle. The latter lies along the lateral 

 border of the fleshy portion and is flattened. It appears on the surface of the mus- 

 cle 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 part of the muscle is related 

 to the pleura, crura of the diaphragm, and sympathetic and splanchnic nerves. 

 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 lum- 

 bar nerves. The lumbar arteries pass through the medial edge. Near its insertion 

 the tendon is crossed medially by the external iliac artery, and laterally by the fem- 

 oral nerve. 



Blood-supply. — Intercostal and lumbar arteries. 



Nerve-supply. — Liunbar nerves. 



2. Psoas major. — 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 transverse processes of the lumbar ver- 

 tebrae and the last two ribs. 



Insertion. — The trochanter minor 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 flat- 

 tened, thick in its middle, thin at its edges. The thoracic part is small, the abdom- 

 inal 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 ihacus. 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. 



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, 

 and the circumflex iliac vessels. 



1 On account of the very slight mobiUty of the sacro-iliao articulation, the muscles of the 

 pelvic girdle are much reduced, and almost all of those which might be included in this group 

 extend to the femur or even to the leg. It seems undesirable, therefore, to attempt a morpho- 

 logical grouping. 



