THE MUSCLES—THE SUBLUMBAR MUSCLES 319 
THE MUSCLES! 
I. THE SUBLUMBAR 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 vertebr. 
Origin.—The bodies of the last three thoracic and first four or five lumbar 
vertebra, 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 1t 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 vertebra, 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.—Lumbar 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. 
tebre 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 iliacus. 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 vertebre, the lumbar 
vertebree, 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. 
1On account of the very slight mobility of the sacro-iliac articulation, the muscles of the 
pelvie 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- 
logieal grouping. 
