The muscles of this group are not confined to the sublumbar region, but
extend 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= (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.
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.
_Action._—To flex the hip joint and to rotate the thigh outward.
_Structure._—The belly of the muscle is so deeply grooved for the psoas
major as to give the appearance of being completely divided into outer
and inner portions. When the psoas is removed, it is seen, however, that
the two heads are not entirely separated. The outer, larger head arises
from the wing of the ilium chiefly; the inner, smaller head arises
chiefly from a small area on the shaft of the ilium, between the psoas
tubercle and the depression for the inner tendon of the rectus femoris,
and from the tendon of the psoas minor. The two portions inclose the
psoas major in front of the hip joint.
_Relations._—Dorsally, the ilium, sacrum, sacro-iliac articulation, the
gluteus medius, the ilio-lumbar and external circumflex vessels;
ventrally, the iliac fascia, inguinal ligament, the psoas major,
sartorius, and abdominal muscles. At the hip joint the chief relations
are: internally, the femoral vessels, the femoral nerve, and the
sartorius muscle; externally, the rectus femoris and tensor fasciæ latæ;
in front, the abdominal muscles; behind, the hip joint.
_Blood-supply._—Lumbar, circumflex iliac, and deep femoral arteries.
_Nerve-supply._—Lumbar and femoral nerves.
4. =Quadratus lumborum.=—This thin muscle lies on the outer part of the
ventral surfaces of the lumbar transverse processes.
_Origin._—The ventral surface of the upper part of the last two ribs and
the lumbar transverse processes.
_Insertion._—The ventral surface of the wing of the sacrum and the
ventral sacro-iliac ligament.
_Action._—Acting together, to fix the last two ribs and the lumbar
vertebræ; acting singly, to produce lateral flexion of the loins.
_Structure._—The muscle is pennate, and is curved with the convexity
outward. It is thin, largely mixed with tendinous fibers, and is, in
general, little developed in the horse in comparison with some of the
other animals (_e. g._, dog, sheep).
_Relations._—Ventrally, the psoas major and the last intercostal and
first three lumbar nerves; dorsally, the last two ribs, the lumbar
transverse processes, and the lateral branches of the lumbar arteries.
_Blood-supply._—Lumbar arteries.
_Nerve-supply._—Lumbar nerves.
5. =Intertransversales lumborum= (M. intertransversarii lumborum).—These
are described with the other spinal muscles. (See p. 238.)