Chapter I: THE SUBLUMBAR MUSCLES (Figs. 450, 456)

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.)