Part 89

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.)

II. THE EXTERNAL MUSCLES OF THE HIP AND THIGH

Under this head will be described the muscles of the outer surface of the pelvis and thigh, and those which form the posterior contour of the latter. They are given in the order in which they may be conveniently examined.

1. =Tensor fasciæ latæ= (Fig. 178).—This is the most anterior muscle of the superficial layer. It is triangular in form, with its apex at the external angle of the ilium.

Origin.—The external angle of the ilium.

Insertion.—The fascia lata, and thus indirectly to the patella, the external straight ligament, and the crest of the tibia.

Action.—To tense the fascia lata, flex the hip joint, and extend the stifle joint.

Structure.—The muscle arises by a rather small head, about two inches (ca. 5 cm.) wide, on the antero-inferior eminence of the external angle of the ilium. Below this the belly spreads out and terminates about midway between the point of the hip and the stifle in the aponeurosis. The belly fuses to a considerable extent with the superficial gluteus. The aponeurosis fuses with the fascia lata, and detaches a lamina which passes with the tendon of insertion of the superficial gluteus to the external border of the femur.

Relations.—Externally, the skin and fascia; internally, the obliquus abdominis externus, the iliacus, superficial gluteus, rectus femoris, and vastus externus, branches of the circumflex iliac, ilio-lumbar, and iliaco-femoral arteries, and the anterior gluteal nerve; anteriorly, the precrural lymph glands. A considerable quantity of connective tissue is found between the deep face of the muscle and the abdominal wall.

Blood-supply.—Circumflex iliac, ilio-lumbar, and iliaco-femoral arteries.

Nerve-supply.—Anterior gluteal nerve.

2. =Gluteus superficialis= (Figs. 178, 199) (Superficial gluteus; gluteus externus).—This muscle lies behind and partly underneath the tensor fasciæ latæ. It is triangular and consists of an anterior and a posterior head united by the gluteal fascia.

Origin.—(1) The external angle and the adjacent part of the external border of the ilium (anterior head); (2) the gluteal fascia (posterior head).

Insertion.—The third (external) trochanter of the femur.

Action.—To abduct the limb, flex the hip joint, and tense the gluteal fascia.

Structure.—The anterior head of the muscle is not completely separable (except artificially) from the tensor fasciæ latæ. The attachment to the border of the ilium is by means of an intermuscular septum, which passes beneath the thick outer border of the gluteus medius. The posterior head arises from the deep face of the gluteal fascia, and so indirectly from the dorsal sacro-iliac ligament. The two heads unite and terminate on a strong flat tendon, which is inserted into the edge of the third trochanter of the femur, beneath the biceps femoris.

Relations.—Superficially, the skin, fascia, and biceps femoris; deeply, the gluteus medius, iliacus, rectus femoris, and branches of the iliaco-femoral artery; in front, the tensor fasciæ latæ; behind, the biceps femoris.