The psoas, or psoas magnus (french: muscle psoas ou grand psoas) - named from the Greek word meaning the muscles of the loins - is thick, rounded, and fusiform.

Origin

Inner part, by five processes which arise from the sides of the intervertebral cartilages which intervene between the bodies of the last thoracic and the five lumbar vertebrae ; and the adjacent part of the sides of the bodies of these vertebras ; and between these processes from (3) tendinous arches which bridge over the sides of the bodies of these vertebrae. Outer part, from the lower i border and the front of the transverse processes of all the lumbar vertebrae.

Insertion

The lower and back part of the lesser trochanter of the femur.

Structure

With the exception of the small tendinous arches which span' the side of each of the four upper lumbar vertebrae from its upper to its lower border, and which give passage to the lumbar vessels, the whole origin of the muscle is fleshy. The fibres pass downwards and forwards in penniform fashion, but with a slight convergence, to the inner side of the tendon, which, beginning in the interior of the muscle about the level of the crest of the ilium, becomes free upon its outer and posterior surface a short distance above Poupart's ligament, while upon its inner surface it receives fibres down to its insertion. The muscle,! having hitherto run in a downward, forward, and slightly outward direction,! changes its course at Poupart's ligament, and passes downwards and backwards to be attached to the lesser trochanter of the femur. In its passage along the, brim of the pelvis and over the lower part of the iliac fossa, the tendon uponj its anterior and outer aspect begins to receive the insertion of the iliacus^ muscle. Between the tendon and the capsule of the hip joint which is in close connection with it, is placed a bursa which frequently communicates through j an opening in the capsule with the interior of the hip joint. !

Nerve-supply

From the anterior primary branches of the second and third | lumbar nerves by filaments which are given off from the lumbar plexus whilst it is passing through the muscle.

Action

The psoas is a powerful flexor of the thigh upon the pelvis, e.g. in walking, running, or going upstairs. The change in the direction of the tendon after crossing the horizontal ramus of the pubes makes its insertion nearly per- pendicular to the axis of the femur. The psoas therefore acts with less mechanical disadvantage than is usual with the muscles of the limbs. It has been sometimes described as an external rotator of the hip ; and its insertion into the lesser! trochanter at the inner side of the femur would appear to favour this view ; but a little consideration will show that, although it is attached on the inner side of the! femur, yet on account of the angle which the neck of the femur forms with its shaft j this point of attachment is really external to the axis about which the rotation! of the femur takes place. It will follow, therefore, that any power of rotation exercised by this muscle will be rather internal than external. j

Acting from below, the psoas will flex the lower thoracic and the lumbar spine upon the pelvis and the pelvis upon the thigh, as when the body is raised from the reclining to the sitting position, or when the trunk is bent forwards in rowing.

Relations

The front and inner surfaces are covered by the iliac fascia, which Ifc the upper part of the muscle is thickened, and forms the ligamentum arcuatum iternum of the diaphragm. In front lie also the peritoneum, the intestines, the renal vessels, the spermatic or ovarian vein, and the ureter. On the outer side is 16 iliacus muscle. In the interior of the muscle is the lumbar plexus, the nerves •cm which run for some distance in its substance. On the inner side lies the external iliac artery ; and behind is the vertebral column, the inner border of the quadratus lumborum, and the brim of the pelvis. In the thigh, after passing oeneath Poupart's ligament, it is covered by the femoral artery, the pectineus lies ilong its inner border, and the capsule of the hip joint lies behind it, together with the intervening bursa.

Variations

Sometimes the part of the psoas which arises from the lower lumbar vertebras forms a distinct muscle. Occasionally fibres from the psoas parvus join the psoas magnus.

Muscles of the front of the thigh.