The psoas parvus - a small muscle, only occasionally present, named from its positic in the loins and its small size - is fusiform and somewhat flattened.

Origin

The side of the intervertebral disc between the last thoracic and the fir lumbar vertebra and the adjacent borders of the bodies of these vertebrse.

Insertion

The ilio-pectineal line.

Structure

Arising fleshy, the fibres converi^e and are inserted in a somewhat penii form manner into the back and inner surface of a tendon which appears about two incln below the origin of the muscle upon its outer and anterior aspect and becomes free aboi the level of the fifth lumbar vertebra. The tendon, a narrow fibrous band, lies upon tl inner aspect of the psoas magnus on the brim of the pelvis and expands at its lowi extremity to be attached along the ilio-pectineal line and the pectineal eminence.

Nerve-supply

By small filaments from the first nerve of the lumbar plexus.

Action

To flex the pelvis upon the thorax; or, taking the pelvis as a fixed point, will flex the lower part of the thoracic spine as well as the lumbar spine upon the pelvi It is a muscle which is well developed in some animals, having for its function the drawii forwards of the lower part of the pelvis, accompanied by the arching of the lumbar spii which is seen when they are running swiftly.

Relations

In front, the iliac fascia, peritoneum, ligamentum arcuatum internm intestines, renal vessels, ureter, external iliac vessels, etc. Behind and externally, the psoas magnus.

Investing the abdominal portion of the ilio-psoas is a strong membrane, call the iliac fascia, which is attached to the crest of the ilium externally and internal to the posterior part of the ilio-pectineal line which forms the brim of the pelvis.

Between these attachments it invests the front of the ihacus and psoas muscles. Above, it is continued upwards as the covering of the latter muscle, at the sides of which it is attached to the transverse processes and bodies of the lumbar vertebrae, as well as to the intervertebral discs and the small tendinous arches which bridge the side of the bodies of these vertebrae. At the diaphragm it is thickened and forms the ligamentum arcuatum internum; near the tips of the transverse processes of the lumbar vertebrae it is attached to the anterior layer of the lumbar fascia. Below, it joins beneath the outer half of Poupart's ligament with the ransversalis fascia, but internal to this it passes downwards into the thigh, forming with the transversalis fascia the sheath of the femoral vessels. Still further downwards, it is continuous with the pubic portion of the fascia lata which invests the pectineus muscle and it also sends backwards a septum between the psoas and the pectineus which is attached to the ilio-pectineal eminence.

Muscles of the front of the thigh.

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