PSOAS 



347 



the an.ule Avhich exists 1)t't\vi'C'U the in'ck and shaft of the femur, the hne of the 

 tendon of the psoas passes either through or external to this axis. The action, 

 therefore of the muscle when the knee is extended will be either to produce no rota- 

 tion at all, or to rotate inwards. When the knee is flexed or when the thigh is 

 In'oken the axis of rotation is altered, and in the latter case the psoas may become 

 an external rotatf)r. It will follow, therefore, that any power of rotation exercised 

 by this muscle will be rather internal than external. 



Acting from below, the psoas will flex the lower thoracic and the luml^ar 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. 



Fig. 286. — Psoas, Iliacus, axd Quadeatus Lumbokum. 



Quadratus lumborum 



Psoas parvus 



Psoas magnus 



— Intertransversalis 

 anterior 



Quadratus lumborum 



Relations. — The front and inner surfaces are covered by the iliac fascia, Avhich 

 at the upper part of the muscle is thickened, and forms the ligamentum arcuatum 

 internum of tlie diaphragm. In front lie also the peritoneum, the intestines, the 

 kidney, ureter, and the renal vessels, and the spermatic or ovarian vessels. On the 

 outer side is the iliacus muscle. In the interior of the muscle is the lumbar i)lexus, 

 the nerves from which run for some distance in its substance. On the inner side lies 

 the external iliac artery; and behind is the inner border of the quadratus lumborum 

 and the l)rim of the pelvis. In the thigh, after passing beneath Poupart's ligament, 

 it is covered by the femoral artery, the pectineus lies along its inner border, and 

 the capsule of the hip-joint lies behind it, together with the intervening bursa. 



Variations.— Souietinies the part of the psoas wliich arises from the lower lumbar vertebrae 

 loruis a distinct muscle. Occasionally fibres from the psoas parvus join the psoas magnus. 



