THE POSTERIOR LIMB 273 



middle of the inguinal ligament to the centre of the ventral 

 surface of the body of the first sacral vertebra. On this 

 dorsal (sacral) continuation of the inguinal ligament some 

 of the superficial fibres of the psoas minor tendon are 

 inserted. 



It is necessary to divide the inguinal ligament and reflect 

 its sacral continuation, together with the tendon of the 

 psoas minor. 



(b) The psoas major. Origin: Internal surfaces of the bases 

 of the last three ribs and bodies of the corresponding 

 thoracic vertebrae; also the lumbar vertebrae. Insertion: 

 Lesser trochanter. 



(c) The iliacus. Origin: Bodies of the last lumbar and first 

 sacral vertebrae, extending to the sacroiliac union and the 

 body of the ilium behind it as far back as the ventral border 

 of the acetabulum. Insertion: With the psoas major on 

 the lesser trochanter. The two muscles together form the 

 iliopsoas. 



The lumbar portion of the lumbosacral plexus, beginning with 

 the fourth lumbar nerve, lies on the ventral surface of the psoas major 

 and between the latter and the iliacus, though, as an individual 

 variation, the fifth lumbar nerve may be the most anterior to appear 

 in this position. The fifth and sixth lumbar nerves usually together 

 give rise to the greater part of the femoral nerve (p. 280), the trunk 

 of which may be traced from a position between the two muscles distad 

 over the dorsal surface of the inguinal ligament to the medial surface of 

 the thigh. The remaining four nerves crossing the dorsal body-wall 

 obliquely are the twelfth thoracic and first three lumbar. 



The psoas major should be freed at its lateral margin 

 and turned toward the median line, the fourth lumbar nerve 

 being divided. 



(d) The quadratus lumborum. Origin: Bodies of the posterior 

 five thoracic vertebrae and the bases of the corresponding 

 five ribs; bodies and transverse processes of the lumbar 

 vertebrae. Insertion: Triangular processes of six lumbar 

 vertebrae and the posterior ventral angle of the iliac wing, 

 together with the adjacent portion of its medial surface. 



