ABDOMINAL CAVITY 549 



the psoas muscle, care must be taken not to injure (i) the sympathetic 

 cord, which lies along its anterior margin ; (2) the lumbo-inguinal nerve, 

 which runs downwards in front of it ; (3) the ilio-inguinal nerve, and the 

 lateral cutaneous nerve of the thigh, which appears at its lateral border, and 

 the femoral nerve, which lies in the interval between it and the iliacus 

 muscle. In the case of the quadratus lumborum, bear in mind that the last 

 thoracic nerve runs laterally in front of this muscle, close to the lower 

 border of the last rib, and that the ilio-hypogastric and ilio-inguinal nerves 

 cross it obliquely at a lower level. 



M. Quadratus Lumborum. This muscle arises from the 

 ilio-lumbar ligament and from the crest of the ilium behind it. 

 It likewise receives two or three slips from the transverse 

 processes of a corresponding number of the lower lumbar 

 vertebrae. Narrowing slightly, as it passes upwards, it is 

 inserted into the medial half of the last rib, behind the arcus 

 lumbo-costalis lateralis, and by four tendinous slips into the 

 tips of the transverse processes of the four upper lumbar 

 vertebrae. 



M, Psoas Magnus (O.T. Psoas). This muscle has three 

 distinct series of origins from the side of the vertebral 

 column: (i) by five fleshy processes from the anterior 

 surfaces and lower borders of the transverse processes of the 

 lumbar vertebrae close to their roots; (2) by five slips, each 

 of which arises from the intervertebral disc and the con- 

 tiguous margins of the bodies of two vertebrae the first slip 

 springing from the last thoracic and the first lumbar vertebrae 

 and the intervening disc, and the last slip from the two lower 

 lumbar vertebrae and their intervening disc ; (3) from the 

 tendinous arches which bridge over the lumbar arteries and 

 protect these vessels from the pressure of the contracting 

 muscle. 



The psoas tapers somewhat as it extends downwards along 

 the brim of the pelvis, and a tendon appears on its lateral 

 border, which affords attachment to the fibres of the iliacus. 

 Passing behind the inguinal ligament, it is inserted into the 

 lesser trochanter of the femur. 



Another muscle, called the psoas parvus, is occasionally present. This 

 springs from the bodies of the last dorsal and first lumbar vertebrae, and 

 the intervertebral disc between them, and, stretching downwards upon the 

 anterior and medial aspect of the psoas magnus, it ends in a tendon which 

 is inserted into the ilio-pectineal eminence and ilio-pectineal line. 



Iliacus. This muscle arises from the upper part of the 

 iliac fossa, the anterior ilio-sacral ligament, and the base of 

 the sacrum. It is inserted into the tendon of the psoas 



