THE ABDOMEN m\ 



and passes out behind Poupart's ligament, having, just prior to 

 this, crossed the deep circumflex iliac artery. On entering the 

 thigh it lies for about \ inch within the crural sheath, and subse- 

 quently, piercing the outer wall of that sheath, it is distributed to 

 the integument over Scarpa's triangle. 



The external cutaneous nerve pierces the outer border of the 

 psoas magnus near its centre, and takes a direction downwards 

 and outwards over the back part of the iliac crest into the iliac 

 fossa. It then crosses the iliacus under cover of the fascia iliaca 

 towards the anterior superior iliac spine, where it emerges on to the 

 thigh behind the outer end of Poupart's ligament. For the distri- 

 bution of the nerve in the thigh, see p. 449. 



The anterior femoral nerve pierces the outer border of the psoas 

 magnus about the level of the back part of the pelvic brim. It 

 then passes forwards, lying deeply between the psoas magnus and 

 iliacus, and emerges on to the thigh behind Poupart's ligament. 

 Whilst in the abdominal cavity it gives branches to the iliacus 

 muscle. The course and distribution of the nerve in the thigh will 

 be found on p. 470. 



The obturator nerve pierces the inner border of the psoas magnus 

 at the back part of the pelvic brim, and it lies upon the ala of the 

 sacrum, having the lumbo-sacral cord on its inner side. Passing 

 deeply behind the common iliac artery it enters the pelvic cavity, 

 and passes along the outer wall a little below the pelvic brim, 

 where it lies above the obturator artery. It then emerges on to 

 the thigh through the obturator canal. For the course and distri- 

 bution of the nerve in the thigh, see p. 474. 



The accessory obturator nerve (when present) pierces the inner border of 

 the psoas magnus close to the main obturator nerve, but, unlike it, does not 

 enter the pelvic cavity. Its course is forwards along the inner border of the 

 psoas magnus underneath the external iliac vessels, and it emerges on to the 

 thigh by passing over the superior pubic ramus beneath the pectineus muscle. 

 Under cover of that muscle it divides into the following three branches : 

 articular to the hip-joint ; muscular to the deep surface of the pectineus ; and 

 a reinforcing branch to join the superficial or anterior division of the main 

 obturator nerve. 



Varieties of the Lumbar Plexus. — These assume the form of two types, high 

 or prefixed, and low or postfixed. In the prefixed type the anterior primary 

 division of the third lumbar is a nervus furcalis, and takes part in the sacral 

 plexus, whilst in the postfixed type the anterior primary division of the fifth 

 lumbar is a nervus furcalis, and takes part in the lumbar plexus. 



Lumbo-sacral Cord. — This is formed by the union of the ventral 

 and dorsal divisions of the descending branch of the fourth lumbar 

 nerve with the ventral and dorsal divisions of the anterior primary 

 division of the fifth lumbar. It is a large double tnmk, which 

 rests upon the ala of the sacrum, being at first under cover of the 

 psoas magnus, and subsequently lying on its inner side, where it 

 has the obturator nerve external to it. In its course it passes 

 behind the common and internal iliac vessels, and in the pelvis, its 

 two divisions having joined those of the anterior primary division 

 of the first sacral nerve, it takes part in the sacral plexus, entering 



