366 NEUROLOGY, OR ANATOMY OF NERVOUS SYSTEM 



The Lumbar Plexus 



The lumbar plexus is formed in the substance of the psoas 

 muscle, in the following manner: Each of the first four lumbar 

 nerves divides into an upper and a lower branch. Just before 

 dividing the first receives the dorsolumbar nerve, and the third 

 and fourth send each a branch to the nerve below. 



The upper branch of the first subdivides into the iliohypo- 

 gastric and ilioinguinal nerves. The lower branch of the first 

 passes downward and subdivides into two branches, one of 

 which unites with the upper branch of the second to form the 

 genitocrural nerve. The other unites with the lower branch 

 of the second to form a cord. This cord passes downward, 

 and gives off the external cutaneous nerve and a branch to 

 the obturator, after which it unites with the upper branches 

 of the third and fourth to form the anterior crural nerve. The 

 lower branches of the third and fourth unite to form the 

 obturator nerve. 



The iliohypogastric escapes at the upper part of the psoas, 

 crosses the quadratus, pierces the transversalis at the iliac 

 crest, and divides, between it and the internal oblique, into 

 two branches. The iliac branch supplies the skin over the 

 glutei, behind the lateral cutaneous of the last dorsal; the 

 hi/pogastric branch communicates with the ilioinguinal, and 

 pierces the oblique inuscles to supply the skin of the pubic 

 and hypogastric regions. 



The ilioinguinal crosses the quadratus and iliacus below 

 the preceding, pierces the transversalis, cpmmunicating with 

 the iliohypogastric, and runs in the inguinal canal, supplying 

 the skin of the groin, scrotum, and penis (the labium in the 

 female) . 



The genitocrural runs downward through and on the psoas 

 muscle, and divides some distance above Poupart's ligament 

 into a genital and a crural branch. The former lies on the 

 external iliac artery, sending filaments around it, and runs 

 with the cord through the inguinal canal to the cremaster 

 muscle; in the female it runs on the round ligament. The 

 crural branch runs under Poupart's ligament into the thigh, 

 sending filaments around the femoral artery, and lying super- 

 ficial to the artery in the femoral sheath. It supplies the 

 skin of the upper thigh, and joins the middle cutaneous. 



