936 THE NERVOUS SYSTEM 



COMMON COMPOSITION 



NERVE. PROXIMAL. ORDINARY. DISTAL. 



Lateral (external) cutaneous 1, 2 L. 1,2, 3 L. 2, 3 L. 



Femoral (anterior crural) . . 1 , 2, 3, 4 L. 2, 3, 4 L. 2, 3, 4, 5 L. 



Obturator 1, 2, 3, 4 L. 2, 3, 4 L. 2, 3, 4 L. 



Furcal 4 L. 4 L. 4 L. 



The lumbar plexus lies in the posterior part of the psoas muscle (fig. 688), in 

 front of the transverse processes of the lumbar vertebra? and the inner border of the 

 quadratus lumborum, and its terminal branches are distributed to the lower part of 

 the abdominal wall, the front and inner part of the thigh, the external genital 

 organs, the front of the knee, the inner side of the leg, and the inner side of the foot. 



" The first and second of the nerves give collateral muscular branches to the quad- 

 ratus lumborum muscle, and the second and third nerves give similar branches to the 

 psoas. The remaining branches of the plexus are terminal branches. The first lumbar 

 nerve, after it has been joined by the communication from the last thoracic nerve, 

 divides into three terminal branches, the ilio-hypogastric nerve, the ilio-inguinal 

 nerve, and a branch which joins the second nerve. The fibres of this latter branch 

 pass mainly into the genito-femoral (genito-crural) nerve, but occasionally some of 

 them enter the femoral (anterior crural) and obturator nerves. The remaining nerves 

 divide into anterior or ventral and posterior or dorsal divisions. The anterior 

 divisions form a portion of the genito-femoral (genito-crural) nerve and the obtura- 

 tor nerve, and the posterior divisions enter the lateral (external) cutaneous and 

 femoral (anterior crural) nerves. 



All the terminal branches of the plexus are formed in the substance of the psoas 

 muscle; four of them, the ilio-hypogastric, the ilio-inguinal, the lateral (external) 

 cutaneous, and the femoral (anterior crural) , leave the muscle at its outer border. 

 The genito-femoral (genito-crural) passes through its anterior surface, and the 

 obturator through its inner border. 



Terminal branches. The Ilio-hypogastric Nerve springs from the first lum- 

 bar nerve, after the latter has been joined by the communicating branch from the 

 last thoracic nerve, as it is in about 50 per cent, of the cases, and it contains fibres of 

 both the last thoracic and the first lumbar nerves. It pierces the outer border of the 

 psoas and crosses in front of the quadratus lumborum (fig. 688), and behind the kid- 

 ney and the colon. At the outer border of the quadratus it pierces the aponeurosis 

 of origin of the transversus abdominis and enters the areolar tissue between the trans- 

 versus and the internal oblique, where it frequently communicates with the last thoracic 

 and with the ilio-inguinal nerve, and it divides into an iliac and a hypogastric branch, 

 which correspond, respectively, with the lateral and anterior branches of a typical 

 spinal nerve. The anterior cutaneous (hypogastric) branch passes forwards and 

 downwards, between the transversus abdominis and the internal oblique muscles, 

 giving branches to both; it communicates with the ilio-inguinal nerve, and, near the 

 anterior superior spine of the ilium, it pierces the internal oblique muscle and con- 

 tinues forwards beneath the external oblique aponeurosis towards the middle line. 

 About 2' 5 cm. (1 in.) above the subcutaneous inguinal ring it pierces the aponeurosis 

 of the external oblique, becomes subcutaneous, and supplies the skin above the 

 symphysis. 



The lateral cutaneous (iliac) branch pierces the internal and external oblique 

 muscles, emerging through the latter above the iliac crest at the junction of its an- 

 terior and middle thirds (fig. 693). It is distributed to the integument of the upper 

 and outer part of the thigh, in the neighbourhood of the gluteus medius and tensor 

 fascia; latse muscles (fig. 691). 



The Ilio-inguinal Nerve arises principally from the first lumbar nerve, but it 

 frequently contains fibres of the last thoracic nerve. It emerges from the outer 

 border of the psoas, at a lower level than the ilio-hypogastric nerve, and passes across 

 the quadratus lumborum (figs. 688 and 689). As a rule, it is below the level of the 

 lower end of the kidney, but it passes behind the ascending or the descending colon 

 according to the side on which it lies, and crosses the posterior part of the inner lip 

 of the iliac crest; it then runs forwards on the upper part of the iliacus, 

 pierces the transversus abdominis near the anterior part of the crest, and com- 

 municates with the anterior cutaneous (hypogastric) branch of the ilio-hypogastric 



