OBTURATOR AND EXTERNAL CUTANEOUS NERVES 805 



ment by piercing the fascia lata. It is distributed to the skin covering the upjter 

 and central j)art of the anterior aspect of the thigh. 



(4) The obturator nerve arises from tiie third and fourth lumbar nerves, and in 

 the majority of cases receives an additional root from tlie second. It passes between 

 the i^soas muscle and the body of the tifth lumbar vertebra. It then runs downwards 

 and forwards outside the internal iliac vessels and the ureter, and below the external 

 iliac vessels to reach the upper margin of the thyroid foramen. In this part of its 

 course it runs parallel to and a little ]:)elow the brim of the pelvis, and is i)laced in 

 the extra-peritoneal fat l)etween the peritoneum and the parietal pelvic fascia. It 

 is accom})anied 1)V the obturator vessels which are placed below it. The nerve then 

 ]»asses through tlie deficiency in the obturator membrane, and divides into tw(j 

 branches, an anterior and a posterior. 



(a) The anterior branch passes across the upper border of the obturator 

 externus, and is directed downwards and inwards l)etween the pectineus and 

 adductor brevis, and divides into the following branches: — (i) A twig for the hip- 

 joint; (ii) a branch for the adductor longus; (iii) a branch to supply the adductor 

 l)revis; (iv) a branch for the gracilis; (v) in rare cases, a twig for the pectineus ; 

 (vi) a cutaneous branch; and (vii) a Ijranch to the femoral artery. The cutaneous 

 branch becomes superficial by passing between the adductor longus and adductor 

 brevis, and is then directed along the posterior border of the sartorius, where it 

 communicates with twigs from the long saphenous and from the posterior Ijranch 

 of the internal cutaneous to form the subsartorial plexus. 



(h) The posterior branch gives off branches to supply the hip-joint and the 

 ol)turator externus, and then pierces the upper part of that muscle and reaches the 

 interval between the adductor brevis and adductor magnus. It runs downwards 

 on the anterior surface of the latter muscle, supplying it with tAvigs, and terminates 

 in an articular twig, the geniculate branch, Avhich is distributed to the knee-joint. 

 (i) The twig to the hip-joint enters the acetabulum by passing through the coty- 

 loid notch. It ramifies in the fat occupying the floor of the acetabulum and in the 

 adjacent synovial memln'ane. (ii) The branch to the ol)turator externus pierces 

 the deep surface of the muscle, (iii) Several large twigs enter the adductor mag- 

 nus. (iv) The geniculate branch pierces the lower part of the adductor magnus 

 and appears on the posterior surface of that muscle close to the opening for the 

 l)opliteal vessels, and descends on the inner side of (sometimes behind) the pop- 

 liteal artery. Having given off a filament which accompanies the superior internal 

 articular artery, it breaks up into terminal twigs which separateh' pierce the pos- 

 terior ligament of the knee-joint. 



An accessory obturator nerve is occasionally present. It arises between the anterior cruial 

 and obturator nerves from the third and fourth lumbar nerves. It runs along the imier 

 border of the psoits, and cros.ses in front of the brim of the pelvis to gain the deep surface of the 

 pectineus. In this situation it breaks up into branches. The largest ofthe.se branches joins the 

 obturator nerve, others enter the capsule of the hip-joint, and a branch is furnished to the pec- 

 tineus. 



The branch which joins the obturator nerve may carry filaments for the adduc- 

 tors longus and brevis, and the gracilis. 



(5) The external cutaneous nerve arises, by two roots, from the second and 

 third lumbar nerves. It riuerges at the outer l)order of the psoas, and crosses 

 ol)li(|uely in front of the iliacus to reach the interval between the anterior superior 

 and anterior inferior spines of the ilium. In this course it runs under cover of the 

 fascia iliaca. It then passes behind Poupart's ligament, crosses in front of the 

 origin of the sartorius, and divides into an anterior and a posterior branch. The 

 posterior branch passes backwards and downwards under cover of the fascia lata, 

 and divides into twigs which supply the integument covering the insertion of the 

 gluteus maximus and the upper and outer part of the thigh. The anterior branch 

 is much larger than the posterior. It runs downwards for several inches in a canal 

 formed by the fascia lata, and enters the superficial fascia at the junction of the 

 upper with the middle third of tlie thigh. It divides into branches which supply 

 the skin of the outer part of the thigh, a few of the terminal twigs reaching as far 

 as the knee and entering into the composition of the plexus patellae. 



