1324 HUMAN ANATOMY. 



5. The External Cutaneous Nerve. 



The external cutaneous nerve (n. cutaneus femoris lateralis) (Fig. 1109) arises at 

 the posterior aspect of the lumbar plexus from the second and, to a less extent, the 

 third lumbar nerve. It may arise from the first and second, from the second alone 

 or may derive a majority of its constituent fibres from the third. It passes obliquely 

 downward and outward beneath the lateral margin of the psoas magnus and o\'er the 

 iliacus muscle, through the iliac fossa, covered by the iliac fascia. After crossing the 

 deep circumflex iliac artery it enters the thigh beneath Poupart's ligament, mesial 

 to the anterior superior spine of the ilium, and passes over, sometimes through or 

 under, the pointed tendinous origin of the sartorius. The nerve then descends in 

 the thigh beneath the fascia lata and soon divides into (a) an anterior and (^) a 

 posterior \.&x\\\\\\3Xhxz.\\Qh (Fig. mo). 



a. The anterior branch (r. anterior) follows a downward course in the thigh in a tubular 

 canal in tlie fascia lata, from which it emerges at a point 10-15 cm. below the anterior superior 

 iliac spine. It continues downward anterior to the vastus externus muscle and is distributed to 

 the integument of the antero-lateral aspect of the thigh as far as the knee. Numerous collateral 

 branches are given off, the majority of which arise from its lateral edge and supply the skin ovei 

 the ilio-tibial band. The main trunk may extend quite to the knee and become a participant in 

 the formation of the patellar plexus. 



b. The posterior branch (r. posterior) passes obliquely backward through the fascia lata 

 and breaks up into several branches which are distributed to the integument over the tensor 

 fasciae femoris and the lower portion of the gluteal region. The uppermost filaments are crossed 

 by twigs from the lateral cutaneous branch of the twelfth thoracic nerve. 



Variations. — The external cutaneous may be associated with the anterior crural until after 

 Poupart's ligament has been passed. A branch of the genito-crural may replace the posterior 

 branch. In one case a branch of the ilio-inguinal took the place of the external cutaneous. 



Three specimens found in the anatomical rooms of the University of Pennsylvania showed 

 decided anomalies. In one the nerve passed beneath Poupart's ligament at a point midway 

 between the anterior superior spine of the ilium and the femoral artery. In another the nerve 

 of the right side resembled in position the one just mentioned, while the left was apparently 

 absent, its place being taken by a branch of the anterior crural. In the third the posterior 

 branch emerged from beneath Poupart's ligament 5 cm. to the inner side of the anterior superior 

 iliac spine. The anterior branch formed a common trunk with the external branch of the mid- 

 dle cutaneous nerve. From the joint trunk a small branch passed to join the internal branch of 

 the middle cutaneous after the latter had pierced the sartorius muscle. 



6. The Obturator Nerve. 



The obturator nerve (n. obturatorius) (Fig. 1109) is composed of fibres which 

 arise from the second, third and fourth lumbar nerves, the fourth supplying the 

 largest and the second the smallest contribution, the latter sometimes being absent 

 entirely. Occasionally additional roots are derived from the first and fifth lumbar 

 nerves, and sometimes the nerve arises, in the high form of plexus, from the first, 

 second and third lumbar nerves. 



The three roots having united in the substance of the psoas magnus, the nerve 

 passes vertically downward and emerges, the only constant branch of the plexus to 

 do so, from the mesial margin of the psoas muscle opposite the brim of the true 

 pelvis. Lying posterior to the common and lateral to the internal iliac vessels, the 

 obturator nerve courses along the antero-lateral wall of the pelvis below the ilio- 

 pectineal line, above the obturator vessels and upon the inner surface of the pelvic 

 fascia. It escapes from the pelvis through the obturator canal in the obturator mem- 

 brane and divides into its terminal branches, either while still within the foramen or 

 shortly after emerging from it. These branches are separated from each other first 

 by the anterior fibres of the obturator externus muscle and later by the adductor 

 brevis muscle. They supply the adductor muscles, the hip and knee joints and the 

 integument of the mesial aspect of the thigh. 



Branches. — The obturator gives of^ : {a) a branch to the obturator extermis 

 muscle and then divides into its terminal branches^ {b^ the anterior and (^) the 

 posterior. 



