THE EXTERNAL CUTANEOUS NERVE. 317 



at the outer side of the femoral artery, it pierces the fascia lata, and supplies the 

 skin on the upper part of the thigh, communicating with the middle cutaneous 

 branch of the anterior crural nerve. While it is passing beneath Poupart's 

 ligament, some filaments are prolonged from this nerve on the femoral artery. 



Varieties. The two divisions of the genito-crural nerve often arise independently from 

 the lumbar plexus (fig-. 205). The genital branch is occasionally derived from the last dorsal 

 and first lumbar nerves (fig-. 205, A). Either branch may proceed wholly from the first or the 

 second lumbar nerve (fig. 205, B). Absence of either division, or more rarely of the whole 

 nerve, may occur : in such cases the fibres usually forming the genital branch are associated 

 with the ilio-inguinal nerve, and those belonging to the crural division with the external 

 cutaneous or anterior crural nerve. Conversely, the genital branch may reinforce or replace 

 the ilio-inguinal nerve, and the crural branch the external or middle cutaneous. The genital 

 branch often gives filaments to the lower fibres of the internal oblique and transversalis 

 muscles. 



External cutaneous nerve. This nerve (n. cufaneus femoris lateralis), 

 arising from the second and third lumbar nerves, emerges from the outer 

 border of the psoas muscle, and crosses the iliacus below the ilio-inguinal nerve, 

 where it is placed beneath the iliac fascia. It passes under Poupart's ligament, 

 and enters the thigh immediately below the anterior superior iliac spine, where it 

 divides into an anterior and a posterior branch distributed to the integument of the 

 outer side of the hip and thigh (fig. 208, 1). 



(a) The posterior branch perforates the fascia lata and subdivides into two or 

 three others, which turn backwards and supply the skin upon the outer surface of 

 the limb, from the upper border of the hip-bone nearly to the middle of the thigh. 

 The highest among them are crossed by the cutaneous branches from the last dorsal 

 nerve. 



(Z>) The anterior branch, the continuation of the nerve, is at first contained in a 

 canal formed in the substance of the fascia lata ; but, about four inches below 

 Poupart's ligament, it enters the subcutaneous fatty tissua, and is distributed along 

 the outer part of the front of the thigh, ending near the knee. The principal offsets 

 spring from its outer side. In some cases, this branch reaches quite down to the 

 knee, and takes part there in the formation of the patellar plexus. 



Varieties. In the normal form of lumbar plexus the external cutaneous nerve is derived 

 mainly from the second lumbar nerve, and receives only a small root from the third. In the 

 high form of plexus it arises entirely from the second lumbar nerve, or from the second and 

 first ; while in the low form its chief root is furnished by the third nerve. The external 

 cutaneous nerve often accompanies, or is united with, the anterior crural trunk to below 

 Poupart's ligament. The posterior branch is sometimes replaced by an offset of the genito- 

 crural nerve. In one instance the place of the external cutaneous nerve was taken by a 

 branch of the ilio-inguinal (M. Griffin). 



Obturator nerve. The obturator nerve arises from the lumbar plexus generally 

 by three roots, which proceed from the second, third and fourth lumbar nerves, and 

 of which that from the third is the largest, while the root from the second nerve is 

 often very small. Having emerged from the inner border of the psoas muscle, 

 opposite to the brim of the pelvis, it runs along the side of the pelvic cavity, above 

 the obturator vessels, as far as the opening in the upper part of the thyroid foramen, 

 through which it escapes from the pelvis into the thigh. In the foramen, it divides 

 into an anterior and a posterior branch, which are separated from one another by 

 some fibres of the obturator externus, and lower down by the short adductor 

 muscle. 



A. The anterior or superficial portion communicates with the accessory obturator 

 nerve, when this is present, and descends over the upper border of the obturator 



VOL. III., PT. 2. X 



