This plexus is produced by the anterior primary divisions of the first four lumbar nerves, the first being reinforced by a communicating twig from the last thoracic. The fourth lumbar, in addition to entering into the composition of the plexus, sends a branch to the fifth lumbar to form the lumbo-sacral cord. The plexus lies in the substance of the psoas muscle, in front of the transverse processes of the lumbar vertebrae,
The ilio-hypogastric and ilio-inguinal are derived from the first lumbar; the genito-crural (femoral) from the first and second; the lateral cutaneous from the second and third; the anterior crural (femoral) from the second, third, and fourth; the obturator from the second, third, and fourth; while muscular twigs are given to the quadratus lumborum and psoas.
(1) From the first, second; and third to the quadratus lumborum.
(2) From the third to the psoas.
(b) Ilio-hypogastric (T. 12, L. 1). Appears at the upper part of the lateral border of the psoas, crosses the quadratus lumborum superficially, pierces the transversalis, proceeds forwards between it and the internal oblique, and divides into an iliac and a hypogastric branch. The former perforates the internal and external obliques to become superficial just above the iliac crest. It traverses the iliac crest a little posterior to the iliac branch of the subcostal nerve, and is distributed to the skin of the buttock. The hypogastric branch runs forwards between the transversalis and internal oblique muscles, furnishing twigs to both, and passes through the external oblique a little above the external (subcutaneous) abdominal ring, to innervate the skin of the abdominal wall just above-the pubes.
(c) Ilio-inguinal (L. 1) is smaller than, the ilio-hypogastric and has a course similar to that nerve, but lies on a slightly lower level. It, however, passes down the inguinal canal, to become superficial by emerging from the external (subcutaneous) abdominal ring. It supplies the integument of the inguinal region and scrotum (labia majora in females), and does not give off an iliac branch.
(d) Genito-crural (femoral) (L. 1, 2) pierces the psoas obliquely, to descend upon the surface of that muscle and divide into a genital and a crural (femoral) branch. The former crosses over the external iliac artery to the inguinal canal. Here it becomes a constituent of the spermatic cord, and supplies the cremaster muscle together with the skin of the scrotum. In females it is associated with the round ligament of the uterus, and is distributed to the skin of the groin. The crural branch passes beneath Poupart's (inguinal) ligament on the lateral side of the femoral artery, to innervate the skin over the upper part of Scarpa's triangle. It communicates there with the medial cutaneous of the anterior crural.
(e) Lateral cutaneous (L. 2, 3). Appears at the lateral margin of the psoas muscle. It crosses the iliac fossa to go beneath Poupart's (inguinal) ligament close to the anterior superior iliac spine, and after a short course, divides into anterior and posterior branches. The anterior is distributed to the skin of the lateral part of the front of the thigh as far as the knee. The posterior branch supplies the integument of the lateral aspect of the buttock and upper part of the back of the thigh.
(f) Anterior Crural (Femoral) (L. 2, 3, 4). Emerges from the lateral border of the psoas in its lower part. It passes beneath Poupart's ligament in the sulcus between the iliacus and psoas muscles-about half an inch on the lateral side of the femoral artery. At the upper part of Scarpa's triangle the nerve divides into superficial and deep portions. The trunk of the nerve supplies branches to the iliacus muscle and femoral artery.
The anterior division gives off the middle and medial cutaneous nerves, and a branch to the pectineus, which runs behind the femoral artery. The middle cutaneous pierces and supplies the sartorius muscle; it then splits into two portions, anterior and posterior, which innervate the skin of the front and medial part of the thigh as far as the knee. The middle cutaneous communicates with the crural branch of the genito-crural, and also with the medial cutaneous. The medial cutaneous after passing down over the femoral artery likewise divides into an anterior and posterior branch. These branches perforate the fascia lata separately, the former supplying the skin down to the knee, while the latter descends on the medial side of the knee to be distributed to the integument of the medial aspect of the upper part of the leg.
The posterior division follows the posterior border of the sartorius. It gives off muscular and articular twigs, and one cutaneous branch, namely, the medial or long saphenous. The muscular branches supply the rectus femoris, vastus lateralis, crureus (vastus intermedius), subcrureus, and vastus medialis. From the nerve to the rectus femoris twigs are furnished to the hip-joint, while from the nerves to the vasti and subcrureus branches are prolonged to the knee-joint.
The long saphenous (L. 3, 4) is the largest branch of the anterior crural, and descends through Hunter's canal, in close contact with the femoral artery-lying first on the lateral side and then crossing over to the medial side of the vessel. It becomes superficial on the medial aspect of the knee, after piercing the aponeurotic roof of Hunter's canal. From thence it is continued along the medial side of the leg with the long saphenous vein, to pass in front of tbe medial malleolus, and terminate about the middle of the medial border of the foot. In addition to branches which supply the skin of the medial surface of the leg and foot, it furnishes a communicating twig to join branches of the obturator and medial cutaneous beneath the sartorius, and a patellar branch which unites with twigs from the medial, middle, and lateral cutaneous nerves on the front of the knee, forming the patellar plexus.
(g) Obturator (L. 2, 3, 4). Emerges on the medial side of the psoas. It runs along the lateral wall of the pelvis, under cover of the external iliac vein, then above the obturator vessels, to traverse the upper part of the obturator foramen. In the foramen it divides into anterior and posterior portions.
The anterior portion enters the thigh above the obturator externus, and passes down over the adductor brevis, but under cover of the pectineus and adductor longus. From it the following branches arise :-
(1) Muscular, to gracilis, adductor longus, adductor brevis, and pectineus.
(2) Articular, to hip-joint.
(3) Communicating twig, to unite with branches of the long saphenous aed medial cutaneous nerves beneath the sartorius.
(4) A terminal branch, which ramifies over the femoral artery.
The posterior division pierces the obturator externus, and runs behind the adductor brevis to the anterior surface of the adductor magnus.
This division gives off:-
(1) Muscular, to the obturator externus and adductor magnus.
(2) Articular, to the knee (the geniculate branch of the obturator), which perforates the adductor magnus to accompany the popliteal artery. It enters the knee-joint through the posterior ligament.
(h) Accessory obturator (L. 3, 4). This nerve is only occasionally present. After running beneath the iliac fascia, and along the medial border of the psoas, it leaves the pelvis by passing over the ascending ramus of the pubes. On reaching the thigh it joins the anterior division of the obturator, supplying the pectineus and the hip-joint.