The lower musculo-eutaneous (small muscu lo-cutaneous—small inguin o-cut aneous—sm all abdominal) is a thin delicate nerve, arising generally from the first lumbar, sometimes from the large musculo-cutaneous, is directed down wards and slightly outwards, along the back part of the psoas, a little in front of the inner border of the quadratus lumborum, crosses the iliacus internus about its upper fourth, and reaches the anterior third of the crest of the ileum. There it is lost by communicating with the large nmsculo-cutaneous, or, as is generally the case, passes after this communication as a very delicate nerve between the internal ob lique and transversalis, supplying the lower part of these rnuscles, but principally the latter, and parallel to Poupart's ligament, per forates the former muscle at the outer ring, and terminates in a manner similar to the pubic or scrotal branch of the upper mus culo-cutaneous, in the scrotum and pubic in tegument.
The genito-crural nerve (external sper matic — internal inguinal) derived front the second lumbar nerve, and sometimes from the communicating branch between the first and second, passes directly forwards to the anterior part of the psoas muscle, along which it de scends vertically to the femoral arch. It lies behind the spermatic vessels, and is crossed by the ureter. Having reached Poupart's ligament, it divides into two branches, an in ternal or genital, and an external or crural. The genital is directed across the external iliac artery (to which it supplies a few filaments) to the chord, lying below it as far as the in ternal ring. Prior to entering the inguinal canal the transversalis and internal oblique re ceive a few reflected branches from it. The nerve then accompanies the chord, crosses the epigastric vessels, supplies the cremaster muscle, runs immediately in front of Gimber nat's ligament, and terminates in the scrotal integument in the male, and labia pudendi in the female, supplying also the integument at the upper and inner part of the thigh, and communicating with the inferior pudendal nerve. The crural branch (femoral-cuta neous), having given off' several delicate fila ments to be distributed to the transversalis and internal oblique, crosses the circumflex ilii vessels, passes underneath Poupart's liga ment, a little to the outside of the femoral artery, pierces the fascia immediately below the ligament, and becomes cutaneous, sup plying the skin of the thigh at the middle part of its upper third. The division of the genito-crural into its terminal branches is subject to considerable variation, sometimes taking place either immediately after it has emerged from within the psoas, or within the psoas directly after its origin front the plexus.
The crural division is at times also extremely small, the external cutaneous then having a more extensive distribution than ordinary.
The external cutaneous (external inguinal) is a branch from the second or from the second and third lumbar, or is occasionally derived from the outer part of the crural nerve. It passes from beneath the outer border of the psoas below its middle, runs across the iliacus towards the space between the two spinous processes or the ilium, lying behind the transversalis fascia. It then passes beneath Poupart's ligament, and divides into an interior and posterior branch. The poste rior passes outwards and backwards over the Fascia, covering the tensor vaginm femoris, and supplies the integument at the upper, outer, and back part of the thigh. The ex tent of distribution of this branch is subject to variation, owing to the circumstance of a corresponding branch being occasionally sup plied either by the great musculo-cutaneous, or by the genito-crural, when the trunk of the external cutaneous itself comes from the anterior crural. In such instances this branch is small and insignificant, ir it exist at all. The anterior branch becoming cutaneous about the upper fifth of the thigh, soon divides into an external and internal, tlirected downwards, over the fascia covering the anterior and outer part of the rectus muscle. The external di vision terminates in the integument at the middle third of the outer part of the thigh ; the internal at the lower third of the thigh, above and to the outside of the patella.
The crural nerve (femoral) is by far the largest branch of the lumbar plexus, and is placed in the substance of the psoas muscle between the external cutaneous, and the ob turator, below the level of the foriner and above that of the latter, from which it diverges at an acute angle. It is formed by the.union of the second mith the outer branch of the third lumbar nerve, by part of the fourth, and generally by their communicating branch. It is destined to supply the integuments of the front of the thigh, and all the muscles at its anterior and outer portion.
Having emerged from the psoas muscle it is directed forwards and outwards between that muscle and the iliacus to Poupart's liga ment, under which it passes, and entering the thigh becomes flattened and expanded, and divides into a series of divergent terminal branches, the trunk occasionally bifurcating before so doing.