Lumbar Plexus

This plexus is formed by the union of the first three and part of the fourth lumbar nerves. It lies in the substance of the psoas muscle in front of the transverse processes of the lumbar vertebras. It has numerous branches, which supply the skin and muscles of the thigh.

The first lumbar trunk breaks up into four branches. The second, third, and fourth trunks each divide into an anterior and posterior division ; the three anterior unite to form the obturator nerve, which issues from the pelvis by the obturator or thyroid foramen. The three posterior unite to form the anterior crural nerve, the largest branch of the plexus, which passes out of the pelvis behind Poupart's ligament, between the psoas and iliacus muscles.

Fig. 28.—Lumbar and Sacral Plexuses.

1, Anterior crural nerve (from 2, 3 and 4, lumbar nerves) ; 2, obturator nerve (from 2, 3 and 4, lumbar nerves) ; 3, lumbar sacral cord (from 4 and 5, lumbar nerves) ; 4, great sciatic nerve (from lumbar sacral cord and 1, 2 and 3, sacral nerves) ; 5, lesser sciatic nerve (from 2 and 3, sacral nerves).

The lumbar plexus is connected by a small branch with the last dorsal nerve, and with the sacral plexus by the lumbosacral cord, which is a large nerve trunk formed by the union of the fourth and fifth lumbar nerves ; it passes over the sacrum and joins the sacral plexus.

Sacral Plexus

The first three sacral nerves, a branch of the fourth, and the lumbo-sacral cord form this plexus. The first sacral, with portions of the second and third nerves and the lumbo-sacral cord, form the sciatic band, which, proceeding to the great sciatic foramen, issues through it from the pelvis as the great sciatic nerve.

The various nerves of these plexuses will be gone into in later chapters.