The anterior divisions of the remaining spinal nerves - viz., five lumbar, five sacral, and one coccygeal- form the lumbo-sacral plexus, which supplies the lower part of the trunk and the lower limb. For convenience of description the plexus is divided into three parts - lumbar, sacral or sciatic, and pudendal.

The lumbar plexus is formed by the first four lumbar nerves and a branch of the twelfth thoracic.

The sacral or sciatic plexus is formed by part of the fourth lumbar, the fifth lumbar, and the first three sacral nerves.

The pudendal plexus is formed by branches from the second and third sacral nerves, the fourth and fifth sacral nerves, and the coccygeal nerve. This plexus supplies mainly the perineum.

The Lumbar Plexus is formed by the anterior divisions of the first three lumbar nerves, a part of the fourth, and a small branch from the twelfth thoracic nerve. It is placed deeply in the substance of psoas just in front of the transverse processes of the lumbar vertebrae. On emerging from the intervertebral foramina, and after communicating with the sympathetic system, the nerves divide to form the plexus.

The first and second nerves divide into upper and lower parts. The upper part of the first nerve joins with the branch from the twelfth thoracic and forms the ilio-inguinal and ilio-hypogastrie nerves. The lower part of the first nerve and the upper part of the second join to form the genito-crural nerve. The lower part of the second nerve, the third nerve, and the upper part of the fourth nerve divide into anterior and posterior parts. The anterior parts join to form the obturator nerve, and the posterior parts form the anterior crural nerve; from the posterior parts of the second and third nerves, branches are given off, which join to form the external cutaneous nerve. Before these divisions take place branches are given off from the lumbar nerves to supply quadratus lumborum and psoas.

Ilio-Hypogastric Nerve, formed by the branch from the twelfth thoracic nerve and the upper part of the first lumbar nerve. It emerges from psoas, and passes between the transversalis and obliquus internus muscles above the crest of the ilium, and becomes cutaneous in the lower part of the anterior abdominal wall.

Fig. 68. - Lumbar Plexus.

Branches

Muscular to abdominal muscles.

Cutaneous

Iliac branch which corresponds to the lateral cutaneous branches of the thoracic nerves, and supplies the skin over the upper part of the buttock. Hypogastric branch, which supplies the skin over the pubis.

Ilio-Ingfuinal nerve in origin and course resembles the iliohypogastric nerve, but pierces the abdominal wall lower down, and becomes cutaneous by passing through the external abdominal ring and spermatic fascia.

Branches

Muscular to the abdominal wall.

Cutaneous to skin over the symphysis pubis, the upper and inner part of Scarpa's triangle, and the upper part of the external genital organs.

Genito-Crural nerve arises from the first and second lumbar nerves, which unite in the substance of psoas. The nerve passes down on the outer side of the external iliac vessels and becomes cutaneous just above Poupart's ligament, and supplies the skin over Scarpa's triangle, external to the ilio-inguinal nerve. A small branch passes upwards to the external genital organs.

External Cutaneous nerve arises from the second and third lumbar nerves. The nerve crosses the iliacus muscle to the anterior superior spine of the ilium. It then pierces the origin of sartorius, and becomes cutaneous a few inches below this point, where it divides into anterior and posterior terminal branches.

Branches

Anterior supplies the skin on the outer side of the front of the thigh almost to the knee.

Posterior supplies the skin on the outer side of the buttock below the great trochanter and the skin of the upper two-thirds of the outer side of the thigh.