Fig. 69. - Sacral Plexus.

A

1,1,1. Nerve to Quadratus Femoris

2, 2, 2. Nerve to Obturator Internus

3, 3, 3. Small Sciatic Nerve

B

4 S. Fourth Sacral, giving a Branch to Sacral Plexus

V. Visceral Branches

M. Muscular Branches T.C. Terminal Cutaneous Branches the long saphenous, internal and middle cutaneous, all branches of the anterior crural nerve, and sometimes a branch of the external cutaneous nerve.

The Sacral or Sciatic Plexus is formed by the anterior divisions of the fourth and fifth lumbar nerves and the first three sacral nerves. The plexus is formed on the anterior surface of pyriformis, all the nerves joining to form a large triangular trunk which passes through the sacro-sciatic foramen as the great sciatic nerve, which supplies the back of the thigh and the whole of the leg and foot (excepting that part of skin supplied by the saphenous nerve); small branches arise from the anterior and posterior surfaces of the plexus to supply the parts in the vicinity. The great sciatic nerve ends at the popliteal space by dividing into tibial and peroneal nerves (internal and external popliteal nerves). In reality this division can be traced all the way up to the plexus, so that the nerve is really constituted in two parts, but for purposes of description it is easier to call the upper part one trunk-namely, the great sciatic.

The plexus is formed by the lower part of the fourth lumbar nerve joining the fifth lumbar nerve, which, known as the lumbosacral cord, passes into the pelvis over the sacro-iliac articulation, and on pyriformis joins the first and second and upper part of third sacral nerves. The three sacral nerves also send branches to the pudendal plexus, so that no distinct line can be drawn between them.

Branches from the plexus (other than great sciatic);

Anterior

Muscular to quadratus femoris, obturator internus and gemelli.

Articular to hip-joint.

Posterior

Muscular to pyriformis and glutei (superior and inferior gluteal nerves).

Articular to knee-joint (via the nerve to biceps).

Cutaneous to skin on back of thigh (small sciatic).

Superior Gluteal nerve arises from the fourth and fifth lumbar and the first sacral nerves. It passes through the sacro-sciatic foramen, with the gluteal artery above pyriformis, and supplies gluteus medius and minimus and tensor fasciae femoris.

Inferior Gluteal nerve arises from the fifth lumbar and first and second sacral nerves. It passes through the sacro-sciatic foramen beneath pyriformis, and supplies gluteus maximus.

Small Sciatic nerve passes through the sacro-sciatic foramen below pyriformis, with the sciatic artery and inferior gluteal nerve. It enters the thigh at the lower border of gluteus maximus, and, piercing the deep fascia, it supplies the skin on the back of the thigh and over the calf of the leg. It is a purely cutaneous nerve.

Branches

Perineal arises at the lower border of gluteus maximus, and, passing inwards, supplies the skin over the perineum and external genital organs.

Gluteal arise beneath gluteus maximus, and, passing round its lower border, supply the skin over the lower half of the buttock.

Femoral in two sets-internal and external-supply the skin on the back of the thigh.