The lumbar plexus lies at the back of the abdominal cavity behind the peritoneum. It is formed by the anterior divisions of the four up per lumbar nerves. C011111111nicating branches connect the plexus with the lumbar ganglia of the sympathetic. Some muscular branches pass directly from the plexus to the quadrates lumbo rum and psoas muscles. Cutaneous branches are the ( 1 ) ilio-hypogas,tric, which sends an iliac branch to the skin of the buttock and a hypoglos. sal branch to the skin of the lower abdomen; (21 the ilio-inguinal, to the skin of the groin; (3) the external cutaneous, to the skin of the outer side of the thigh. .Mixed branches of the plexus are (1) the genito-erural, to the skin of the groin and to the eremaster muscle; (2) the an terior crural, a large and important brand which supplies the sartorins. the pectineus, the psoas iliacus. and the extensors of the knee joint. be sides giving off three cutaneous branches, the internal cutaneous to the skin of the inner aspect of the thigh. a middle cutaneous to the skin of the front of the thigh, and the long saphe nous to the skin of the inner side of the knee. leg, and foot ; (3) the obturalor, which supplies the obturator externns, the adductor longus gracilis, and adductor =gnus. Branches of this nerve sometimes pass to the adductor brevis and pectinous. It forms the main nerve supply of the hip and knee joints and sometimes sends a cutaneous branch to the skin of the lower and inner part of the thigh. There is sometimes an accessory ohturato• nerve to the pectinous and hip joint. It is usually connected with the obturator.
The lumbo-sacral cord is formed by the union of the fifth lumbar nerve with a braneb from the fourth lumbar. It gives rise to a communicating branch to the fifth lumbar ganglion of the sym pathetic, and a muscular branch, the superior gluteal, to the gluteus merlins and mininms and the tensor fasciae femoris, after which the cord joins the sacral plexus.
The sucral plexus is formed by the union of the lumbo-sacral vord with the first, second, third. and part of the fourth sacral nerves. It is a large plexus situated in the cavity of the pelvis. Like the other plexuses. it gives rise to rommunicating muscular and mixed branches. The communicating branches pass to the upper sacral sympathetic ganglia. The muscular branches supply the quadlratus femoris, the ob turator interims, the genielli, the py'rifo•mis, and a part, of the gluteus maximus. 'flu mixed nerves are: ( 1 ) The small sciatie, which supplies part of the gluteus maxiinus and the skin of the hack part of the thigh, buttoek, popliteal space, and part of the leg. liy means of the long pudendal
branch it also supplies the skin of the perineum. (2) The pudic supplies the muscles and skin of the external generative organs. (3) The great sciatic. This nerve is the largest nerve of the body. It passes down the back of the thigh, giv ing off branches to the adductor magnus and the hamstring muscles. Above the popliteal space the nerve divides into the internal and the ex ternal popliteal branches. The internal poplitcal sends a branch to the knee joint, supplies the poplitens muscle. and the musehds of the calf of the leg. It furnishes a branch called the eom municans tibialis, which joins the communicans peronei to form the external saphenous nerve, which supplies the outer side of the foot. Be low, the internal polditeal is continued into the posterior tibial, which supplies the long flexor of the toes, the tibialis postieus, and the skin of the heel. In the foot it divides into the internal and the external plantar nerves. The former supplies the skin of the sole of the foot, and the great, second, third, and inner side of the fourth toes. the flexor hrevis pollieis. the flexor hrevis digitormn, the abductor pollieis, and the Iwo in ner himbrical muscles: the latter supplies the skin of the little and outer side of the fourth toes. and the mostly, of the sole of the foot not supplied by the internal plantar. The external popliteal sends brandies to the knee joint. gives off the communicans peronei to the skin of the outer side of the back of the leg, supplies the peronei and brevis, and divides into the external cutaneous and anterior tibial nerves. The former supplies the skin of the upper surface of the foot and of all the toes except the outer side of the little toe and ail sides of the great and second toes. The :unterio• tibial sup plies the extensors of the toes. the tibialis affli ct's, and peronens tertius, and ends as n cutadie ous branch to the skin of the adjacent sides of the great and second toes.
The sacro-cuerygral plexus is a small plexus formed by the fourth and fifth sacral and the coccygeal nerves. It sends communicating branches to the sacral and coecygeal ganglia of the sympathetic, cutaneous branches to the skin of the arms and eocesgeal region, muscular hranebes to the external anal sphincter, the leva tor ani, and eoceygeal muscles, and branches to the pelvic plexus of the sympathetic, whence they supply the rectum and bladder.