Page:The New International Encyclopædia 1st ed. v. 14.djvu/424

NERVOUS SYSTEM AND BRAIN. 374 NERVOUS SYSTEM AND BRAIN. supiilies the obturator txternus, the adductor lonj;is gracilis, and adductor niagnus. Branches ol this nerve sometimes pass to the adductor brevis and pectineus. 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 obturator nerve to the pectineus and hip joint. It is usually connected with the obturator. The Uimlo-sacral cord is formed by the union of the fifth lumbar nerve with a branch from the fourtli lumbar. It gives rise to a communicating branch to the liftli lumbar ganglion of the sym- Ijathetic, and a muscular branch, the superior gluteal, to the gluteus medius and minimus and the tensor fasci;B femoris, after which the cord joins the sacral ple.us. The sacral plexus is formed by the union of the lumbo-sacral cord with the first, second, third, and ])art 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 communicating muscular and mi.xcd brandies. The communicating liranchcs ])ass to the upper sacral sympathetic ganglia. The muscular branches supply the quadratus femoris, the ob- turator internus, the geiiiclli, the pyriformis, and a part of the gluteus niaximus. The mixed nerves are: ( 1 ) The small sciatic, which supplies part of the gluteus maximus and the skin of the back part of the thigh, buttock, popliteal space, and ])art of the leg. By means of the long pudendal branch it also supplies the skin of the perineum. (2) The pudic supiilies the muscles and skin of the external generative organs. (3) The great sciatic. This nerve is the largest nerve of tho body. It passes down the back of the thigh, giv- ing off branches to the adductor magnus and the hamstring muscles. Above the poiiliteal space the nerve divides into the internal and the ex- ternal popliteal branches. The internal popliteal sends a branch to tho knee joint, supplies the popliteus muscle, and the muscles of the calf of the leg. It furnishes a branch called the com- municans tibialis, which joins the communicans pcronei to form the external saphenous nerve, which supplies the outer side of the foot. Be- low, the internal popliteal is continued into the posterior tibial, which supplies the long fle.xor of the toes, the tibialis posticus, and the skin of the heel. In the foot it diviilcs 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 brevis poUicis. the (lexor brevis digit(num, the abductor polliiis, and the two in- ner liiiiibrical muscles; the latter supplies the skin of the little an<l outer side of the fourth toes, and the muscles of the sole of the foot not supplied by the internal plantar. The external popliteal sends branches 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 longus and brevis, and divides into the external cutaneous and anterior tibial iiervcs. The former supplies the skin of the upper surface of the foot and of all the toes except the outer side of the little too and adjacent sides of the great and second toes. The anterior tibial sup- plies the extensors of the toes, the tibialis anti- cus, and peroneus tertius, and ends as a cutqfie- ous branch to the skin of the adjacent sides of the great and second toes. The sucro-coccygral plexus is a small plexus formed by the fourth and fifth sacral and the coccygeal nerves. It sends communicating branches to the sacral and coccygeal ganglia of the sympathetic, cutaneous branches to the skin of the arms and coccygeal region, muscular branches to the external anal sphincter, the leva- tor ani, and coccygeal muscles, and branches to the pelvic plexus of the sympathetic, whence they supply the rectum and bladder. Peripheral Teh.mix.tion-s of Spixal Nerves. The modes of terminations of these processes are extremely varied and complicated. The termina- tions are always 'free' in the sense that while possibly sometimes penetrating cells, they are never directly continuous with their protoplasm. The motor nerve fibres end in voluntary and in- voluntary muscle. On its way to a muscle a motor fibre, which, as noted above, is the a.xone of an anterior horn cell, may divide into several branches, a single cell thus innervating more than one muscle fibre. On reaching a voluntary muscle the bundle of nerve fibres breaks up to ■L.sac.c ■Sup-glut. Qu.femr THE SACRAL PLEXUS OP THE RIOHT SIDE FBOU BEHIND. L. sac. c, himbo-sacral ford : Sup. filtit.. fluperior glu- teal : /n/^ ^/Hf.. iufLTior gliitt'ai: I'er.. perforating: Sta. scia., small sciatic ; I'opl.. poplit»?al : P.vr., nerves to p.vrl- fonnla; Per. cat., perforating^ cutaneous: Obt. int., obtu- rator Internus; Qa. fern., quadrator femoris; Pu., pudic. form a ple.xus in the connective tissue which sur- rounds the bundles of muscle fibres. I'roin this plexus nerve fibres pass to the individual muscle fibres. Having arrived at the muscle fibre in which it terminates, the nerve fibre loses its medullary sheath, and its neurilemma fuses with the sarcolenima. The naked a.xone then (accord- ing to most observers) penetrates the sarcolenima and terminates in a more or less elaborate expan- sion known as a 'motor end plate' The sensory or afferent jiart of the spinal nerves is made by the peripheral processes of the spinal ganglion cells. In the skin, and in those mucous membranes which are covered with squamous epithelium, the nerve fibres lose their medullary sheaths in the subepithelial tissue, an<l, penetrating the epithelial layer, split up into niinuto fibrils which pass in between the cells and terminate there, often in little knoli- like swellings. In aildition to such comparatively sini|de nerve endings, there are also found in the skin and mucous membranes, especially where sensation is most acute, much more elaborate