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Ilia-inguinal, which supplies the skin of the groin; this nerve is by some said to send a branch to the internal oblique muscle 3 0, External Cutaneous, which supplies the skin on the outer aspect of the thigh. The Mixed branches are as follows :—a, Genl'to-crural, which supplies the cre-'' master muscle, and a cutaneous branch to the skin of the groin. b, Anterior Crural, a large nerve which enters the thigh by passing behind Poupart’s ligament, and supplies the great extensor muscles of the knee-joint, and also the sartorius, the psoas-iliacus and the pectineus, which act as flcxors of the hip-joint ; it gives off the following cutaneous branches :—An internal cutaneous to the skin of the inner side, a middle cutaneous to the skin of the middle of the front of the thigh, and the long saphenous nerve, which supplies the skin of the inner side of the knee-joint, the inner side of the leg and the foot. 0, Obturator nerve, which leaves the pelvis through the obturator foramen, and supplies the obturator externus and adductor muscles of the thigh, and sends a branch to the pectineus ; it also supplies the hip and knee joints, and not unfrequently gives a branch to the skin of the lower part of the inner side of the thigh. d, An Accessory Obturator nerve is sometimes present, which goes to the pectineus, to the hip-joint, and also joins the obturator nerve.

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Flo. (id—Lumbar. sacral, and sacro-coccygcu. plexuses. DXII, the lowest thoracic nerve of the lmercnstul series: Li to iv, the nerves of the lumbar plexus; V, the fifth lumbar, with 8, the lumbo-sacrul cord; Si to IV, sacral nerves going to form the sacral plexus;~ V and Cl, the micro-coccygeal plexus; a, chain of ganglia of the sympathetic system, showing the communicating branches with the spinal nerves: 4:, the last oi these ganglia, called coccygeal ganglion, or ganglion impar; b, position of solar plexus; 1, liio-hypogustric nerve; 2, illoinguinnl; 3, external cutaneous; 4, gcnito~crurnl; l5, anterior crurnl; 6, obturstor; 7, superior glunnal.

The Lumbo-s/lcral Cord is formed of the fifth lumbar nerve and of a branch from the fourth lumbar. It joins the sacral plexus. Before the junction it gives origin to a communicating and a muscular branch. The Communicating joins the fifth lumbar ganglion of the sympathetic. The Muscular. branch, named the superior glutwal nerve, supplies the glutzeus medius and minimus and the tensor fasciae femoris muscle.

The Sacral plexus is situated in the cavity of the pelvis, and is the largest of all the plexuses. It is formed by the

junction of the lumbo-sacral cord, the first, second, third, and part of the fourth sacral nerves, and appears as aflattened mass in front of the sacrum. It gives origin to communicating, muscular, and mixed branches. The Communicating branches join the upper sacral ganglia of the sympathetic system. The Muscular branches supply the upper fibres of the glutzeus maximus, the pyriformis, gemelli, quadratus femoris, and obturator internus muscles. The lllixed nerves are as follows :——a, Pud-I'c, which supplies the muscles and skin of the external organs of generation. 6, Small Sciatic, which supplies not only the lower fibres of the glutaaus maximus muscle, but the skin of the buttock, the back of the thigh, of the popliteal space, and of the leg; it also gives a long pudendal branch to the skin of the perineum. 0, Great Sciatic; this is the largest nerve in the body. It leaves the pelvis through the great sciatic foramen, and passes down the back of the thigh, when it divides into external and internal popliteal branches. Before dividing it supplies the hamstring muscles, and gives a branch to the adductor magnus. The external popliteal branch gives offsets to the knee joint, passes down the outer side of the leg, supplies the peronei longus and brevis, gives off the communicana peronei branch to the skin of the outer side of the back of the leg, and ends as the external cutaneous nerve for the dorsum of the foot and the dorsal surfaces of all the toes, except the outer side of the little and the adjacent sides of the great and second toes. The internal popliteal branch gives offsets to the knee-joint, and supplies the communicans tibialis nerve, which joins the communicans peronei, and forms with it the external sapbenous nerve that passes to the outer side of the foot and little toe. The internal popliteal also supplies the muscles of the calf and the popliteus muscle, and is prolonged downwards as the posterior tibial nerve. The anterior tibial passes to thev front of the leg, supplies the tibialis anticus, peroneus tertius, and extensor muscles of the toes, and terminates as the cutaneous digital nerve for the adjacent sides of the great and second toes. The posterior tibial nerve passes down the back of the leg, supplies the tibialis posticus and long fiexors of the toes, gives off a cutaneous branch to the skin of the heel, and terminates by dividing into the internal and external plantar nerves. The internal plantar nerve supplies the skin of the sole and sends digital branches to the skin of the great, second, third, and tibial side of the fourth toes; it also supplies the abductor pollicis, fiexor brevis digitorum, fiexor brevis pollicis, and two inner lumbrical muscles. The external plantar nerve supplies digital branches to the skin of the little and fibular sides of the fourth toes, and branches to all the muscles of the sole of the foot which are not supplied by the internal plantar nerve.

The Sacro-Coccygcal is the smallest plexus belonging to the anterior divisions of the spinal nerves. It is formed by a part of the fourth sacral, the fifth sacral, and the coccygeal nerves. It lies in front of the last sacral and the first coccygeal vertebrae, and gives origin to communicating, visceral, muscular, and cutaneous branches. The Communicating branches join the lower sacral and the coccygeal ganglia of the sympathetic system; the Visceral pass to the pelvic plexus of the sympathetic, and through it to the bladder and rectum ; the Muscular to the levator ani, coccygeus, and sphincter ani externus muscles ; the Cutaneous to the skin about the anus and tip of the coccyx.

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Medulln oblongate.

continuous with the spinal cord through the foramen magnum. The cerebellum lies above, and immediately behind the medulla oblongata, with which it is directly continuous. The pons lies above and in front of the medulla, with which it is directly continuous. The cerebrum is the highest division, and lies above both pens and cerebellum, with both of which it is directly continuous. Several figures of the brain are given in Plate XVIII.

The MEDULLA OBLONGATA rests upon the basioccipital. It is somewhat pyramidal in form, about 1} inch long, and 1 inch broad in its widest part. It is a bilateral organ, and is divided into a right and a left half by shallow anterior and posterior median fissures, continuous with the corresponding fissures in the spinal cord; the posterior fissure ends above in the 4th ventricle. Each half is subdivided into elongated tracts of nervous matter. Next to, and parallel with the anterior fissure is the anterior pyramid (Pl. XVIII. figs. 1 and 2, P). This pyramid is continuous below with the cord, and the place of continuity is marked by the passage across the fissure of three or four bundles of nerve fibres, from each half of the cord to the opposite anterior pyramid; this crossing is called the (lecussation of the pyramids. To the side of the pyramid, and separated from it by a faint fissure, is the olivary fasciculue, which at its upper end is elevated into the projecting oval-shaped olivary body (Pl. XVIII. figs. 1 and 2,0). Behind the olive, and separated from it by a faint groove, is the strong tract named restiform body; as it ascends from the cord it diverges from its fellow in the opposite half of the medulla oblongata. By this divergence the central part of the medulla is opened up, and the lower half of the cavity of the 4th ventricle is formed. Internal to the restiform body is the posterior pyramid, which is continuous with the postero-median column, and bounds the postero-median fissure. Where the restiform bodies diverge from each other, there also the posterior pyramids diverge outwards from the sides of the postero-median fissure. At the upper part of the floor of the 4th ventricle a longitudinal tract of nerve fibres, the fasciculus teres, ascends on each side of its median furrow (Fig. 68, 7). Slender tracts of nerve fibres, the arczl/brm fibres, arch across the side of the medulla immediately below the olive ; and white slender tracts emerge from the median furrow of the 4th ventricle, pass outwards across its floor, and form the strice medullares or acousticw, the roots of origin of the auditory nervc(Fig. 68,

The medulla oblongata, like the spinal cord, with which it is continuous, consists both of grey and white matter. But the exterior of the medulla is not so exclusively formed of white matter as is the outer part of the cord, for the divergence from each other of the restiform bodies and posterior pyramids of opposite sides opens out the central part of the medulla, and allows the grey matter to become superficial on the floor of the 4th ventricle. The nerve fibres which enter into the formation of the pyramids and the other tracts just described, are partly continuous below with the columns of the spinal cord, and are prolonged upwards either to the pons and cerebrum, or to the cerebellum, or they partly take their rise in the medulla oblongata itself from the cells of its grey matter. As the medulla is a bilateral organ, its two halves are united together by commissural fibres, which cross obliquely its mcsial plane from one side to the other, and as they deemsate in that plane, they form a well-marked mesial band or rap/zé. Further, the medulla is a centre of origin for several pairs of the more posterior encephalic nerves, and for the vasomotor nerves. In the passage upwards through the medulla of the columns of the cord, 0. rearrangement of their fibres takes place ; just as in a great central railway station, the rails, which enter it in one direction, intersect and are rearranged before they emerge from it in the opposite

direction. The fibres of the posterior median column of the cord are prolonged upwards as the posterior pyramid. The fibres of the posterior column of the cord are for the most part prolonged upwards into the restiform body, though some fibres pass to the front of the medulla to participate in the decussation of the anterior pyramids. The lateral column of the cord divides into three parts: a, the greater number of its fibres pass inwards across the anterior median fissure, to assist in forming the anterior pyramid of the opposite side, so as to produce the decussation already referred to; b, others join the restiform body; c, others form the fasciculus teres situated on the floor of the 4th ventricle. The anterior column of the cord also divides into three parts: a, some fibres form the arciform fibres and join the restiform body ,- 6, others assist in the formation of the olivary fasciculus; 0, others are prolonged upwards in the anterior pyramid of the same side (Fig. 67).

The anterior pyramid consists partly of fibres of the anterior column of the cord of the same side, partly of decussating fibres of the anterior commissure, partly of decussating fibres from the posterior columns and posterior cornu of grey matter, but principally of the decussating fibres of thelateral column of the opposite side of the cord. The fibres of the anterior pyramid are prolonged through the pons to the cerebrum. Owing to the decussation of the lateral columns of the cord in the formation of the pyramids, the motor nerve fibres from one-half of the brain are transmitted to the opposite side of the cord, so that injuries afl'ecting one side of the brain occasion paralysis of the motor nerves arising from the opposite half of the cord. The olivary fasciculus is formed partly of fibres of the anterior column of the same side, and partly of fibres arising from the grey matter of the olive. It is continued upwards through the pons to the cerebrum. The restiform body is formed principally of fibres of the posterior column of the same side, but partly of fibres of the lateral column, and also of the arciform fibres from the anterior column, and from the grey matter of the superior and inferior olives. As the restiform body is continued upwards to the cerebellum, and forms its interior pedunclcs, the arcil'orm fibres have been called by Solly the superficial cerebellar fibres of the medulla. Through the restiform body the cerebellum is connected with the posterior, lateral, and anterior columns of the cord as well as with the olivary nuclei in the grey matter of the medulla oblongata. The posterior pyramid consists of the posterior median column of the cord, and is prolonged through the pons to the cerebrum. The fasciculus teres is formed of a small part of the lateral column of the cord, and is also prolonged through the pons to the cerebrum.

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Fro. 67.-Dlngrammstlc dissection of the mednlla oblongnra and pens to show the course of the fibres. a, superficial, a’, decptransvl-rse fibres of the pans; b, b, anterior pyramids ascending at b! through the pens; c, c, olirnry bodies; 1.’. olirsry fssclculus in the par-3; d. d, anterior columns of cord; e, inner part 0! the right column joining the antcnhr pyramid; /, the outer part going to the olivsry iasciculus; g, lateral column of cord ; II, the pm“! which dccussntcs at l‘, the deems \tlon of the pyramids; l, the part whlch joins the restiform body ; m. that which forms Illa fasclrulus trrcs; n, arciform fibres. 1 and 2. sensory snd motor roots of filth non-c; 3. slxih nerve; 4, portio durs; 5. pm-rln intermcdia; 6, portlo mollis of seventh nerve; 7I glean-pharyngeal; 8, plncummgnstrlc; 9, spinal accessory of eight nerve; 10, hypo— glosml nerve.

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ENCYCLOPEDIA BRITANNICA. NINTH EDITION.

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