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 muscle, and another (posterior) to the skin of the thigh just below the groin.

The external cutaneous nerve (L.2, 3) supplies the skin of the outer side of the thigh, while the anterior crural (L.2, 3, 4) innervates the muscles on the front of the thigh, the skin on the front and inner side of the thigh, through its middle and internal cutaneous branches, and the skin of the inner side of the leg and foot through the internal saphenous branch. At first sight it is difficult to understand how the anterior crural nerve, which supplies the skin of the front of the thigh, is a posterior secondary division of the lumbar plexus, but the explanation is that the front of the human thigh was originally the dorsal surface of the limb bud, and the distribution of the nerve is quite easily understood if the position of the hind limb of a lizard or crocodile is glanced at. The fourth lumbar nerve is sometimes called the nervus furcalis, because, dividing, it partly goes to the lumbar, and partly to the sacral plexus (fig. 8), though, when the plexus is prefixed, the third lumbar may be the nervus furcalis, or, when it is postfixed, the fifth lumbar. Under ordinary conditions the descending branch of the fourth lumbar nerve joins the fifth, and together they make the lumbo-sacral cord, which, with the first three sacral nerves, forms the sacral plexus. This plexus, like the others, contains anterior and posterior secondary divisions of its spinal nerves, and it resembles the brachial plexus in that the lowest nerve to enter it contributes no dorsal secondary division.

All the constituent nerves of the plexus run into one huge nerve, the great sciatic, which runs down the back of the thigh and, before reaching the knee, divides into external and internal popliteal nerves. These two nerves are sometimes separate from their first formation in the plexus, and may always be separated easily by the handle of a scalpel, since they are only bound together by loose connective tissue to form the great sciatic nerve. When they are separated in this way it is seen that the external popliteal is made up entirely of posterior (dorsal) secondary divisions (see fig. 9), and is derived from the fourth and fifth lumbar and first and second sacral nerves, while the internal popliteal is formed by the anterior (ventral) secondary divisions of the fourth and fifth lumbar and first, second and third sacral nerves. The external popliteal nerve supplies the short head of the biceps femoris (see ), and, just below the knee, divides into anterior tibial and musculo-cutaneous branches, which both supply the dorsal surface of the leg and foot. The anterior tibial nerve is chiefly muscular, innervating the muscles in front of the tibia and fibula as well as the extensor brevis digitorum pedis on the dorsum of the foot, though it gives one small cutaneous branch to the cleft between the first and second toes. The musculo-cutaneous nerve supplies the peroneus longus and brevis muscles, and the rest of the skin of the dorsum, of the foot, and lower part of the leg, while the skin of the upper part of the dorsum of the leg, below the knee, is supplied by the external popliteal before its division. The internal popliteal nerve, after supplying the hamstrings, is continued into the calf of the leg as the posterior tibial and innervates all the muscles on this, the ventral, surface. Behind the inner ankle it divides into the external and internal plantar nerves, from which the muscles and skin of the sole are supplied. A little above the knee each popliteal nerve gives off a contribution to help form the external or short saphenous nerve. That from the internal popliteal is called the communicans tibialis, while that from the external popliteal is the communicans fibularis. These join about the middle of the back of the calf, and the, now formed, short saphenous nerve runs down behind the outer ankle to supply the outer side of the foot. Sometimes it encroaches on the dorsum of the foot, replacing part of the musculo-cutaneous, though, when this is the case, its dorsal contribution from the external popliteal (communicans fibularis) is always larger than usual. To return to the sacral plexus: branches are given off from the anterior secondary divisions to the short external rotator muscles of the hip (pyriformis, quadratus femoris, &c.), while from the posterior secondary divisions come the superior gluteal (L. I.S. 4, 5) and the inferior gluteal (L.5, S. 1, 2) to the muscles of the buttocks.

In modern descriptions the lower branches of the lumbo-sacral plexus are grouped into a pudendal plexus, and the plan, though open to criticism on morphological grounds, has such descriptive advantages that it is followed here. Contributions from the first, second, third and fourth sacral, and the coccygeal nerve, form it, and these contributions are almost all anterior (ventral) secondary divisions. The branches of this plexus are the small sciatic, pudic, visceral, perforating cutaneous, muscular and sacro-coccygeal nerves. The small sciatic (S.1, 2, 3) is partly dorsal and partly ventral in its origin and distribution; it supplies the skin of the perineum, buttock and the back of the thigh. The pudic nerve (S.2, 3, 4) helps to supply the skin and muscles of the perineum and genital organs. The visceral branches form the pelvic stream of white rami communicantes (see ); they run from the second and third or third and fourth sacral nerves to the pelvic plexuses of the sympathetic system. The perforating cutaneous nerve (S.2, 3) pierces the great sacro-sciatic ligament and supplies the skin over the lower internal part of the buttock. The muscular branches (S.3, 4) supply the external sphincter, levator ani and coccygeus.

The sacro-coccygeal nerve (S.4, 5, Coc.1) runs down on each side of the coccyx to supply the adjacent skin, and represents the ventrolateral nerve of the tail of lower mammals.

NERVI, a coast town of Liguria, Italy, in the province of Genoa from which it is 7 m. S.E. by rail (also electric tramway), 82 ft. above sea-level. Pop. (1901) 3480 (town); 6317 (commune). It is much frequented as a winter resort. It is surrounded with groves of olives, oranges and lemons, and its villas have beautiful gardens. It is moister and less dusty than the western Riviera, and is especially in favour with those who suffer from lung complaints. At Quarto, 2 m. N.W., 1000 Garibaldians (1 mille) embarked for Marsala in 1860.

NERVOUS SYSTEM. The nervous system forms an extremely complicated set of links between different parts of the body, and is divided into (A) the central nervous system, composed of (1) the brain, and (2) spinal cord; (B) the peripheral nervous system, consisting of (1) the cranial nerves, (2) the spinal nerves, (3) the various sense organs, such as the eye, ear, olfactory organ, taste organ and tactile organs, and (4) the motor end plates; (C) the sympathetic system. The anatomy and physiology of many of these parts are treated in separate articles (see, , , , , , , , , ). The object here is to deal with anatomical points which are