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 poplit eal 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 (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 MUSCULAR SYSTEM), 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 popltteal nerve, after supplying the ham strings, 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.
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. 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, etc.), while from the posterior secondary divisions come the superior gluteal (L. 5, S. I) and the inferior gluteal (L. 5, S. I, 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) second ary divisions. The branches of this plexus are the small sciatic, pudic, visceral, perforating cutaneous, muscular and sacro-coccyg eal nerves. The small sciatic (S.I, 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 NERVOUS SYS TEM) ; 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 exter nal sphincter, levator ani and coccygeus.
The sacro-coccygeal nerve (S.4, 5; Coe. I) runs down on each side of the coccyx to supply the adjacent skin, and represents the ventro-lateral nerve of the tail of lower mammals.