7. N. obturatorius ([Fig. 162], g).—The obturator nerve arises from the connecting band between the sixth and seventh nerves. It lies at first laterad, then dorsad, of the common iliac vein, and passes almost directly caudad to the cranial border of M. obturator internus (15). Here it turns sharply laterad about the pubis, and passes through the obturator foramen. It gives twigs to the obturator externus muscle, and divides into several branches, which innervate the adductor femoris, adductor longus, pectineus, and gracilis.

[Fig. 162].—The Lumbar and Sacral Nerves, as seen in a View of the Dorsal Wall of the Abdominal Cavity, after Removal of the Viscera.

On the right side the iliopsoas, psoas minor, and quadratus lumborum have been removed. The symphysis pubis has been split and the two parts divaricated, to show the pelvic cavity; on the right side parts of the innominate bones and the levator ani and coccygeus muscles have been removed. II-VII, second to seventh lumbar nerves; 1-3, first to third sacral nerves. a, second lumbar nerve; b, lateral and medial branches of third lumbar nerve; c, N. genitofemoralis (c′, its lateral branch; c″, its medial branch); d, N. cutaneus femoralis lateralis; e, muscular branch of N. femoralis; f, N. femoralis; g, N. obturatorius; h, N. ischiadicus or great sciatic nerve; i, N. gluteus superior; j, N. gluteus inferior; k, N. cutaneus femoris posterior; l, N. pudendus; m, N. hemorrhoidalis inferior; n, part of the sympathetic (incompletely shown, merely to indicate connections with the spinal nerves); 4, M. transversus abdominis; 5, 6, cut edges of M. obliquus internus and externus; 7, dorsal border of the diaphragm; 8, M. iliopsoas (mostly removed on the right side); 9, M. psoas minor; 10, M. pyriformis; 11, M. levator ani; 11′, M. iliocaudalis; 12, M. flexor caudæ longus; 13, M. flexor caudæ brevis; 14, edge of pubic symphysis; 15, cut end of M. obturator internus; 16, cut ramus of ischium; 17, cut ramus of pubis.

D. Sacral Nerves and Sacral Plexus

([Fig. 162]).—A large band passes caudad from the sixth and seventh lumbar nerves to connect with the sacral nerves; this constitutes the so-called lumbosacral cord. The sacral nerves are three in number. Their dorsal rami pass dorsad out of the two posterior sacral foramina and the foramen caudad of the sacrum, to the muscles and skin dorsad of the sacrum. The ventral rami pass from the two anterior sacral foramina and from the foramen between the last sacral and first caudal vertebræ. The first is the largest; it joins the lumbosacral cord to form the great sciatic nerve (N. ischiadicus) (h) and the superior (i) and inferior (j) gluteal, and sends a branch caudad to connect with the second sacral. The second and third sacral nerves are small; they unite with the connecting branch from the first to form a network from which arises the pudendus (l), cutaneus femoris posterior (k), hemorrhoidalis inferior (m), and a small branch to the great sciatic (h).

The nerves arising from the sacral plexus are the following:

1. N. gluteus superior ([Fig. 162], i; [Fig. 163], j).—The superior gluteal nerve arises from the lumbosacral cord and the first sacral nerve; it passes dorsad in the notch between sacrum and ilium, caudolaterad of the first anterior sacral foramen, curves over the dorsal border of the ilium between the gluteus medius and pyriformis muscles ([Fig. 163], 7), passes between the gemellus superior (6) and gluteus minimus (5) and ventrad of the latter, and reaches the medial surface of the tensor fasciæ latæ, which it innervates. In its course it sends branches to the gluteus medius, gluteus minimus (5), and gemellus superior.

2. N. gluteus inferior ([Fig. 162], j; [Fig. 163], i).—A small nerve from the lumbosacral cord and the first sacral nerve. It passes dorsocaudad, lying on the dorsal surface of the great sciatic nerve ([Fig. 162], h). After leaving the pelvis by the great sciatic notch it divides beneath the pyriformis ([Fig. 163], 7) into two branches; one goes to M. caudofemoralis (4), the other to M. gluteus maximus (3).

3. N. ischiadicus ([Fig. 162], h; [Fig. 163], a).—The great sciatic nerve, the largest nerve in the body, arises from the lumbosacral cord (formed chiefly by the sixth and seventh lumbar nerves), the first sacral nerve and a small branch from the second sacral. It passes caudodorsad and leaves the pelvis by passing across the great sciatic notch, between M. pyriformis and M. gemellus superior. It passes across the tendon of M. obturator internus ([Fig. 163], 9) near the insertion, and here sends caudad a large muscular branch (b) beneath the biceps. This branch innervates the biceps, semitendinosus (17) and semimembranosus (16). Other twigs in this region pass to the quadratus femoris (10) and tenuissimus. The great sciatic nerve (a) now passes across the quadratus femoris (10), adductor femoris (15), and semimembranosus (16), lying beneath the biceps femoris. It thus enters the popliteal space and approaches the popliteal artery and vein. Here it gives off one or two small muscular branches to the distal part of the biceps, and sends a slender branch, N. suralis (c), along the medial surface of the biceps to the lateral surface of the lateral head of M. gastrocnemius (20). The sural nerve (c) becomes cutaneous at the distal (ventral) border of the biceps, and divides three or four centimeters proximad of the ankle into two branches. One passes over the tendon of Achilles (27) to the proximal end of the calcaneus and ramifies in this region. The other passes onto the lateral surface of the foot, and supplies the integument in this region over the tarsus and part of the metatarsus.