The =great sciatic nerve= (N. ischiadicus) (Figs. 451, 455, 526), the
largest in the body, is derived chiefly from the last lumbar and the
sacral roots of the lumbo-sacral plexus, but may receive a fasciculus
from the third sacral nerve also. It emerges through the great
sacro-sciatic foramen as a broad flat band—blended at first with the
posterior gluteal nerve—which passes downward and backward on the lower
part of the sacro-sciatic ligament and on the origin of the deep gluteus
muscle. It turns downward in the hollow between the trochanter major and
the tuber ischii over the gemellus, the tendon of the obturator
internus, and the quadratus femoris. In its descent in the thigh it lies
between the biceps femoris externally and the adductor, semimembranosus,
and semitendinosus internally, and it is continued between the two heads
of the gastrocnemius as the tibial nerve. Its chief branches are as
follows:
1. In the pelvic part of its course the sciatic nerve supplies small
branches to the obturator internus, gemellus, and quadratus femoris; the
branch to the obturator internus reaches the muscle by passing through
the anterior end of the lesser sacro-sciatic foramen.
2. As the sciatic nerve turns down behind the hip joint it gives off a
large branch (Ramus muscularis proximalis) which divides to supply the
semimembranosus and the short heads of the biceps femoris and
semitendinosus.
3. The =posterior cutaneous nerve= of the leg or =external saphenous
nerve= (N. cutaneus suræ posterior) is detached from the sciatic about
the middle of the thigh. It receives a fasciculus from the peroneal
nerve and descends with the recurrent tarsal vein on the outer face of
the gastrocnemius to the distal third of the leg. Here it perforates the
deep fascia and ramifies under the skin on the outer surface of the
tarsus and metatarsus (Fig. 525).
4. The =peroneal= or =external popliteal nerve= (N. peronæus) (Figs.
455, 460, 527) is a large trunk which arises from the great sciatic
nerve very shortly after the latter emerges from the pelvic cavity. It
descends with the parent trunk to the origin of the gastrocnemius; here
the peroneal nerve deviates outward and forward across the external face
of the gastrocnemius under cover of the biceps femoris, and divides at
the origin of the lateral extensor muscle into superficial and deep
branches. The collateral branches include a twig to the biceps femoris,
the reinforcing fasciculus detached about the middle of the thigh to the
posterior cutaneous nerve, and, lower down, the =external cutaneous
nerve= of the leg (Ramus cutaneus suræ lateralis). The latter, also
known as the peroneal cutaneous nerve, emerges between the middle and
posterior divisions of the biceps femoris at the level of the stifle
joint and ramifies under the skin. The =superficial peroneal= or
=musculo-cutaneous nerve= (N. peronæus superficialis) furnishes branches
to the lateral extensor, and descends in the furrow between that muscle
and the anterior extensor, perforates the deep fascia of the leg, and
ramifies under the skin on the front and the outer face of the tarsus
and metatarsus. The =deep peroneal= or =anterior tibial nerve= (N.
peronæus profundus) is the direct continuation in point of size of the
peroneal trunk. It dips in between the lateral and anterior extensors of
the digit, gives branches to these muscles and the tibialis anterior,
and descends in front of the intermuscular septum which separates the
former. It continues downward behind the tendon of the anterior extensor
and divides on the front of the hock into internal and external
branches. The =internal branch= (Ramus medialis) passes down under the
skin on the anterior face of the metatarsus and supplies the cutaneous
fibers of this region. The =external branch= (Ramus lateralis) furnishes
a twig to the extensor brevis muscle and descends with the great
metatarsal artery. It supplies the skin on the outer face of the
metatarsus and the fetlock.
In the greater part of its course in the leg the deep peroneal nerve
is separated from the anterior tibial vessels by the tibialis anterior
(muscular part of the flexor metatarsi). It should be noted that the
outer of the two veins which almost always accompany the artery here
is frequently very large, and the layer of muscle which intervenes
between it and the nerve is often exceedingly thin. In the distal part
of the leg the nerve is in direct contact with the vein, and on the
front of the hock it lies behind the vessels.