The anterior rami of S1, S2, and S3 emerge from the anterior (pelvic) foramina of the sacrum and proceed laterally on the anterior surface of the piriformis muscle. The lumbosacral trunk joins the sacral roots and fuses with S1. The sacral nerve plexus lies in the posterior wall of the pelvic cavity (see Fig. 8-8).
All the roots, including L4 and L5 (contained in the lumbosacral trunk), split into anterior and posterior divisions. However, the separation of these divisions can be observed only through careful dissection of cadavers. The anterior and posterior divisions converge laterally to form the sciatic nerve. This large nerve trunk leaves the pelvis through the greater sciatic foramen. The sciatic nerve is actually composed of two nerves, the common fibular nerve and the tibial nerve, which usually separate from each other just above the knee (Table 8-4). Sometimes, however, these two nerves may exit independently from the plexus and leave the pelvis as separate nerves. The posterior divisions of L4, L5, S1, and S2 form the common fibular nerve, and the corresponding anterior divisions, plus the anterior division of S3, form the tibial nerve. The posterior division of S3 is represented in minor, cutaneous branches of the plexus.
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