Individual Variation.—None of significance in any of the three species.
Sciatic Nerve, [Figs. 6], [7], [8], [9]
T. pallidicinctus
Description.—The sciatic nerve passes through the anterior part of the ilio-ischiatic fenestra. Several branches diverge from the nerve immediately after it emerges from the fenestra. The main trunk of the nerve then extends distally through the thigh deep to M. extensor iliofibularis and superficial (lateral) to Mm. flexor ischiofemoralis, caudofemoralis, adductor superficialis, and femorocruralis. The main trunk subdivides into two large nerves—peroneal and tibial—that are adjacent and bound to each other throughout the thigh; the peroneal nerve lies anterior to the tibial. At the distal end of the thigh the main trunk splits grossly into two large branches that diverge and enter the shank. This division does not represent the separation between peroneal and tibial nerves, as is sometimes assumed; the anterior branch includes a part of the tibial nerve as well as the entire peroneal nerve.
A longitudinal groove is visible grossly on the lateral surface of the main trunk, except at the proximal end; distally a second groove is visible posterior to the first one ([Fig. 6]). The long anterior groove indicates the boundary between the peroneal and tibial nerves; this groove may disappear distally, although the posterior groove is always visible distally. The posterior groove, which is continuous with the division of the sciatic nerve into anterior and posterior branches, represents the boundary between two divisions of the tibial nerve. (This is discussed in detail below.) In the middle of the thigh the peroneal and tibial nerves are enclosed in separate connective tissue sheaths, although the two sheaths are fused together; the point of fusion is marked by the anterior groove. If the two sheaths are slit open, the two nerves can be removed and can be seen to be entirely separate. In the proximal part of the main trunk the peroneal and tibial components are enclosed in a single sheath and appear as an undivided trunk; but if the sheath is removed, the two components can be pulled apart rather easily, although there may be some intermingling of a few fibers. This separation can be extended to a point proximal to the origin of all the branches of the sciatic nerve; thus it can be determined which branches arise from the peroneal component and which from the tibial. (These branches arise from the sciatic nerve as, or immediately before, the nerve passes through the ilio-ischiatic fenestra; since this level of the intact nerve could not be adequately observed, it was necessary to cut the nerve inside the pelvis and pull the intrapelvic part of the nerve out through the ilio-ischiatic fenestra. In doing this, care had to be taken to avoid damaging the most proximal branches.)
Three main branches arise from the peroneal component (apart from the main trunk) and two from the tibial. Including the peroneal and tibial components of the main trunk, the sciatic nerve can be divided into seven major divisions—anterior peroneal, middle peroneal, dorsal peroneal, posterior or main peroneal (contributes to main trunk), anterior or main tibial (contributes to main trunk), middle tibial, and posterior tibial. Farther distally, the posterior peroneal division becomes the peroneal nerve and the anterior tibial division becomes the tibial nerve. For descriptive purposes, the term peroneal (or tibial) nerve will be applied only where the nerve is enclosed in its own sheath, but regardless of whether or not the sheath is fused with another; proximal to this, where the separation may not be precise, the terms peroneal (or tibial) division or component will be used.
The small anterior peroneal division arises from the anterior edge of the sciatic nerve. Immediately after emerging from the ilio-ischiatic fenestra, the division turns anteriorly and passes deep to M. piriformis, to which the division gives a twig (in some cases more than one twig), then continues forward to supply the posterior part of M. gluteus profundus.
The middle peroneal division branches into two parts. One part penetrates the deep surface of the anteroproximal part of M. extensor iliofibularis. The other part emerges between the proximal ends of Mm. extensor iliofibularis and vastus lateralis and penetrates the deep surface of M. extensor iliotibialis lateralis.
The dorsal peroneal division arises from the posterodorsal part of the peroneal component, then angles posteriorly, crossing the dorsal surface of the anterior tibial division and superficially appears to arise from the tibial component. The dorsal peroneal division usually subdivides into two unequal branches, both of which penetrate the deep surface of the proximal end of M. extensor iliofibularis.
The large middle tibial division soon subdivides into two branches that pass posterodistally lateral to M. flexor ischiofemoralis. One branch (usually the anterior one) passes lateral to M. caudofemoralis (both heads) and emerges between Mm. extensor iliofibularis and flexor cruris lateralis and enters the skin. The other branch passes deep to M. caudofemoralis pars iliofemoralis, and divides into several branches. Several tiny branches penetrate the deep surface of M. caudofemoralis pars iliofemoralis. Another branch also enters the substance of the latter and emerges from the ventral edge of it, giving a twig to pars caudifemoralis, then passes lateral to pars caudifemoralis and enters M. flexor cruris lateralis. Still another branch passes deep to both heads of M. caudofemoralis and enters the anterior part of M. flexor cruris medialis.