The small posterior tibial division arises from the posterior edge of the sciatic nerve. The division diverges from the remainder of the nerve, as the latter passes through the ilio-ischiatic fenestra, and penetrates the dorsal surface of M. flexor ischiofemoralis.
Below the middle of the main trunk a bundle of fibers of moderate size separates from the anterior edge of the tibial nerve, leaves the tibial sheath, and enters its own sheath, lying superficially between the tibial and peroneal sheaths ([Fig. 6]). At the distal end of the thigh the sheath enclosing this bundle of fibers remains fused with the posterior edge of the peroneal nerve and passes with the latter (diverging from the remainder of the tibial nerve) through the tendinous guide loop for M. extensor iliofibularis, and then diverges from the peroneal nerve. Since this bundle of fibers is distributed with the peroneal nerve, and since the origin of the bundle may be easily overlooked, it has sometimes been misinterpreted as a branch of the peroneal nerve, whereas it almost certainly is a branch of the tibial nerve; this bundle will here be termed the paraperoneal branch of the tibial nerve.
A small but long branch separates from the posterior edge of the proximal end of the tibial nerve or from the tibial component proximal to this and extends distally for some distance adjacent to the tibial nerve, then passes posterodistally between Mm. extensor iliofibularis and flexor cruris lateralis and supplies the skin.
A small branch separates from the anterior edge of the peroneal nerve a short distance above the distal end of the main trunk and passes distolaterally between Mm. extensor iliotibialis lateralis and extensor iliofibularis and supplies the skin.
A twig comes off the medial surface of the tibial nerve near the distal end of the main trunk, passes anteriorly deep to the peroneal nerve, and penetrates the lateral surface of M. femorocruralis; in some cases two twigs enter this muscle.
Individual Variation.—In one leg ([Fig. 7]), the twig to M. caudofemoralis pars caudifemoralis arises more proximally than usual and perforates pars iliofemoralis independently of the branch to M. flexor cruris lateralis. The nerve supplying M. flexor cruris lateralis does not perforate M. caudofemoralis pars iliofemoralis, but passes deep to it in three legs. In half the legs, the paraperoneal branch of the tibial nerve, after extending a short distance in its own sheath, enters the sheath of the peroneal nerve and appears grossly to unite with it; if, however, the sheath is slit open, the paraperoneal branch can be easily pulled apart from the posterior edge of the peroneal nerve; the paraperoneal branch is again enclosed in its own sheath at the distal end of the thigh. In one leg, the cutaneous branch of the peroneal nerve perforates the posteroproximal part of M. gastrocnemius pars externa; in three others, this branch is absent. In one of these last three legs ([Fig. 7]), the distal cutaneous branch of the tibial nerve is also absent. In three legs (of different specimens), a minute twig from the middle tibial division passes posteriorly deep to M. caudofemoralis pars caudifemoralis toward the tail ([Fig. 7]); this twig joins the pudendal plexus in one leg; in the other two the twig could not be traced to its termination. Minute twigs come off the peroneal nerve near the middle of the thigh and enter M. extensor iliofibularis in some legs. In a few cases, a minute nonmuscular twig arises from the peroneal nerve near the distal end of the main trunk and passes anteriorly deep to M. vastus lateralis pars postica ([Fig. 7]).
T. cupido
Individual Variation.—In several legs, the nerve supplying M. flexor cruris lateralis does not perforate M. caudofemoralis pars iliofemoralis, but passes deep to it. The branch to M. flexor cruris medialis arises from the posterior (rather than the middle) tibial division in one instance ([Fig. 8]). In one leg, a minute twig from the middle tibial division passes posteriorly and joins the pudendal plexus; in another, a similar twig is present but could not be traced to its termination. In some specimens, minute twigs come off the peroneal nerve near the middle of the thigh and enter M. extensor iliofibularis. In one leg, a nonmuscular twig arises from the base of the cutaneous branch of the peroneal nerve and passes anteriorly deep to M. vastus lateralis pars postica. In another leg ([Fig. 8]), a tiny additional twig arises from the posterior edge of the tibial nerve and subdivides, one branch joining the cutaneous branch of the middle tibial division and the other joining the distal cutaneous branch of the tibial nerve.
P. p. jamesi