Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 2

The slender dorsal division pa.s.ses ventral to M. iliotrochantericus medius and dorsal to the inguinal ligament and penetrates the ventral surface of M. iliacus.

The small, short posterodorsal division penetrates the ventral surface of M. iliotrochantericus medius.

INDIVIDUAL VARIATION.--In two legs the anterior division gives a twig or two twigs to M. extensor iliotibialis lateralis. The dorsal division may fuse proximally with either the anterior or middle division, thus appearing to be a branch of one of these divisions. In one leg (fig.

5A), there are two separate branches (both fused with the middle division) to M. iliacus. On both sides of one specimen (fig. 5A), the anteriormost branch of the middle division, which supplies M. extensor iliotibialis lateralis, gives off a twig that anastomoses with the branch of the anterior division that supplies M. extensor iliotibialis anticus. On both sides of another specimen, the anterodorsal division pa.s.ses lateral to the anterior end of M. iliotrochantericus medius instead of through the femoral notch, which is lacking.

_T. cupido_

INDIVIDUAL VARIATION.--In three legs, the anterior division gives twigs into M. extensor iliotibialis lateralis. The dorsal division is fused proximally with the middle division in one instance. In three cases, a twig from the middle division anastomoses with the branch of the anterior division supplying M. extensor iliotibialis anticus. In the example shown in fig. 5B, a twig comes off the cutaneous branch of the anterior division, perforates the ventral part of M. iliacus, and rejoins the cutaneous branch. In both legs of one specimen, the cutaneous branch of the anterior division perforates the anterior edge of M. extensor iliotibialis lateralis instead of pa.s.sing between the latter and M. extensor iliotibialis anticus. The posteriormost branch of the middle division, which terminates in M. femoritibialis internus, perforates the medial part of M. vastus medialis in one leg. In another leg, one of the branches to the fused Mm. vastus lateralis and vastus medialis sends a twig into M. extensor iliotibialis lateralis.

_P. p. jamesi_

INDIVIDUAL VARIATION.--In three legs, the anterior branch of the anterior division is cutaneous and the posterior branch supplies M.

extensor iliotibialis anticus. The dorsal division may fuse proximally with either the anterior or middle division. In one leg (fig. 4B), there are two branches to M. iliacus, one a.s.sociated with the anterior division and one with the middle division.

=_Obturator Nerve_=

_T. pallidicinctus_

DESCRIPTION.--The long slender obturator nerve pa.s.ses along the oblique iliac crest and divides into several branches immediately before reaching the obturator foramen. One or two branches, which do not pa.s.s through the foramen, penetrate the superficial surface of M. obturator pars postica. Several small branches (variable in number and arrangement) pa.s.s through the obturator foramen and supply pars ventralis, pars dorsalis, and pars antica of M. obturator. When pars ventralis and pars dorsalis are fused, one branch perforates the proximal end of this ma.s.s and reaches pars antica. One large branch pa.s.ses through the obturator foramen dorsal to the tendon of M.

obturator pars postica, then turns ventrally, pa.s.sing lateral to the latter; the branch pa.s.ses between Mm. adductor superficialis and adductor profundus and gives twigs to each of these two muscles.

INDIVIDUAL VARIATION.--None of significance in any of the three species.

=_Sciatic Nerve_=, Figs. 6, 7, 8, 9

_T. pallidicinctus_

DESCRIPTION.--The sciatic nerve pa.s.ses 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 a.s.sumed; 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 pa.s.ses 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 pa.s.ses 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 pa.s.s posterodistally lateral to M. flexor ischiofemoralis. One branch (usually the anterior one) pa.s.ses lateral to M. caudofemoralis (both heads) and emerges between Mm. extensor iliofibularis and flexor cruris lateralis and enters the skin. The other branch pa.s.ses 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 pa.s.ses lateral to pars caudifemoralis and enters M.

flexor cruris lateralis. Still another branch pa.s.ses deep to both heads of M. caudofemoralis and enters the anterior part of M. flexor cruris medialis.

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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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, pa.s.ses 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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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 pa.s.ses 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_

INDIVIDUAL VARIATION.--In both legs of one specimen, the branch to M.

flexor cruris medialis arises from the posterior (rather than the middle) tibial division; in three legs, this branch arises as an independent division of the tibial nerve (fig. 6). (Only in one leg does this branch arise as in _T. pallidicinctus_.) The branch to M. flexor cruris medialis perforates the lateral part of M. flexor ischiofemoralis in one instance. In all legs except one (nerve possibly destroyed), a second twig to M. flexor ischiofemoralis arises from the branch to M.

flexor cruris medialis (fig. 6). In one leg (fig. 9), an additional branch, arising as an independent division of the sciatic nerve, enters M. extensor iliofibularis distal to the point of entrance of the dorsal peroneal division; this extra branch arises posterior (adjacent) to the dorsal peroneal division, but it could not be determined with certainty whether it arises from the peroneal or tibial component. A minute twig from the branch to M. flexor cruris medialis pa.s.ses posteriorly and joins the pudendal plexus in one leg (fig. 6); in another, a similar twig is present but could not be traced to its termination. In nearly all the legs, minute twigs come off the peroneal nerve near the middle of the thigh and enter M. extensor iliofibularis (fig. 6). In both legs of one specimen, the paraperoneal branch enters the peroneal sheath (although separable from the peroneal nerve). The distal branch to M.

femorocruralis gives off a long twig to M. gastrocnemius pars media in one instance (fig. 6).

=_Peroneal Nerve_=, Fig. 10

_T. pallidicinctus_

DESCRIPTION.--The branch that is given off in the thigh has been discussed above. The peroneal nerve pa.s.ses, with the paraperoneal branch of the tibial nerve, through the guide loop for M. extensor iliofibularis. The peroneal nerve diverges from the paraperoneal branch and pa.s.ses along the anterior (proximal) edge of the tendon of M.

extensor iliofibularis medial to the common tendon of the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II and lateral to the common tendon of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III.

The peroneal nerve soon gives off a spray of branches that supplies the following: femoral head of M. tibialis anticus, tibial head of M.

tibialis anticus (branch pa.s.ses deep to femoral head), M. extensor digitorum longus (branch pa.s.ses deep to tibial head of M. tibialis anticus), and M. peroneus longus. A part of the nerve may or may not pa.s.s through a notch in the proximal end of the lateral head of M.

flexor digitorum longus. The nerve then extends distally along the anterolateral edge of the latter muscle and subdivides into two long branches. Gadow (1891) termed these branches the superficial peroneal and the deep peroneal; his terminology will be used here.

The superficial peroneal branch, after giving off, near its proximal end, one or two twigs into M. peroneus brevis, pa.s.ses lateral to the retinaculum for the tendon of M. tibialis anticus, then across the intratarsal joint lateral to the latter, then lateral to the insertion of M. tibialis anticus, where the branch subdivides. One of the two resulting branches gives one or two twigs into M. extensor brevis digiti IV, then terminates nonmuscularly in the digits. The other branch pa.s.ses between the main and accessory insertions of M. tibialis anticus and joins the branch of the deep peroneal which supplies M. abductor digiti II. (See next paragraph.)

The deep peroneal branch pa.s.ses through the retinaculum for the tendon of M. tibialis anticus, lying lateral, then deep, then medial to the latter; it crosses the intratarsal joint medial to the latter.

Immediately above the insertion of M. tibialis anticus, the deep peroneal branch divides, one branch pa.s.sing on each side of the main insertion. The branch pa.s.sing lateral to the main insertion pa.s.ses between the latter and the accessory insertion (medial to the latter) and is joined by a branch of the superficial peroneal nerve. This fused branch extends distally between Mm. extensor hallucis longus and extensor brevis digiti IV and medial to M. extensor brevis digiti III, giving twigs into the latter and into M. abductor digiti II before terminating nonmuscularly in the digits. The branch of the deep peroneal nerve that pa.s.ses medial to the main insertion of M. tibialis anticus gives one or two twigs into the proximal head of M. extensor hallucis longus, then terminates nonmuscularly in the digits.

INDIVIDUAL VARIATION.--In four legs, the branch of the superficial peroneal nerve that usually joins the lateral branch of the deep peroneal nerve is lacking (fig. 10B). In these legs it can be seen that Mm. extensor brevis digiti III and abductor digiti II are supplied by the deep peroneal nerve.

_T. cupido_

INDIVIDUAL VARIATION.--In two legs, the same branch that gives twigs into the proximal head of M. extensor hallucis longus also sends a twig into the distal head of this muscle (fig. 10C).