into six major divisions--anterior, middle, posterior, anterodorsal,
dorsal, and posterodorsal. The anterodorsal and posterodorsal divisions
are short, failing to extend so far laterally as the inguinal ligament;
the posterodorsal division is also small and is usually covered by other
divisions and is not visible when viewed from the ventral side.
The anterior division passes ventral to Mm. iliotrochantericus medius
and iliacus and dorsal to the anterior end of the inguinal ligament. The
division branches into two parts. The anterior part extends around the
posterior border of M. extensor iliotibialis anticus and sends several
twigs to the lateral surface of this muscle. The posterior part passes
between the proximal parts of Mm. extensor iliotibialis anticus and
extensor iliotibialis lateralis and supplies the skin.
The middle division passes ventral to Mm. iliotrochantericus medius and
iliacus and dorsal to the inguinal ligament. The division branches into
a large but variable number of parts. A variable number of branches
(usually two) pass posterior to M. extensor iliotibialis anticus and
penetrate the medial surface of M. extensor iliotibialis lateralis.
Several branches supply the fused Mm. vastus lateralis and vastus
medialis. The posteriormost branch of this division passes between Mm.
ambiens and vastus medialis, giving twigs to the lateral surface of M.
ambiens, and sometimes also to the medial surface of M. vastus medialis,
and terminates in M. femoritibialis internus.
The posterior division, which does not subdivide, spirals completely
around M. psoas (passing in turn anterior, dorsal, posterior, and
ventral to it) and gives twigs into this muscle. This nerve then extends
distally into the proximal part of the shank and there has a nonmuscular
termination.
The short, thick anterodorsal division, partly covered by the anterior
division, turns dorsally and passes through the femoral notch of the
ilium and penetrates the deep surface of M. gluteus profundus.
The slender dorsal division passes 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
passes 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 passing 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 associated with the anterior
division and one with the middle division.
=_Obturator Nerve_=
_T. pallidicinctus_
DESCRIPTION.--The long slender obturator nerve passes along the oblique
iliac crest and divides into several branches immediately before
reaching the obturator foramen. One or two branches, which do not pass
through the foramen, penetrate the superficial surface of M. obturator
pars postica. Several small branches (variable in number and
arrangement) pass 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 mass and reaches pars antica. One large branch
passes through the obturator foramen dorsal to the tendon of M.
obturator pars postica, then turns ventrally, passing lateral to the
latter; the branch passes 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 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.
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_
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 passes 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 passes, 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 passes 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 passes deep to femoral head), M. extensor
digitorum longus (branch passes deep to tibial head of M. tibialis
anticus), and M. peroneus longus. A part of the nerve may or may not
pass 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, passes 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 passes
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 passes 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 passing on each side of the main
insertion. The branch passing lateral to the main insertion passes
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 passes 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).
_P. p. jamesi_
INDIVIDUAL VARIATION.--None of significance.
=_Tibial Nerve_=, Fig. 11
_T. pallidicinctus_
DESCRIPTION.--The branches given off in the thigh have been discussed in
the account of the sciatic nerve. At the distal end of the thigh the
peroneal nerve and the paraperoneal branch of the tibial nerve diverge
from the remainder of the tibial nerve and pass through the tendinous
guide loop for M. extensor iliofibularis whereas the remainder of the
tibial nerve does not. This main part of the tibial nerve immediately
divides into three main divisions--lateral, posterior, and medial.
The lateral division passes between Mm. flexor perforatus digiti IV and
gastrocnemius pars externa and subdivides into two branches, one of
which penetrates the medial surface of M. gastrocnemius pars externa.
The other branch passes deep to the latter and sends twigs into the
posterior head of M. flexor perforans et perforatus digiti II, then
passes deep to the latter and enters M. flexor perforans et perforatus
digiti III.
The posterior division sends a branch into the medial head of M. flexor
perforatus digiti IV, then passes between the latter and the medial head
of M. flexor perforatus digiti III, and extends distally giving off
twigs to each of the three heads of M. flexor perforatus digiti IV, to
each of the two heads of M. flexor perforatus digiti III, and to each of
the three heads of M. flexor perforatus digiti II. The number and
arrangement of these twigs is variable.
The medial division passes medial to the medial head of M. flexor
perforatus digiti III, sends a twig to the lateral surface of M.
gastrocnemius pars media, then passes into the shank musculature between
Mm. plantaris and flexor hallucis longus, and sends a branch along the
medial edge of M. flexor hallucis longus that gives several twigs into
this muscle before terminating nonmuscularly. A small branch extends to
M. popliteus, another to M. plantaris, and another to the posterior head
of M. flexor digitorum longus. A nonmuscular branch passes between the
medial and posterior heads of M. flexor digitorum longus and extends
distally deep to this muscle. A long branch gives off near its proximal
end a variable number of twigs that pass deep to M. plantaris and enter
M. gastrocnemius pars interna; the branch then extends distally along
the lateral edge of M. plantaris and terminates nonmuscularly.