Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 3

_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 pa.s.s 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 pa.s.ses 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 pa.s.ses deep to the latter and sends twigs into the posterior head of M. flexor perforans et perforatus digiti II, then pa.s.ses 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 pa.s.ses 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 pa.s.ses medial to the medial head of M. flexor perforatus digiti III, sends a twig to the lateral surface of M.

gastrocnemius pars media, then pa.s.ses 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 pa.s.ses 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 pa.s.s 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.

The paraperoneal branch diverges from the peroneal nerve, pa.s.sing medial and then distal to the insertion of M. extensor iliofibularis, whereas the peroneal nerve pa.s.ses proximal and then lateral to this insertion.

The paraperoneal branch pa.s.ses deep to the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II and superficial to the tendon of the anterolateral head of M. flexor perforatus digiti IV and then pa.s.ses distally along the anterolateral borders of the latter and the lateral head of M. flexor perforatus digiti III and the posterolateral border of M. flexor digitorum longus. This branch is thus separated from the peroneal nerve by M. flexor digitorum longus and by the fibula; the branch pa.s.ses along the lateral surface of the tibial cartilage, continues lateral to the hypotarsus, then turns medially before extending distally between Mm. abductor digiti IV and flexor hallucis brevis, sending twigs into each of these muscles and a long twig into M. lumbricalis before terminating nonmuscularly.

INDIVIDUAL VARIATION.--In T.p. 3L,R (fig. 11B), an extra branch arises from the tibial nerve as a separate (fourth) division; it enters the medial head of M. flexor perforatus digiti IV and also gives off a twig that anastomoses with the posterior division (left leg) or with the first branch of the posterior division (right leg). In T.p. 3R (fig.

11B), a large extra branch arises from the proximal part of the medial division and pa.s.ses medial and then deep to the medial head of M. flexor perforatus digiti III, perforates the tendinous part of the medial head of M. flexor perforatus digiti II, and joins the posterior division (lateral to the medial head of M. flexor perforatus digiti III). A similar branch is found in T.p. 3L except that it arises from the proximal part of the posterior (rather than the medial) division. In T.p. 3R (fig. 11B), the branch to M. gastrocnemius pars externa arises so far proximally that it appears as a separate (fifth) division of the tibial nerve. In two legs, the branch of the medial division that supplies M. gastrocnemius pars media sends a twig into the distal end of M. femorocruralis (fig. 11A).

_T. cupido_

INDIVIDUAL VARIATION.--In one leg, an extra branch of the medial division arises immediately distal to the branch to M. gastrocnemius pars media and enters the proximal end of the medial head of M. flexor perforatus digiti III. In one instance, the branch to M. gastrocnemius pars interna pa.s.ses through a gap in the origin of M. plantaris rather than distal to the origin of the latter.

_P. p. jamesi_

INDIVIDUAL VARIATION.--The branch to M. gastrocnemius pars interna gives a minute twig to the deep surface of the free belly of M. plantaris in one leg.

[Ill.u.s.tration: FIG. 2. Ventral views of the lumbosacral plexus of _Tympanuchus pallidicinctus_. Sympathetic ganglionated chain removed.

Numbers indicate synsacral spinal nerves. 2. A. T.p. 1L. B. T.p. 2L.]

[Ill.u.s.tration: FIG. 3. Ventral views of the lumbosacral plexus.

Sympathetic ganglionated chain removed. Numbers indicate synsacral spinal nerves. 2. A. _Tympanuchus cupido pinnatus_ 3L. B. _Pedioecetes phasianellus jamesi_ 4L.]

[Ill.u.s.tration: FIG. 4. Semidiagrammatic ventral views of the femoral nerve, showing the distribution of the branches. 3. 1,2, M. extensor iliotibialis anticus; 3, cutaneous; 4-6, M. extensor iliotibialis lateralis; 7,8, M. iliacus; 9, M. gluteus profundus; 10-12, fused Mm.

vastus lateralis and vastus medialis; 13,14, M. vastus medialis; 15, M.

ambiens; 16, M. femoritibialis internus; 17, nonmuscular; 18, M. psoas; 19, M. iliotrochantericus medius. A. _Tympanuchus cupido pinnatus_ 3L.

B. _Pedioecetes phasianellus jamesi_ 3L.]

[Ill.u.s.tration: FIG. 5. Semidiagrammatic ventral views of the femoral nerve, showing the distribution of the branches. 3. 1,2, M. extensor iliotibialis anticus; 3, cutaneous; 5,6, M. extensor iliotibialis lateralis; 7,8, M. iliacus; 9, M. gluteus profundus; 10,11, fused Mm.

vastus lateralis and vastus medialis; 13, M. vastus medialis; 15, M.

ambiens; 16, M. femoritibialis internus; 17, nonmuscular; 18, M. psoas; 19, M. iliotrochantericus medius. A. _Tympanuchus pallidicinctus_ 2L. B.

_Tympanuchus cupido att.w.a.teri_ 1R.]

[Ill.u.s.tration: FIG. 6. Semidiagrammatic dorsolateral view of the sciatic nerve of _Pedioecetes phasianellus jamesi_ 3R, showing the distribution of the branches. 2-1/2. 1, M. gluteus profundus; 2, M. piriformis; 3, M. extensor iliotibialis lateralis; 4-7, M. extensor iliofibularis; 8, M. flexor cruris medialis; 9, cutaneous; 10, to pudendal plexus; 11, M.

flexor cruris lateralis; 12, M. caudofemoralis pars caudifemoralis; 13-15, M. caudofemoralis pars iliofemoralis; 16,17, M. flexor ischiofemoralis; 18,19, M. femorocruralis (branch of tibial nerve); 20, cutaneous; 21, M. gastrocnemius pars media (branch of tibial nerve); 22, cutaneous.]

[Ill.u.s.tration: FIG. 7. Semidiagrammatic dorsolateral view of the sciatic nerve of _Tympanuchus pallidicinctus_ 2L, showing the distribution of the branches. 2-1/2. 1, M. gluteus profundus; 2, M. piriformis; 3, M.

extensor iliotibialis lateralis; 4, 7, M. extensor iliofibularis; 8, M.

flexor cruris medialis; 9, cutaneous; 10, to pudendal plexus; 11, M.

flexor cruris lateralis; 12, M. caudofemoralis pars caudifemoralis; 13-15, M. caudofemoralis pars iliofemoralis; 17, M. flexor ischiofemoralis; 18, M. femorocruralis (branch of tibial nerve); 22, cutaneous; 23, nonmuscular (branch of peroneal nerve).]

[Ill.u.s.tration: FIG. 8. Semidiagrammatic dorsolateral view of the sciatic nerve of _Tympanuchus cupido pinnatus_ 3L, showing the distribution of the branches. 2-1/2. 1, M. gluteus profundus; 2, M. piriformis; 3, M.

extensor iliotibialis lateralis; 4,7, M. extensor iliofibularis; 8, M.

flexor cruris medialis; 9, cutaneous; 11, M. flexor cruris lateralis; 12, M. caudofemoralis pars caudifemoralis; 13, M. caudofemoralis pars iliofemoralis; 17, M. flexor ischiofemoralis; 18, M. femorocruralis (branch of tibial nerve); 20, cutaneous; 22, cutaneous.]

[Ill.u.s.tration: FIG. 9. Semidiagrammatic dorsolateral view of the sciatic nerve of _Pedioecetes phasianellus jamesi_ 3L, showing the distribution of the branches. 2-1/2. 1, M. gluteus profundus; 2, M. piriformis; 3, M. extensor iliotibialis lateralis; 4,5,7, M. extensor iliofibularis; 8, M. flexor cruris medialis; 9, cutaneous; 11, M. flexor cruris lateralis; 13,14, M. caudofemoralis pars iliofemoralis; 16,17, M. flexor ischiofemoralis; 18,19, M. femorocruralis (branch of tibial nerve); 20, cutaneous; 22, cutaneous.]

[Ill.u.s.tration: FIG. 10. A,B. Semidiagrammatic drawings of the peroneal nerve of _Tympanuchus pallidicinctus_ 1L, showing the distribution of the branches. 2. C. Semidiagrammatic drawing of the distal part of the peroneal nerve of _Tympanuchus cupido att.w.a.teri_ 1R, showing the distribution of the branches. 2. 1,2, M. tibialis anticus (tibial head); 3,4, M. tibialis anticus (femoral head); 5, M. extensor digitorum longus; 6, nonmuscular; 7,8, M. peroneus longus; 9, M. peroneus brevis; 10,11, M. extensor hallucis longus (proximal head); 12, M. extensor hallucis longus (distal head); 13-15, nonmuscular (to toes); 16, M.

abductor digiti II; 17, M. extensor brevis digiti III; 18, M. extensor brevis digiti IV.]

[Ill.u.s.tration: FIG. 11. A,B. Semidiagrammatic drawings of the tibial nerve (excluding the paraperoneal branch) of _Tympanuchus pallidicinctus_, showing the distribution of the branches. 2. A. T.p.

1L. B. T.p. 3R. C. Semidiagrammatic drawing of the distal part of the paraperoneal branch of the tibial nerve of _Pedioecetes phasianellus jamesi_ 2L, showing the distribution of the branches. 2. 1, M.

femorocruralis; 2, M. gastrocnemius pars media; 3, M. popliteus; 4, M.

plantaris; 5, M. flexor digitorum longus; 6-8, nonmuscular; 9-11, M.

gastrocnemius pars interna; 12,13, M. flexor hallucis longus; 14-16, M.

flexor perforatus digiti IV (medial head); 17, M. flexor perforatus digiti III (medial head); 18-20, M. flexor perforatus digiti II; 21, M.

flexor perforatus digiti IV (lateral head); 22-24, M. flexor perforatus digiti IV (anterolateral head); 25, M. flexor perforatus digiti III (anterolateral head); 26, M. flexor perforans et perforatus digiti III; 27,28, M. flexor perforans et perforatus digiti II; 29, M. gastrocnemius pars externa; 30,31, M. abductor digiti IV; 32,33, M. flexor hallucis brevis; 34,35, nonmuscular (to toes).]

MUSCLES

In the accounts of the muscles the name used by Hudson, _et al._ (1959) for each muscle is given in parentheses after the name used by me if the two differ.

In the account of each muscle, the description of the condition found in most specimens of the Lesser Prairie Chicken (_T. pallidicinctus_) is given first. This is hereafter referred to as the typical condition for _T. pallidicinctus_. Then any individual variations found within this species are given. Under the heading _T. cupido_ any constant differences between this species and typical _T. pallidicinctus_ are given first, and any individual variations found within the species _T.

cupido_ (both subspecies considered together) are given second. Under the heading _P. p. jamesi_ any constant differences between this subspecies and the typical condition for _T. pallidicinctus_ (thus these differences are not necessarily constant between the two genera) are given first, and any individual variations found within the subspecies _P. p. jamesi_ are given second.

In the bird embryo, according to the studies of Romer (1927) and Wortham (1948), the muscles within each segment of the leg differentiate from distinct dorsal or ventral mesenchymal ma.s.ses. Presumably these represent the primitive dorsal extensor and ventral flexor muscle ma.s.ses. The list below indicates the ontogenetic origin of the avian leg muscles, according to the studies of Romer and Wortham. The individual muscles are discussed in the order in which they are listed below.

Dorsal muscles of thigh

M. extensor iliotibialis lateralis M. extensor iliofibularis M. extensor iliotibialis anticus M. piriformis M. ambiens M. gluteus profundus M. vastus lateralis M. iliacus M. vastus medialis M. iliotrochantericus medius M. femoritibialis internus M. psoas

Ventral muscles of thigh

M. flexor cruris lateralis M. adductor superficialis M. flexor cruris medialis M. adductor profundus M. caudofemoralis M. obturator M. flexor ischiofemoralis M. femorocruralis

Ventral muscles of shank

M. gastrocnemius M. flexor perforatus digiti III M. flexor perforans et perforatus M. flexor perforatus digiti II digiti II M. flexor hallucis longus M. flexor perforans et perforatus M. plantaris digiti III M. flexor digitorum longus M. flexor perforatus digiti IV M. popliteus