Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 9

INDIVIDUAL VARIATION.--The anterior edges of the two adductor muscles may be fused together. In one leg, the entire muscle is indistinguishably fused with M. adductor profundus and they appear as a single muscle.

=_M. Adductor Profundus_= (M. adductor longus et brevis, pars interna), Figs. 13, 15, 17, 18

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Broad; on medial surface of thigh immediately posterior to femur; bounded posteriorly by M. flexor cruris medialis (medial to anteroproximal corner of latter), anteriorly by M.

femoritibialis internus (anterior edge overlapped by latter), and laterally by Mm. adductor superficialis and femorocruralis; proximal end tendinous (except anterior edge), fused to proximal fleshy end of M.

adductor superficialis.

ORIGIN.--The muscle arises tendinously from the ventral edge of the ischium extending from the posterior border of the obturator foramen to the ventral ischiatic tubercle and (anterior edge) fleshily from the lateral surface of the pubis ventral to the obturator foramen; the origin is adjacent (ventral) to the origin of M. adductor superficialis.

INSERTION.--The attachment is fleshy and tendinous from the posterior intermuscular line and (proximally and distally) from a narrow adjacent area. Proximally there are often two approximately parallel lines a short distance apart, representing points of attachment of the lateral and medial edges of the muscle; if there is only one line proximally, it may represent the attachment of either the lateral or medial edge of the muscle; distally there is usually only one line, representing the lateral edge of the muscle. The distal end extends onto the posterior surface of the proximal part of the internal condyle, and is adjacent (lateral) to the origin of M. femoritibialis internus, adjacent (medial) to Mm. adductor superficialis and femorocruralis, and adjacent (proximal) to M. gastrocnemius pars media.

INNERVATION.--See M. adductor superficialis.

INDIVIDUAL VARIATION.--The anterior edges of the two adductor muscles are strongly fused together in some cases.

_T. cupido_

INDIVIDUAL VARIATION.--The anterior edge may be fused with that of M.

adductor superficialis. The distal end is sometimes slightly fused with M. gastrocnemius pars media. In one leg, the proximal two thirds of the insertion is entirely tendinous, whereas in another the distal end of the insertion is tendinous.

_P. p. jamesi_

INDIVIDUAL VARIATION.--The anterior edge (in one leg the entire muscle) in some legs fuses with that of M. adductor superficialis.

=_M. Obturator_= (M. obturator externus + M. obturator internus), Figs.

16, 17, 18, 19C, D, 20K, L, M

I am adopting the single name M. obturator for the complex that Fisher (Fisher, 1946; Fisher and Goodman, 1955) subdivides into Mm. obturator externus and obturator internus. The reasons for this change are given in the section on terminology.

For ease of description, it is desirable to apply names to the subdivisions of M. obturator. It has been customary to divide the obturator complex into two parts--an obturator internus and an obturator externus; the latter has often been further subdivided. The evidence given below demonstrates that a primary division of the complex into only two parts is unsatisfactory.

I strongly suspect that comparable parts of the obturator complex have been considered a part of the "internus" in some birds and a part of the "externus" in others. In their work on the Galliformes, Hudson, _et al._ (1959) subdivide the obturator complex into only two divisions--obturator externus and obturator internus. The extrapelvic part of this complex that arises from the rim of the obturator foramen and inserts in common with the stout tendon of the main intrapelvic part of the obturator internus is considered by them to be a part of the obturator internus. Their obturator externus lies anterior and deep to the extrapelvic part of the obturator internus and inserts separately from the latter. (I also have found this same arrangement in _Tympanuchus_ and _Pedioecetes_.)

Berger (1952), in his description of the Black-billed Cuckoo (_Coccyzus erythrophthalmus_), also divides the obturator complex into an obturator internus and an obturator externus; the latter he subdivides into a dorsal and a ventral part. He states (p. 530) that he did not find any measurable differences in myology between _C. erythrophthalmus_ and _C.

america.n.u.s_. In order better to compare this arrangement with that in _Tympanuchus_, I have examined two specimens of _C. america.n.u.s_. My findings in the latter differ from Berger's description (p. 541) in one respect. Whereas Berger states that the dorsal and ventral parts of M.

obturator externus are distinct except at their origin, I find them fused for their entire length; the muscle fibers that connect these two parts lie deep to the tendon of M. obturator internus. The origin of all parts of the complex in _Coccyzus_ is similar to that in _Tympanuchus_.

The only notable difference in configuration is that the part in _Coccyzus_ that appears to correspond to the obturator externus of Hudson, _et al._ (1959) is not separate from the remainder of the extrapelvic part of the muscle. Berger (1952) considers all parts of the muscle having an extrapelvic origin to make up the obturator externus.

It appears to me that the dorsal part and a part of the ventral part of the obturator externus of Berger correspond to the extrapelvic fleshy part of the obturator internus of Hudson, _et al._

From my limited study, it seems to me to be desirable to recognize four subdivisions of the obturator complex, for which I propose the terms pars antica, pars dorsalis, pars ventralis, and pars postica. These parts exhibit various degrees of fusion in different groups of birds and some parts appear to be absent in certain birds. A study of a wide variety of birds will be required to determine whether or not a subdivision into the four parts proposed here is suitable for birds as a whole.

Applying these terms to _Coccyzus_, pars postica is equivalent to the entire obturator internus of Berger (1952). Pars dorsalis is apparently equivalent to the dorsal part of Berger's obturator externus. The ventral part of the obturator externus of Berger represents the fused pars antica and pars ventralis.

The main parts of the obturator muscle appear to be pars postica and pars antica. Pars dorsalis and pars ventralis are more variable; in _Coccyzus_ these two parts are closely a.s.sociated with pars antica whereas in _Tympanuchus_ they are most closely a.s.sociated with pars postica. Apparently pars dorsalis and pars ventralis may be absent in some birds.

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Deeply situated immediately posterior to head of femur; part extending through obturator foramen and lying inside pelvis; extrapelvic part deep to Mm. flexor ischiofemoralis and piriformis; muscle partially divisible into four parts--pars antica, pars dorsalis, pars ventralis, and pars postica (fig. 20K); _pars postica_: mostly inside pelvis; much larger than other parts; broad (narrow anteriorly); on medial surface of ischium; composed of several fascicles; anterior end forming narrow, heavy tendon (with some fleshy fibers on posterior part of deep surface) pa.s.sing through obturator foramen; anteriormost fleshy fibers of ventralmost fascicle fused with pars ventralis; _pars ventralis_: essentially extrapelvic (see origin); mostly ventral to tendon of pars postica; superficial to pars antica; fused to anterior fleshy part of pars postica; anterodorsal edge usually adjacent to, and often slightly fused with, ventral edge of pars dorsalis (deep to tendon of pars postica); _pars dorsalis_: entirely extrapelvic; mostly dorsal to tendon of pars postica; superficial to dorsal part of pars antica; _pars antica_: extremely short but relatively thick; entirely fleshy; entirely extrapelvic; between obturator foramen and head of femur; anterior surface adjacent to articular capsule; almost completely covered by other parts of muscle; proximal end of posterior surface often slightly fused with adjacent parts of pars ventralis and pars dorsalis.

ORIGIN.--_Pars postica_: This arises fleshily from the medial surface of the entire ischium except the posterior end, from the dorsomedial and medial surfaces of the anterior half of the pubis as far forward as the obturator foramen, from the internal ilio-ischiatic crest, from the medial surface of the ilium for a short distance posterior to this crest, and from the iliac recess; the posteroventral corner usually arises from the medial surface of the ischiopubic membrane. _Pars ventralis_: This arises fleshily from the dorsomedial edge of the ventral border of the obturator foramen (fig. 20M) and (narrowly) from the anterior border of the foramen; this part may or may not arise from the lateral surface of the anteroventral border of the foramen and is usually adjacent along the anterior border of the foramen to pars dorsalis; _pars ventralis_ is continuous along the ventral border of the foramen with the intrapelvic origin of pars postica. _Pars dorsalis_: This arises fleshily from the lateral surface of the anterodorsal border of the foramen (fig. 20L) and may extend posteriorly along the dorsal border of the foramen. _Pars antica_: This arises fleshily from the depresssed area anterior to the obturator foramen (adjacent to pars dorsalis and pars ventralis); the posteroventral corner may arise from the lateral surface of the anteroventral border of the obturator foramen (ventral to the anterior end of pars ventralis; fig. 20L).

INSERTION.--_Pars postica_: Several tendinous bands (intrapelvic) converge and coalesce, forming a single strong tendon that pa.s.ses through the obturator foramen and attaches to the lateral surface of the femoral trochanter a short distance posterior to the insertion of M.

gluteus profundus and proximal to the insertion of M. flexor ischiofemoralis. _Pars ventralis_: The attachment is fleshy and tendinous to the ventral edge and the deep surface of the tendon of pars postica. _Pars dorsalis_: The attachment is fleshy and tendinous to the dorsal edge of the tendon of pars postica. _Pars antica_: The attachment is fleshy to the posterior surface of the proximal end of the femur several mm. posterior to the insertion of pars postica; the lateral edge attaches to the obturator ridge.

INNERVATION.--The muscle is supplied by the obturator nerve; several twigs, which do not pa.s.s through the obturator foramen, penetrate the anterior part of the medial surface of pars postica; several twigs pa.s.s through the obturator foramen and supply pars dorsalis, pars ventralis, and pars antica.

INDIVIDUAL VARIATION.--In some cases the origin of pars postica does not include the dorsal end of the internal ilio-ischiatic crest nor the ilium posterior to it. Tiny but distinct accessory slips are sometimes present. In one leg a tendinous slip of pars antica extends beyond the remainder of the muscle and inserts independently on the trochanter close to the insertion of pars postica. In another leg, a fleshy and tendinous slip of pars antica attaches to the deep surface of the insertional tendon of pars postica. In still another leg, a fleshy and tendinous slip of pars dorsalis inserts adjacent (anterior) to the dorsal edge of the insertion of pars antica.

_T. cupido_

INDIVIDUAL VARIATION.--The variations are similar to those given above for _T. pallidicinctus_ except that there is no slip of pars antica attaching to the tendon of pars postica.

_P. p. jamesi._

INDIVIDUAL VARIATION.--There are variations similar to those given above for _T. pallidicinctus_ except that there is no independent slip of pars antica attaching on the trochanter close to the insertion of pars postica. Pars dorsalis may be quite small. In several legs, pars dorsalis is more closely a.s.sociated with pars antica than with pars postica; in one of these, pars dorsalis is indistinguishably fused with pars antica (inserting with the latter) except for a few fibers which insert with pars postica.

=_M. Femorocruralis_= (M. accessorius semitendinosi), Figs. 14, 15, 16, 17

Fisher (Fisher, 1946; Fisher and Goodman, 1955) considers this muscle as an accessory head of M. flexor cruris lateralis. The reasons for this change in terminology are given in the section on terminology.

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Short and broad; posterior to distal part of femur; deep to Mm. extensor iliofibularis and vastus lateralis pars postica; bounded posteriorly by M. flexor cruris lateralis, medially by Mm. adductor superficialis and adductor profundus, and distally by M. gastrocnemius pars media; fused to a variable degree with the latter (in some cases these two muscles fused firmly together, appearing as single muscle); distal and medial to proximal end of M.

flexor perforatus digiti IV.

ORIGIN.--The muscle arises fleshily (thin proximally, thick distally) from the posterior surface of approximately the distal half of the femur between the posterior and posterolateral intermuscular lines. The ventral end is continuous with the origin of M. gastrocnemius pars media, adjacent (medial) to the origin of M. vastus lateralis pars postica, and adjacent (lateral) to the insertions of Mm. adductor superficialis and adductor profundus.

INSERTION.--The attachment is to the tendinous raphe in common with M.

flexor cruris lateralis (which see).

INNERVATION.--One or two tiny branches come off the tibial nerve near the distal end of the main trunk of the sciatic nerve, pa.s.s anteriorly deep to the peroneal nerve, and penetrate the lateral surface.

INDIVIDUAL VARIATION.--In two legs, the branch of the medial division of the tibial nerve which supplies M. gastrocnemius pars media sends a twig to the lateral surface of the distal end of M. femorocruralis (in addition to the usual innervation).

_T. cupido_

INDIVIDUAL VARIATION.--None of significance.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The muscle is much wider, extending farther proximally on the femur.

INDIVIDUAL VARIATION.--None of significance.