Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 6

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

=_M. Extensor Iliofibularis_= (M. biceps femoris), Figs. 12, 14, 16, 17

The term extensor in the name of this muscle does not refer to its function. Howell (1938) used the term extensor to indicate derivation of the muscle from the primitive dorsal extensor muscle ma.s.s. (Likewise he used the term flexor to indicate derivation from the primitive ventral flexor muscle ma.s.s.)

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Deep to M. extensor iliotibialis lateralis and posterior to femur; broad proximally and narrow distally; posterior to M. vastus lateralis and anterior to proximal part of M.

flexor cruris lateralis (superficial to distal part of latter); anteroproximal part aponeurotic, fused to deep surface of aponeurosis of M. extensor iliotibialis lateralis; proximal part of aponeurosis of M.

extensor iliofibularis also fused to dorsal edges of underlying Mm.

gluteus profundus and piriformis.

ORIGIN.--The posterior part is fleshy from the ventromedial surface of the entire lateral iliac process; the anterior part is aponeurotic from the posterior part of the anterior iliac crest.

INSERTION.--The tendon forms along the posterodistal edge of the belly and continues beyond the end of the belly as a cylindrical tendon that pa.s.ses through the tendinous guide loop (the belly terminates approximately at the level of the guide loop), then extends anterodistally into the shank musculature; the tendon pa.s.ses between the medial and lateral heads of M. flexor perforatus digiti IV, between the medial and lateral heads of M. flexor perforatus digiti II, lateral to the common tendon of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III, and between the posterior and lateral heads of M. flexor digitorum longus, attaching to the fibular tubercle.

The tendinous guide loop has three arms--proximal femoral, distal femoral, and fibular; the proximal and distal femoral arms unite posterior to the tendon of M. extensor iliofibularis; the proximal arm is medial to, and the distal arm is lateral to, the latter; the fibular arm joins the distal edge of the distal arm lateral to the tendon of M.

extensor iliofibularis. The proximal arm extends anteroproximally lateral to the medial head of M. flexor perforatus digiti IV and medial to M. vastus lateralis pars postica, attaching to a narrow line on the anterolateral surface of the femur a short distance proximal to the external condyle and adjacent (posterior) to the origin of M. vastus lateralis pars postica. The distal arm extends anteriorly medial to the posterior head of M. flexor perforans et perforatus digiti II and medial to M. vastus lateralis pars postica, attaching in common with the tendon of origin of M. gastrocnemius pars externa to a small oval area on the posterolateral surface of the femur a short distance proximal to the fibular groove; the arm is also fused to the underlying articular capsule. The fibular arm (broadest of the three) pa.s.ses deep to, and fused with, the common tendon of origin of the lateral heads of Mm.

flexor perforatus digiti IV and flexor perforatus digiti II, superficial to the common tendon of origin of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III, and deep to the tendon of M. ambiens, attaching broadly to a narrow line on the anterolateral surface of the proximal part of the fibula; the arm is also fused to the underlying articular capsule.

INNERVATION.--A branch of the middle peroneal division of the sciatic nerve sends twigs to the deep surface of the anteroproximal part; the dorsal peroneal division of the sciatic nerve penetrates the deep surface of the proximal end.

INDIVIDUAL VARIATION.--In some instances a variable number of twigs arises from the peroneal nerve near the middle of the thigh and enters the deep surface of the muscle. They are difficult to expose without breaking and may have been overlooked in some specimens.

_T. cupido_

INDIVIDUAL VARIATION.--The same variation is found as in _T.

pallidicinctus_ (see above). In one leg, the tendon of insertion bifurcates into proximal and distal arms before attaching.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--It arises from the ventral rather than the ventromedial surface of the lateral iliac process (there is no ventromedial surface to this process).

INDIVIDUAL VARIATION.--In nearly all of the legs, minute twigs to M.

extensor iliofibularis come off the peroneal nerve near the middle of the thigh. The insertional tendon tends toward doubleness in two legs.

=_M. Piriformis_= (M. gluteus medius et minimus), Figs. 16, 20H, I

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Small, thin, and triangular; lateral to ant.i.trochanter and posterior part of trochanter; deep to M. extensor iliofibularis and posterior (adjacent) to M. gluteus profundus; distal half (or more) tendinous.

ORIGIN.--The muscle arises fleshily from the posterior end of the anterior iliac crest (ventral to the origins of Mm. extensor iliotibialis lateralis and extensor iliofibularis) beginning adjacent to the posterior end of M. gluteus profundus.

INSERTION.--The flat tendon narrows, overlaps the anteroproximal corner of insertion of M. flexor ischiofemoralis, and attaches to the lateral surface of the proximal part of the femur immediately anterior to the insertion of M. flexor ischiofemoralis and posterior to the proximal end of M. vastus lateralis; the attachment is posterodistal to the insertion of M. iliotrochantericus medius and posteroproximal to the insertion of M. iliacus.

INNERVATION.--The small anterior peroneal division of the sciatic nerve turns anteriorly immediately after emerging from the ilio-ischiatic fenestra and pa.s.ses deep to M. piriformis, giving twigs to the deep surface.

INDIVIDUAL VARIATION.--In both legs of one specimen, the insertion does not overlap the insertion of M. flexor ischiofemoralis. The posteroproximal corner of the muscle is tendinous in one leg.

_T. cupido_

INDIVIDUAL VARIATION.--The anterior border is somewhat fused with the posterior edge of M. gluteus profundus in one leg, while in another there is a slight gap between the origins of M. gluteus profundus and M.

piriformis. In one leg, the posterior edge of the origin is aponeurotic.

On both sides of one specimen, an accessory tendinous band arises several mm. posterior to the main part of M. piriformis and joins the proximal part of the insertional tendon, thus forming a Y-shaped unit (fig. 20I); the accessory tendon arises from the anterior end of the lateral iliac process (left side) or from the anterior part of the lateral iliac fossa (right side). The insertion may be proximal (rather than posterior) to the proximal end of M. vastus lateralis. In one leg, the insertional tendon is partly fused to the insertional tendon of M.

flexor ischiofemoralis.

_P. p. jamesi_

INDIVIDUAL VARIATION.--There is often a gap between the origins of M.

gluteus profundus and M. piriformis. In one leg (fig. 20H), the posteroproximal corner of the muscle is aponeurotic. The insertion is often proximal (rather than posterior) to the proximal end of M. vastus lateralis. In one instance, the insertion does not overlap the insertion of M. flexor ischiofemoralis.

=_M. Gluteus Profundus_= (M. iliotrochantericus posterior), Figs. 14, 16

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Large and thick; covering dorsolateral surface of entire preacetabular part of ilium; deep to Mm.

extensor iliotibialis lateralis and extensor iliotibialis anticus; bounded posteriorly by M. piriformis and ventrally by M. iliacus; ventral edge fused with anterior part of latter and with proximal accessory aponeurosis of M. extensor iliotibialis lateralis; tough sheet of fascia strongly fused to anterior two thirds of lateral surface; posterior to this, fascia overlying muscle but not attaching to it; posterior half of fascia fused to overlying aponeurosis of M. extensor iliotibialis lateralis; deep surface of muscle somewhat fused to proximal part of M. iliotrochantericus medius.

ORIGIN.--The superficial surface is tendinous from the entire anterior iliac crest except the posterior end and from the crest forming the anterior and anterolateral edges of the ilium; the muscle arises fleshily from the entire dorsolateral surface of the preacetabular ilium as far posteriorly as the level of the pectineal process; the dorsal edge is adjacent (anterior) to the origin of M. piriformis.

INSERTION.--The attachment is by a short, wide, thick tendon to a curved line (convex anteriorly) on the lateral surface of the femoral trochanter.

INNERVATION.--The anterodorsal division of the femoral nerve turns dorsally through the femoral notch of the ilium and penetrates the deep surface of the ventral part of the muscle midway of its length; the anterior peroneal division of the sciatic nerve pa.s.ses deep to M.

piriformis and terminates near the posterodorsal edge of M. gluteus profundus.

INDIVIDUAL VARIATION.--On both sides of one specimen, the branch from the femoral nerve pa.s.ses lateral to the extreme anteroproximal corner of M. iliotrochantericus medius instead of through the femoral notch.

_T. cupido_

INDIVIDUAL VARIATION.--In one leg, the insertional tendon is strongly fused to the insertional tendon of M. iliotrochantericus medius.

_P. p. jamesi_

INDIVIDUAL VARIATION.--None of significance.

=_M. Iliacus_= (M. iliotrochantericus anterior), Figs. 13, 14, 15, 16, 17

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Adjacent ventrally to ventrolateral edge of M. gluteus profundus; lateral edge much thicker than medial edge; deep to M. extensor iliotibialis lateralis and anterolateral to M.

iliotrochantericus medius; distal (posterior) end pa.s.sing between proximal ends of Mm. vastus medialis and vastus lateralis pars lateralis; insertion overlapped by latter; dorsal surface of anterior part fused with ventrolateral edge of M. gluteus profundus and with ventral surface of proximal accessory aponeurosis of M. extensor iliotibialis lateralis; ventral surface partly fused with distal accessory aponeurosis of latter muscle.

ORIGIN.--The origin is fleshy and tendinous from the lateral edge of the anterior part of the ilium.