Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse (Tympanuchus and Pedioecetes)

by E. Bruce Holmes

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 associated with pars antica whereas in Tympanuchus they are most closely associated 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. 20]K); 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) passing 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. 20]M) 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. 20]L) 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. 20]L).

Insertion.—Pars postica: Several tendinous bands (intrapelvic) converge and coalesce, forming a single strong tendon that passes 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 pass through the obturator foramen, penetrate the anterior part of the medial surface of pars postica; several twigs pass 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.