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

by E. Bruce Holmes

[NERVES]

For each nerve (or plexus) the condition found in most specimens of the Lesser Prairie Chicken (T. pallidicinctus) is described first. Following this, variations from the typical T. pallidicinctus condition are given for T. pallidicinctus, then for T. cupido (both subspecies considered together), and finally for P. p. jamesi.

Lumbosacral Plexus, [Figs. 2], [3]

T. pallidicinctus

Description.—Eight spinal nerves contribute to the lumbosacral plexus. These are the second through the ninth synsacral spinal nerves (S2 to S9). The entire ventral ramus of each of these nerves, excepting S2 and S9, contributes to this plexus. The ventral ramus of S2 divides into two branches, only the posterior of which contributes to the plexus; the anterior branch directly innervates muscles of the abdominal wall (as does the entire ventral ramus of S1). The ventral ramus of S9 divides into two branches, only the anterior of which contributes to this plexus; the posterior branch contributes to the more posteriorly situated pudendal plexus.

Each root of the plexus corresponds to a single spinal nerve except one spinal nerve (S5—the furcal) that contributes a root to both the femoral nerve and the sciatic nerve; thus typically the plexus has nine roots (but see below). The four anteriormost roots (S2 to S5) contribute to the femoral nerve, although the contribution from S2 is small. S3 and S4 contribute to the obturator nerve. The five posteriormost roots (S5 to S9) contribute to the sciatic nerve, although the contribution from S9 is relatively small.

Individual Variation.—In all specimens (of all species) examined, the right and left sides of the plexus in any one individual were practically identical. In T.p. 2 ([Fig. 2]B), there appear to be two furcal nerves; S5 is typical, but a small branch of S4 apparently also contributes to the sciatic nerve. In T.p. 5, S9 is unique in dividing into three branches; the anterior two join the sciatic nerve separately; the posterior one joins the pudendal plexus as usual.

T. cupido

Individual Variation.—S2 or S5, or both, may contribute to a limited extent to the obturator nerve. In T.c.p. 3 ([Fig. 3]A) and T.c.a. 1 and 2, much of the plexus has shifted one segment anteriorly, relative to the synsacral vertebrae (the so-called prefixed condition); the roots of the femoral nerve are S2, S3, and S4 (all large); the furcal nerve is S4 (in T.c.a. 1, S5 gives an extremely small root to the femoral nerve, thus making two furcal nerves); six roots (S4 to S9) contribute to the sciatic nerve; S3 and S4 remain as the main contributors to the obturator nerve except in T.c.a. 2 in which only S2 and S3 contribute to it.