Part 15 (1/2)

INNERVATION.--The paraperoneal branch of the tibial nerve sends one or two twigs into the proximal part of the muscle (but distal to the hypotarsus).

INDIVIDUAL VARIATION.--In two legs, the muscle arises in part from the distal end of the lateral calcaneal ridge. The individual variation is insignificant in _T. cupido_ and _P. p. jamesi_.

DISCUSSION AND CONCLUSIONS

_a.n.a.lysis of Individual Variation_

Considerable individual variation occurs in both the muscles and the nerves of the leg of the three species studied. The amount of variation reported by a worker depends in large part on the degree of variation that he considers significant.

Individual variation in the muscles and in the nerves will be discussed separately; that of the muscles (excluding innervation) will be considered first.

Muscles

Considering the number, rather than degree, of variations, the most variable muscles are: Mm. flexor digitorum longus, obturator, caudofemoralis, and extensor hallucis longus. The first-mentioned muscle exhibits 14 different variations in the specimens studied. Mm. vastus lateralis, flexor perforans et perforatus digiti II, and piriformis also showed a considerable number of variations. The following muscles did not exhibit any variations considered significant in this study: Mm.

vastus medialis, femoritibialis internus, flexor perforatus digiti III, extensor brevis digiti III, and abductor digiti IV.

Muscles showing a great _degree_ of individual variation included the following: M. extensor proprius digiti III was present in two legs of _Pedioecetes_ but absent in the other legs studied. A fleshy muscle slip connected M. caudofemoralis pars caudifemoralis with the tendinous raphe between Mm. flexor cruris lateralis and femorocruralis in two legs, whereas in others this connection was tendinous or even absent altogether. M. caudofemoralis pars caudifemoralis had a tendinous area within the belly in only three legs. A vinculum connected the insertional tendons of Mm. flexor perforans et perforatus digiti II and flexor perforatus digiti II in only one leg. The fleshy belly of M.

iliotrochantericus medius was completely split into two parts in one leg. M. flexor cruris lateralis had an accessory slip arising from the caudal musculature in one leg.

Certain individual variations reported in the accounts of the muscles formed a graduated series, as far as degree is concerned, from the typical to the extreme condition. Therefore it was difficult or impossible in some cases to state whether or not certain specimens exhibited such a variation. Elimination of the doubtful instances of variation leaves a total of 50 different variations (excluding variations between species) which can be attributed to a definite number of specimens. The remainder of the discussion of individual variation in the muscles concerns these 50 variations. See table 3.

The typical condition of any structure is considered to be the condition of that structure in the majority of the legs studied. Some conditions considered as typical in the present study might not be so considered if a larger number of specimens had been studied. If exactly half of the legs of one species shows a particular condition of a structure, the condition typical for this species is considered (for purposes of the following discussion) to be that found in the majority of the legs of the other species.

In all instances except two (of 50) the typical condition of the muscles in _T. pallidicinctus_ was also the typical condition in _T. cupido_.

The majority of the legs in _T. cupido_ had an additional dorsal slip on the tendon of M. flexor digitorum longus in digits II and III. In all instances except seven the typical condition in _T. pallidicinctus_ was also the typical condition in _Pedioecetes_. In these seven instances a variation in the former was the typical condition in the latter. These were: an additional dorsal slip on the tendon of M. flexor digitorum longus in each of three digits, a vinculum between the latter and M.

flexor perforatus digiti IV, a partly fleshy insertion of M. flexor cruris medialis, an unossified lateral branch of the insertional tendon of M. extensor digitorum longus, and an independent insertion of the distalmost fibers of the distal head of M. extensor hallucis longus. For all characters except the number of the dorsal slips on the tendon of M.

flexor digitorum longus in digits II and III, the typical condition in _T. pallidicinctus_ was also the typical condition for all species considered together. To facilitate comparison, in the following discussion all of the above-mentioned characters are considered in all species as variants from the typical condition.

Certain legs showed a greater number of variations from the typical condition than did others. The majority of legs showed from four to seven variations in the muscles of the leg. The extremes were P.p. 1L, which showed 11, and T.c.p. 2L, which exhibited only one variation.

Twenty-three of the 50 variations were found in only one leg (out of 23). It would be expected that if additional specimens were studied, more kinds of variations would be found. Nine variations were found in only two legs, five in three legs, five in four legs, and four in five legs. One variation was found in nine legs, one in ten legs, and two in 12 legs; the last four variations were in the number of dorsal slips of the insertional tendon of M. flexor digitorum longus in digits II, III, and IV and in the ossification of the insertional tendon of M. extensor digitorum longus.

Five of the variations were found only in specimens in which only one leg was dissected. Considering only those eight specimens in which both legs were dissected, five of the 45 variations were found in both legs of each specimen exhibiting the variation; 28 variations were found in only one leg of each specimen exhibiting the variation; 12 variations were found in both legs of some specimens but in only one leg of other specimens. Of the six muscle features showing the greatest degree of individual variation (described previously), only two (both pertaining to M. caudofemoralis) were found in both legs of the specimens exhibiting the variation.

For one leg (the one showing the most variations) of each specimen of which both legs were studied, the number of variations that this leg had in common with every other leg (of all species) was determined. Then the number of variations in common between the two legs of one individual was compared with the number of variations in common between one leg of this individual and each leg of every other individual. See table 4. One leg of six of the eight specimens showed at least as many variations in common with a leg of another individual as with the other leg of the same individual. The two exceptions were T.p. 2R and T.c.a. 1R. Thus for most specimens there was as much variation in the muscles between the right and left legs of one individual as there was between individuals.

Of the 50 muscle variations seven were found only in _T. pallidicinctus_ (eight legs), 16 were found only in _T. cupido_ (nine legs), and ten were found only in _Pedioecetes_ (six legs). Two were found in both species of _Tympanuchus_ (but not in _Pedioecetes_). Fifteen were found in both _Tympanuchus_ and _Pedioecetes_; of these, five were found in all three species studied, eight were shared by _T. pallidicinctus_ and _Pedioecetes_, and two occurred in _T. cupido_ and _Pedioecetes_.

Nerves

The lumbosacral plexus, femoral nerve, sciatic nerve, and tibial nerve all showed numerous individual variations. The peroneal nerve, however, was relatively constant. Variations in the obturator nerve were considered to be insignificant. See table 5.

In all instances except one (of 40) the typical condition in _T.

pallidicinctus_ was also the typical condition in _T. cupido_. In most of the legs of the latter the nerve to M. flexor cruris lateralis did not perforate M. caudofemoralis. In all instances except four the typical condition in _T. pallidicinctus_ was also the typical condition in _Pedioecetes_. These exceptions were: prefixation of the lumbosacral plexus, six roots of the sciatic nerve, femoral nerve formed mainly from S2 to S4 and two twigs to M. flexor ischiofemoralis. In all instances the typical condition in _T. pallidicinctus_ was also the typical condition for all species considered together.

Certain legs showed a greater number of variations from the typical condition of the nerves than did others. The greatest number of variations was shown by P.p. 3L, which had 12. T.p. 1R and T.c.p. 1L both showed only one.

All six variations in the lumbosacral plexus were found on both sides of each specimen exhibiting the variation. In marked contrast to the other nerves, there was no significant variation in the lumbosacral plexus between the right and left sides of one individual. (This might not always be true, however, if a larger number of specimens were studied.) Of the variations in the lumbosacral plexus, one was found in only one specimen (of 15), one was found in three specimens, one in four specimens, two in six specimens, and one in seven specimens. Of the 34 variations found in the other nerves, 14 were found in only one leg (of 23), six occurred in two legs, four in three legs, three in four legs, three in five legs, two in six legs, one in seven legs, and one in nine legs.