Thus, the divided or undivided conditions of the m. obturator externus and the pars interna of the m. gastrocnemius seem to be correlated with the degrees of strength of certain movements of the leg. It is conceivable that these differences in structure are correlated with the manner in which food is obtained, the birds having the bipartite muscles being those which spend the most time on the ground searching and scratching for seeds and other sorts of food. Yet, in Leucosticte, a cardueline, and in Calcarius, an emberizine, whose foraging habits are rather similar, the structure is unlike. Leucosticte does resemble the emberizines and also Piranga and Spzia in the extension of a band of muscle fibers from the pars interna of the m. gastrocnemius around the front of the knee. A band of muscle fibers of this sort strengthens the knee joint and gives still more strength to the pars interna. This condition has been reported in a number of birds by Hudson (1937) and is, in all probability, an adaptation for greater strength of certain leg movements. The development of this band in Leucosticte seems to parallel that in the other birds studied and does not indicate relationship, since in Leucosticte this band arises from the undivided muscle which (as stated above) resembles only the posterior portion of the bipartite muscle described for the other birds. In the latter, the muscular band arises from the anterior part of the muscle.
Minor differences in muscle pattern, like those already mentioned, are consistent also between subfamilies, but correlation of these minor differences with function is difficult. There is the implication, however, that in all the groups except the carduelines and ploceids, the emphasis is on greater strength and mobility of the leg. In the carduelines that were studied the origin of the m. sartorius does not extend so far craniad as in the other species. In the latter, at least half of the origin is from the last one or two free dorsal vertebrae; in the carduelines no more than one third of the origin is anterior to the ilium. It is conceivable that the more craniad the origin, the stronger the forward movement of the thigh would be.
In Passer, Estrilda and Poephila, and in all the cardueline finches examined, the bellies of the m. flexor perforans et perforatus digiti II and the m. flexor perforans et perforatus digiti III are more intimately connected than they are in the other species studied. Thus, the amount of independent action of these muscles in Passer, in the estrildines, and in the carduelines probably is reduced.
In Passer, the estrildines, and the carduelines the edges of the sheathlike tendon of insertion of the m. perforatus digiti III are thickened; as a result the insertion appears superficially to be double but closer examination reveals that there is a fascia stretched between the thickened edges. In the other species examined, the insertion is sheathlike throughout and there are no thick areas. I cannot explain this on the basis of function. The difference, however, is obvious and constant.
Aside from the differences noted above, there were variations of muscle pattern that seem to be significant only in Vireo olivaceus. In this species the central, aponeurotic portion of the m. iliotibialis is absent. The origin of the m. adductor longus et brevis is from the dorsal edge of the ischiopubic fenestra and not from the membrane covering this fenestra. The origin of the pars posticus of this muscle, furthermore, is fleshy and not tendinous as it is in the other species. The m. flexor perforatus digiti II is larger and more deeply situated in Vireo and has, furthermore, no connection with the m. flexor hallucis longus. The latter muscle is smaller and weaker than in any of the other species and has only one (the posterior) head of origin. The m. flexor hallucis brevis, on the contrary, is larger than in the other birds, compensating, probably, for the small m. flexor hallucis longus. In those differences, however, which separate the carduelines and ploceids from the other birds studied, Vireo resembles, in every instance, the richmondenines, emberizines, tanagers, warblers, and blackbirds.
On the basis of differences in leg-musculature the species which are now included in the Family Fringillidae may be separated into two groups. One group includes the richmondenines and the emberizines; the other, the carduelines. The muscle patterns of the legs of the birds of the first group are indistinguishable from those of Seiurus, Icterus, Molothrus, and Piranga, and except for the differences noted are similar to those in Vireo. The carduelines, on the other hand, are similar in every point of leg-musculature to the ploceids which were studied. Thus, the heterogeneity of the Family Fringillidae, as now recognized, is emphasized by differences in the muscle patterns of the leg.
Comparative Serology