F. Ya. Dzerzhinsky
Biological Faculty, Moscow Lomonosov State University, Leninskie Gory, 119992 Moscow
e-mail: dzer@soil.msu.ru
A.N. Severtsov, when introduced the concepts of ecto- and entosomatic organs, illustrated them by three examples of adaptations, namely by the blindness of some subterranean vertebrates, by the "suckers" on the geckos' fingers and toes, and by the flattened body shape of skates and rays. Morphology of the pterygoideus muscle in some song-birds, which is one of the most complicated jaw muscles, is considered here in the framework of those concepts. This muscle displays mosaic variation among bird taxa while the arrangement of skeletal and ligamentous components is rather monotonous. The ventromedial portion of the muscle predominates in the Vietnamese babblers (Garrulax leucolophus, Alcippe peracensis, and Yuhina zantholeuca; fam. Timaliidae), in the leafbird Aegithina lafresnayei (fam. Chloropsidae), and in the drongo (fam. Dicruridae). One part of this portion, m. retractor palatini, is relatively big in the starling, Gracula religiosa (fam. Sturnidae), and in the dipper, Cinclus cinclus (fam. Trogloditidae). Both muscular bulks (entosomatic organs after Severtsov) affect clearly and directly properties of the bill (ectosomatic organ) without any visible differences in its construction. They provide the upper jaw with additional force (directed downward while grasping a prey) and so fasten it against the passive deviation during tearing or pulling off the food item (the mandible does not accept those forces). These two versions of the muscle morphology are parallel in the functional sense and are independent in respect to their evolutionary origin. The ventromedial portion, judging by its attachment to the mandible close to the center of the quadrato-mandibular joint, is neutral in respect to its adduction, and the m. retractor palatini is not connected to it. Thus, muscular portions under consideration, in spite of their position deep in the feeding apparatus, behave as ectosomatic organs. Deviations in their morphology are not the answer to any antecedent modification of the bill shape or of arrangement of the skeleto-ligamentous apparatus components. Instead, they appear to be primary results due to immediate action of the natural selection to provide a direct functional effects.
