Respective role of the axial and appendicular systems in relation to the transition to limblessness.

In lower quadrupedal vertebrates locomotor efficiency seems to result from the associate movements of the axial and appendicular systems, which are totally independent in structure and embryological origin. The curvature of the trunk, produced by a standing wave, magnifies the propulsive action of the limbs. In intermediate forms, the association of an elongate trunk with limbs reduced in size brings about functional consequences which may be noticeably diverse according to the degree of trunk elongation and limb reduction. According to environmental constraints, animals search for better locomotor efficiency, which implies the maintenance or breakage of this association of both locomotor systems. In some cases, limb action on the ground is added to the axial wave action through a perfect mutual adjustment of rhythmic activity, until mechanical inefficiency of the limbs is reached by possible loss of contact with the ground. In other cases, the limbs dragged on the ground during the stance phase act against the axial action or, on the contrary, are inhibited by the axial system. A review of available data tries to contribute to an understanding of the respective roles of both systems in the transition to limblessness.

[Biomechanics of the skull and the beak in Ramphastos (Ramphastidae, Aves) I. Description of anatomical elements]. / Biomécanique du crâne et du bec chez Ramphastos (Ramphastidae, Aves). I. Description des éléments anatomiques.

The different cranial kinetic systems described in Birds are limited in Ramphastos by the dorsal and backward position of the nostrils, the shape of the fronto-nasal suture, the lateral fit-in between nasal, lacrymal and frontal bones, and the divergent directions of the 2 otic processes of the quadrate, 5 ligaments joint the posterior skull elements to the quadrate and the mandible. Small sesamoid bones strengthen the external jugo-mandibular ligament in its retro-articular portion. The nomenclature of the aponeuroses is not useful by itself to compare the organizations and homology of the diverses muscular layers in the different birds groups. The differences observed in the relative position of the muscular bundles have to be related to the shape and particular relationships between skeletal structures. We have to consider the osteo-muscular systems as a whole and analyse them from a functional viewpoint.

[Biomechanics of the skull and the bill in Ramphastos (Ramphastidae, Aves). II. Analysis of the conditions of movement of the bill]. / Biomécanique du crâne et du bec chez Ramphastos (Ramphastidae, Aves). II. Analyse des conditions du mouvement du bec.

As all the anatomical conditions are not present to permit any movement between the cranial segments, the bill aperture and closure by the rotation on the quadrato mandibular joint is, in Ramphastos , the only action studied in relation to muscles organization and bone architecture by torques analysis. From a mechanical advantage viewpoint, the beginning of the closure seems to be performed by velocity muscles, while the action is ended by power muscles. The stresses issued by the muscles actions are balanced by bone trabeculae and other muscles, and may be considered as responsible of the mechanical stiffness of the whole bill system.