A contribution to the functional morphology of the mammalian carpus

Abstract

While the positional relationships of the mammalian carpal bones were fully discussed by palaeontologists around 1900, the dynamic role of the carpus has been almost ignored. The roles of the forelimb in locomotion and of the carpus within the forelimb are briefly considered. There is a tendency for the wrist to be rigid during retraction, in contrast to the activity of the tarsus, and it acts as a hinge only when the limb is off the ground, folding the limb during protraction. The position close under the mid-line in which the feet are placed is emphasised, since this means that the lower limb segments must swing out sideways to pass the contralateral limb. Some ulnar deviation must accompany flexion at the carpus to achieve this. The form and function of the carpus in various groups of mammals is examined. Function is determined largely by manipulative studies, and described as degrees of flexion and deviation at the two main carpal joints. Thus in Carnivora, the proximal joint is both the main flexion hinge and a deviational joint, while the mid-carpal joint is solely a flexion hinge. In these and many other quadrupedal mammals with proximal joints of similar function, the scaphoid and lunar bones of the primitive carpus are fused. In ungulates, both joints are solely flexion hinges, and the bones of the proximal row must remain separate to provide the twisting movement which gives the deviational component. In primates both joints give moderate flexion and deviation. The isolated position of the Monotremes is emphasised.