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FORM AND FUNCTION OF THE HOMINOID FOOT
This dissertation examines the relationship of form and function in the foot of four species of hominoid; Homo sapiens, Pan troglodytes, Gorilla gorilla, and Pongo pygmaeus. The study is both analytic and comparative; that is, objectives include both analysis of the funtioning of the anatomical system of the foot, and comparison of this system in several closely related organisms exhibiting very different types of positional behavior. The investigation utilizes the joint-link approach of Dempster, and focuses on the functional complex rather than the single structure. Analysis of a particular complex, such as a joint, integrates anatomical data relating to muscle and ligament size and disposition and joint configuration, with osteometric data pertaining to the size of articular surfaces and the shape and proportions of the osseus structures. Several new osteometric measurements have been derived which are more relevant in assessing function than previously used linear measures.^ The four species with which this study is concerned exhibit very different types of positional behavior. Chimpanzees exhibit the widest range of behavior, being at home in the trees or on the ground. Gorillas are more terrestrial but are also capable of arboreal activities. The foot of these African pongids is more "generalized" than that of humans or orangs. The mobile tarsus can be stabilized in certain positions, and the powerful hallux can be used in a wide range of prehensile and non-prehensile activities.^ Humans use the foot as a base of support and as a propulsive organ in bipedal striding and running. The stabilized tarsus and adducted hallux are used as levers in raising the center of gravity and are unsuited for activities in which the foot is used as a prehensile organ. The high, narrow tarsus, which adds resiliency to this part of the foot, necessitates a number of "balancing" adaptations to maintain body weight over the base of support and prevent uncontrolled movement and consequent strain at the talo-crural joint, in particular. For similar reasons, the long axes of the tibiae are parallel, and the plane of the talo-crural and subtalar joints are perpendicular to the line of gravity during bipedal activities.^ Orangs are highly arboreal, usually supporting the body using the limbs in tension in a small branch setting. The highly prehensile foot emphasizes the lateral digits rather than the hallux in grasping the substrate. The tarsus is extremely mobile, especially in inversion, in order to allow foot placement in a wide variety of positions. Increased mobility, which occurs at the talo-crural as well as the subtalar and transverse tarsal joints, is a result of differences between this and the other species in both osseus and soft-part (especially ligamentus) morphology. As a consequence, the tarsus is ill-suited for compressive weight-bearing, or for use as a lever for the plantar flexors of the foot in this species. In keeping with the emphasis on digital prehension, the lateral metatarsals and phalanges are extremely elongated, and the hallux is reduced in the orang.^ When the total morphological pattern of the foot is considered, it is apparent that humans and African pongids are more similar to each other than they are to orangs. Somewhat different conclusions may be drawn if single structures only are compared. Generalizations regarding both habitus and heritage are commonly made from limited comparisons of anatomical structures. Problems with this practice are discussed in this dissertation. ^
DAVID NEIL GOMBERG,
"FORM AND FUNCTION OF THE HOMINOID FOOT"
(January 1, 1981).
Electronic Doctoral Dissertations for UMass Amherst.