The pelvis in the bipedalism of primates.

Abstract

AbstractThe pelvis in the “Normalstellung” tends in man to tilt backwards. In primates, the pelvis shows a marked tendency to tilt into quadruped posture. In both the bodyweight intensifies those tendencies. Consequently, assuming erect posture and maintaining bipedal balance encounter, in primates, constant resistance from pelvis and bodyweight.In gibbons and ponginae loss of the tail, lengthening of the sacrum and broadening of the ilium combined with a pronounced development of the sacro‐tuberous ligament furnish the gluteal muscles with a more powerful reflectory moment on pelvis and body. Moreover, the development of a short head of the femoral bicipital muscle, made the hip‐joint to a greater degree independent of the knee‐joint. Thus, maintenance of bipedal balance is better developed in gibbons and ponginae than in monkeys.In man broadening of the sacrum altered the position of the acetabulum. Together with ensuing changes of the ilium and the ischium this brought about in the pelvis a balance relationship so that the bodyweight not only causes no resistance to standing upright but even facilitates it.The essence of bipedalism, however, is not only the capacity to rise upright and to stand, but also to walk, that is, to keep the body balanced in the frontal plane on one leg. It is the ventralwards broadening of the ilium, and the sidewards bend of its middle segment, that allow, the strongest part of, the gluteus medius (and minimus) to produce a powerful abducent moment on the pelvis. The lack of a corresponding osteomuscular structure in monkeys as well as in gibbon and ponginae compels these primates to move faster and run straddle‐legged, in a more or less forward‐bending posture.Lastly it is found that in gibbons and ponginae, not only is the ilium markedly broadened laterally, but also that its sacral part is turned completely sidewards. This indicates that bipedalism in man has developed along quite other lines than in gibbons and ponginae.