LAWS OF BONE ARCHITECTURE 225 



plex to analyze. For this reason the effect of the action of the 

 muscles will not be in^'estigated. further in this study. 



B. The effect of the body weight. The center of gravity of 

 the body in the normal, erect, standing position passes slightly 

 posterior to the plane passing through the centers of the heads 

 of both femurs, and slightly anterior to the centers of bearing 

 at the knee-joint. This is readily verified by a study of the 

 relations of the ilio-femoral ligament, which holds the weight of 

 the body balanced upon the heads of the femurs; and by the great 

 preponderance of tendon strength at the knee-joint which is 

 arranged centrally and posteriorly to the center of gravity of the 

 lower end of the femur. The eccentric position of the center of 

 gravity line of the body with respect to these two joints is one 

 of considerable mechanical importance; excessive or suddenly 

 applied loads reaching the femur through the acetabula are 

 cushioned by the tensing of the muscles and ligaments at these 

 joints. 



In the standing position the weight of the trunk is transmitted 

 in a vertical line passing through the acetabulum and the outer 

 condyle. So the erect, standing position is maintained not 

 only by the approximation of the bones, as would be the case 

 if the perpendicular line through the acetabulum passed in 

 succession through the center of the knee-joint, the axis of the 

 tibia and the center of the ankle-joint, but also by the action 

 of the muscles and ligaments at these joints. This action of 

 the muscles and ligaments at these joints on the medial side 

 brings the resultant pressure line in the leg to pass close to the 

 centers of the knee- and ankle-joints. The eccentricities at 

 these joints vary in different individuals and usually produce 

 slight effect upon the principal stresses that normally occur in 

 the femur. For this reason they will not be considered in the 

 analysis of the loads. 



3. Magnitude and direction of the external load. The weight 

 of the body acts in a vertical direction and in the erect, stand- 

 ing position is equally divided between the two femurs. In 

 walking and running the whole body weight is alternately trans- 

 ferred from one foot to the other, the center of gravity of the 



THE AMERICAX JOURNAL OK ANATOMY, VOL. 21. NO. 2 



