THE GENERAL MECHANISM OF THE HUMAN HIP-JOINT. 513 



action. The anterior and posterior bands of this ligament are 

 reciprocally related to the anterior and posterior areas of the 

 femoral head, and of the acetabulum, as the ilio-femoral and 

 pubo-ischial femoral are. 



On the Movements of the Hip-Joint. The movements of 

 the elbow and ankle-joints take place in an anterior and a 

 posterior conical screw combination in each joint. These com- 

 binations are both dexiotrope or both scoeotrope in each joint, 

 according to the side to which the joint belongs. The axes 

 of the fundamental cones lie more or less obliquely across the 

 joint, one in front of the other the apices of the cones 

 pointing, the one outwards, the other inwards. The so-called 

 movements of flexion and extension in these joints that is, 

 presumed movements in the same antero-posterior plane are, 

 in fact, movements of flexion and extension produced by 

 combined gliding and rolling along one conical helicoid course 

 in the anterior half, and along a second in a reverse direction 

 in the posterior half of the articular path. As, however, the 

 axes of rotation of both screw combinations are so nearly 

 coincident with the axes of the presumed hinge movement, 

 the actual variation from such a movement is not at first 

 obvious. For the same reasons the movements are princi- 

 pally screwing or gliding, with a minimum of rolling ; the 

 gaping, therefore, is comparatively slight. It is evident that 

 the path described by a point in either of the segments of the 

 limb, between which the joint is placed, must be a double 

 helix that is, two conical helices, corresponding respectively 

 to the anterior and posterior screw combinations of the joint 

 oscultating with one another. It must also be evident from 

 the double-threaded form of the screwed surfaces of these 

 joints, and the peculiar configuration of their opposite arti- 

 cular surfaces, that they do not admit of the movements 

 technically termed adduction, abduction, and rotation. 



The anterior and posterior screw combinations of the knee- 



2 L 



