TIIK GENERAL MECHANISM OK THE HUMAN HIP-JOINT. 513 



caction. The anterior and posterior bands of litis ligament are 

 riiciproeally related to the anterior and posterior areas of the 

 femoral head, and of the acetabulum, as the ilio-fem(jral and 

 pubo-ischial femoral are. 



On titv Mtnriitcnfs of (lie Hip-Joint. — The movements of 

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

 posterior conical screw cond)ination in eacli joint. These com- 

 binations are both de.xiotrope or both scceotrope in each joint, 

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

 of the fundamental cones lie more or less oblicpiely across the 

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

 l>ointing, 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 lielicoid 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 tlic 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 e\'ident 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 fonn of the screwed surfaces of these 

 joints, and the peculiar configuration of their opposite arti- 

 cidar surfaces, that thev dn not admit of the movements 

 technically termed adduction, abduction, and rotation. 



The anterior and posterior screw combinations of the kneo- 



