MECHANISM OF THE KNEE-JOINT. 245 



the lateral parts of fibro-cartilage and ligamentous texture 

 (semilunar discs and lateral ligaments). The nuts consist of 

 corresponding portions of the femoral condyles. There is 

 actually retained, however, for the service of the joint, only so 

 much of each combination as is necessary for the required 

 movements. The base of the tap is connected to the vertex 

 of the nut by the crucial ligaments, which form the apex of 

 the former, and when the combination is screwed home these 

 ligamentous bundles are in a state of tension. In the process 

 of unscrewing, the ligamentous bundles of the tap become, on 

 account of their mode of attachment to the vertex of the nut, 

 successively relaxed from the point downwards ; while this 

 graduated relaxation of the ligaments provides for the tension 

 necessary for the continued gliding screw-movement. The 

 successive relaxations of those ligamentous bundles, which, 

 having served their purpose, are no longer required, permit a 

 movement to be superadded to this form of organic screw, 

 which the artificial screw does not admit of. The relaxation 

 of the vertex of the tap permits the two threads of the nut to 

 roll as well as glide along. The nut rolls as well as glides on 

 its convex condyloid surfaces. But as only a limited extent 

 of the cartilaginous surface of the tap is adapted to the carti- 

 laginous surface of the nut, the latter would speedily roll and 

 glide off the former, if the latter were not prolonged in the 

 required direction. The rolling movement of the convex 

 margin of the nut is further provided for by the interposition 

 of the tough and elastic semilunar discs ; as, moreover, the 

 rolling motion increases from the axis to the periphery of the 

 combination, it takes place principally on these discs, while 

 the gliding or proper screwing motion, increasing proportion- 

 ally towards the axis, takes place chiefly between the opposite 

 cartilaginous surfaces of the central part of the condyles of 

 the two bones, and to the greater extent between the central 

 margins of the femoral condyles and the surface of the spine 

 of the tibia. 



