MECHANISM OF THE KNEE-JOINT. 245 



the latuial parts ui' fibro-cai-tilage and ligamentous texture 

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

 corresponding ])ortions of the femoral condyle.s. There Is 

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

 nmch of each combination as is necessaiy for the required 

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

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

 the former, and when the combination is screwed home these 

 ligamentous bundles are in a state of tension. In the proc&ss 

 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 ; wliile 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 theii* puqjose, are no longer required, permit a 

 movement to be superadded to this form of organic screw, 

 which the artificial screw does not admit of. Tlie relaxation 

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

 roU as well as gUde 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 oft' 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 gi'eater extent between the central 

 margins of the femoral condyles and the surface of the spine 

 of the tibia. 



