MECHANISM OF THE KNEE-JOINT. 243 



The result of these two combinations is, that in complete 

 extension the anterior combination is screwed home, while 

 the posterior is unscrewed. In complete flexion, again, the 

 anterior combination is unscrewed and the posterior is home. 

 For the purpose of understanding how the screw-movements 

 of the femur and tibia round two axes alternately, both of 

 which are only slightly, if at all, inclined to the axis of the 

 limb, should result in the so-called flexion and extension at 

 the knee, it is necessary to consider some of the properties of 

 conical screws, and of the peculiar modification of them which 

 is presented by the mechanism of the knee-joint. A conical 

 screwed nut and tap, if both constructed of comparatively 

 unyielding materials, do not coincide, and consequently do 

 not afford any serviceable result as a screw-combination till 

 they have been screwed home. Conical screw-combinations 

 are consequently rarely employed in the arts ; and when they 

 are made use of, the male or female screw, more generally the 

 latter, consists of a yielding, elastic, and tough material. The 

 opposed threads of a conical screw-combination constructed of 

 such materials are throughout congruent with one another ; 

 and that amount of friction consequently procured, which 

 constitutes an element of the productive effect of a cylindrical 

 screw-combination ; and the total absence of which renders a 

 conical screw-combination of unyielding materials inefficient 

 till screwed home. In the knee-joint the concave screws on 

 the articular femoral surface are comparatively unyielding ; 

 while the convex screws $n the tibial surface are only par- 

 tially cartilaginous, but mainly flexible, elastic, and tough. 

 Their movements, therefore, are precise, and when screwed 

 home the opposite elements of each combination become fixed. 

 In screwing and unscrewing a conical combination of unyield- 

 ing materials up to the completion of the one process, and 

 from the commencement of the other, the concave and convex 

 screws are non-coincident. Both processes may be conceived 



