242 MECHANISM OF THE KNEE-JOINT. 



gliding and rolling movements of conical screwed surfaces 

 upon one another. To adapt this hypothesis to the structure 

 of the joint and the general character of the curves, it was 

 necessary to assume that the anterior areas of the combined 

 femoral condyles are portions of a double-threaded conical 

 nut ; the anterior parts of the combined tibial condyles, and 

 the corresponding part of the intercondyloid spine, with other 

 structures to be afterwards mentioned, are portions of a cor- 

 responding double-threaded conical tap. It was also necessary 

 to assume that the combined posterior areas of the femoral 

 condyles are portions of a second conical nut, and the com- 

 bined posterior portions of the tibial condyles and spine the 

 corresponding tap. The hypothesis, however, in addition re- 

 quired that the anterior screw-combination should be opposed 

 to the posterior ; if the anterior is a right-handed, the pos- 

 terior must be a left-handed screw. 



In the absence of numerical values for the co-ordinates of 

 the curves (which, however, I do not despair of procuring), I 

 cannot make a definite statement ; but the general character 

 of the curves observed, and the corresponding movements and 

 structure of the joint, leave little doubt in my mind that the 

 flexion and extension, combined gliding and rolling movements 

 of the knee are performed between two conical double-threaded 

 screw-combinations, an anterior and a posterior the anterior 

 being a left-handed screw, and the posterior a right-handed 

 screw in the right knee-joint ; the anterior a right-handed, 

 and the posterior a left-handed screw in the left knee-joint. 



The movements which take place round these two com- 

 binations are alternate those round the anterior completing 

 extension and commencing flexion ; those round the posterior, 

 completing flexion and commencing extension of the joint. 



The movements round the anterior combination are more 

 extensive and important than those round the posterior ; and 

 in the ordinary use of the joint are alone employed. 



