ACTING FACETS OF ARTICULAK SURFACES. 251 



pass from the positive to the negative* phase without losing 

 entirely their stability is by a combination of the primary and 

 secondary movements for any attempt to glide the one 

 element over the other, except by the double movement, im- 

 mediately destroys the congruence of the couple. It will also 

 be observed that the twin-elements, when in their positive 

 position, are fixed or adjusted in their proper localities ; and 

 that the successive extents of congruence during the action of the 

 couple are directly as the approximation to the positive, and 

 inversely as the approximation to the negative phase. 



16. The successive extents of congruence appear to be 

 determined by the successive adaptation of corresponding por- 

 tions of curvature on the opposite elements, as these elements 

 pass through their opposite movements ; for it appears as if 

 the gliding of the twin-elements across the thread not only 

 successively accommodates their transverse, but also their 

 longitudinal lines of curvature as they respectively meet one 

 another. 



17. During the gliding in the lines of the transverse cur- 

 vatures in passing from the positive to the negative phase, 

 the two bones on which respectively the twin-elements are 

 situated gradually incline on one another towards that side of 

 the articular couple on which the proximal margins of the 

 elements are situated. As it will appear in the sequel that in 

 some couples the proximal margins are on the concave, but 

 in others on the convex sides of the couple, the inclination of 

 the two bones will be to the concave side in the former, to 

 the convex side in the latter. 



18. During the gliding in the lines of longitudinal cur- 

 vature in passing from the positive to the negative phase, the 

 two bones incline towards the distal extremity of the couple 

 if that couple has a concave proximal side, and towards the 

 proximal extremity of the couple if that couple has a convex 

 proximal side. 



