ACTING FACETS OF AUTICULAU SURFACES. 249 



a facet by the more limited sweep and apparently greater 

 rapidity of its longitudinal curvature. 



10. It is evident, therefore, that these helical lines of cur- 

 vature cannot be conceived as developed on a cylindrical 

 surface ; they must lie in some surface of a conical form. 

 The lines may, however, be provisionally assumed as conical 

 helices of a given curvature. 



11. The lines of movement across the thread are also curves 

 of double curvature. The curvature of these transverse lines, 

 like that of the longitudinal, while it maintains its character 

 throughout, is more limited in extent, with less sweep, and 

 therefore apparently more rapid as it approaches the proximal 

 margins of the elements. The transverse lines also have 

 their centres of curvature towards the proximal extremities 

 of the facets. The concave margnn also is of less extent than 

 the convex, and the proximal extremity narrower than the 

 distal. 



12. It also follows that the margins of the twin-facets 

 must be concave towards the centres of longitudinal curva- 

 ture — i.e. the marginal outlines of the two facets respectively 

 must be, as they are in fact, concave on one side, convex on 

 the other. 



18. As already stated, the actual movement of the twin- 

 elements of an ai-ticidar couple is the resultant of their 

 primary and secondary movements. This residtant movement 

 may be effected by either of the two elements on the other, or 

 by both simultaneously. The resultant or actual movement 

 of an articular couple occurs alternately in opposite direc- 

 tions along the same path ; when completed in one direction 

 the articular elements are in a maximum of contact ; and 

 when the movement is repeated in the opposite direction, 

 they are in the position of minimum contact. I tind it con- 

 venient, in treating of the movements of joints, to employ the 

 terms positive and negative for the two opposite relative posi- 



