248 CURVATURES AND MOVEMENTS OF THE 



to be complete — Le. unrestricted — and therefore congruent or 

 in contact tliroiighout at the commencement of action, the 

 congruence becomes a minimum at the close of action ; and the 

 twin elements may even be completely separated from one 

 another in the subsequent movements of the joint. The two 

 movements are simultaneous, so that the actual movement is 

 their resultant. 



7. The two movements determine the contours of the 

 facets. The most extended movement, which determines the 

 length of the facets, I shall term the primary movement, or 

 (for reasons to be afterwards stated) the movement along the 

 thread. The two extremities of the facets will be referred to 

 aspvximal and distal. The other movement I shall term 

 secondary, or the movement across the thread. This movement 

 determines the breadth of the facets and their two margins 

 respectively ; subject, however, like the lengths and extremities 

 of the facets, to the restrictive modification of the two margins 

 of each element respectively, the one has a higher teleological 

 import than the other. I shall refer to the former as the 

 proximal, to the latter as the distal, margin. 



8. When, therefore, the elements of a couple are in their 

 fundamental positions — i.e. when they are in contact through- 

 out — their extremities and margins are also coincident respect- 

 ively ; but in consequence of the frequent restriction of por- 

 tions of one element, the corresponding portions of the other 

 are not in contact with correlative portions of surface, but with 

 synovia, synovial pads, or elastic menisci. 



9. The lines of the thread, or of primary movement, which 

 extend from the proximal to the distal extremities of the 

 facets, are curves of double curvature of a helical form. The 

 curvature of these lines, while maintaining its general cha- 

 racter throughout, diminishes in extent or sweep, appearing 

 more rapid as it approaches the proximal extremities of the 

 facets ; and therefore I distinguish the proximal extremity of 



