260 CURVATURES AND MOVEMENTS OF THE 



with increasing and diminishing series of contacts, and in the 

 same manner as a single couple ; but with this advantage, that 

 while the single couple, in its negative phase and in its minor, 

 degrees of contact, is supported on one side only of the axis, 

 an articular combination is supported by two equal contacts 

 on opposite sides of the axis. An articular combination is in 

 fact equivalent to a double-threaded screw, as a single articular 

 couple may be conceived as a single-threaded screw. 



33. The theory of the geometrical development of articular 

 surfaces, which I have endeavoured to express in this com- 

 munication, necessarily involves the possibility of dexiotrope 

 and scceotrope i.e., right and left-handed articular couples and 

 combinations. The two forms depend upon the direction in 

 which the generative curve revolves around the axis, moving 

 with the hands of a watch to the right in a dexiotrope, 

 against the hands of a watch in a scseotrope couple or combi- 

 nation. We find accordingly that the corresponding articular 

 couples and combinations on opposite sides of the body are 

 opposed to one another in the direction of their winding. 

 Thus, for example, it was pointed out in my former communi- 

 cation (p. 242) that the anterior articular combination in the 

 knee-joint is left-handed in the right knee and right-handed 

 in the left, and that the same relations obtain in the posterior 

 articular combinations of opposite knee-joints. In succeeding 

 communications I will point out that articular combinations 

 with opposite windings, on opposite sides of the body, similar 

 to those in the knee-joint, exist in the ankle and tarsal, and 

 in the elbow and carpal joints ; and that the hip and shoulder 

 joints consist of single-threaded couples, but also with opposite 

 windings on opposite sides of the body. 



34. The nature of that more or less marked restriction or 

 curtailment of one or both elements of a couple, already 

 adverted to in paragraph 5, may now be more fully examined. 

 This restriction or curtailment consists essentially in the 



