ACTING FACETS OF AJiTlCULAR SURFACES. 247 



are coiiseqiu-utly the fundameutal facets of tlie articular 

 areas. 



The object of the present communication is to put on re- 

 cord the general result of the observations I have made on 

 the configuration and movements of central articular facets 

 since the date of the paper above referred to ; and to submit 

 a theoiy of their probable geometrical character. 



4. The central facet on one articular area of a joint is 

 adapted to a corresponding central facet on the coiTesponding 

 opposite articular area of that joint. Tlie surface-curvatures 

 of the two facets are similar, so that the facets themselves may 

 be considered reciprocally as cast and mould of one another. 

 I shall have occasion hereafter to employ the teim articular 

 couple to designate collectively two opposite corresponding 

 facets of any kind ; and the individual facets of such couples 

 I shall term twin-elements. 



5. The twin elements are not invariably developed to the 

 same extent, one of them being generally only a portion of the 

 entire facet, the deficient portion being supplied by yielding 

 and elastic structure, and which may be replaced in the exa- 

 mination of certain joints by a cast in wax or plaster of the 

 corresponding portion of the opposite facet. This restriction 

 or curtailment of one of the twin elements is referable to that 

 principle of constructive economy w^hich is evinced in the 

 arrangements of organic mechanism, in the midst of their 

 general complexity ; and it will, moreover, appear in another 

 section of this communication that the retention of the whole 

 of both the twin elements is not essential to their pecidiar 

 mode of action. I find it convenient to employ the terms 

 reserved and restricted to distinguish the articular elements in 

 these respective conditions. 



6. If the two elements of an articular couple be observed 

 during action, they will be seen to glide past one another in 

 two directions in such a manner that, assuming both of tlieiu 



