246 CURVATURES AND MOVEMENTS OF THE 



XL— ON THE CUKVATUEES AND MOVEMENTS OF 

 THE ACTING FACETS OF ARTICULAR SURFACES.* 



1. The opposite gliding surfaces of joints employed by me- 

 chanicians are surfaces of revolution ; and consequently all 

 sections of these surfaces, at right angles to their axes of 

 rotation, are circular arcs. In all uncompounded artificial 

 joints, therefore, except those with spherical surfaces, the 

 movements are limited to rotation in opposite directions round 

 a single axis. 



2. In organic joints, on the contrary, the opposite gliding 

 surfaces are not surfaces of revolution ; they are not cylindrical, 

 conical, or spherical, in the geometrical sense of the terms. 

 In no instance, as far as I have observed, is a section at right 

 angles to the assumed or so-called axis of rotation, the arc of 

 a circle ; nor, as it appears to me, is it possible to associate 

 the characteristic curvature of the path described by a given 

 point in the bone or limb to which the joint appertains, with 

 articular surfaces of revolution. 



3. As stated in my former communication " On the Me- 

 chanism of the Knee-Joint," t the opposed surfaces of organic 

 joints are not continuous but faceted areas ; and of the 

 various kinds of facets indicated by me as existing on opposite 

 articular surfaces, those termed "acting facets" determine the 

 movements of the bones to which the joint appertains, and 



* This memoir had evidently been carefully prepared for publication, but 

 had not been sent to press. It is now published, therefore, for the first time. — 

 Eds. 



+ Proc. Rorjal Soc. EdinhurgJi, Jan. 18, 1858 ; and No. X. of this volume. 



