258 CURVATURES AND MOVEMENTS OF THE 



spiral, as it simultaneously glides along and revolves around 

 the axial line, to do so with its polar extremity directed either 

 towards or away from that axis. In the former case, the 

 proximal or polar margins of the resulting elements will 

 constitute the concave side of the couple, which will be con- 

 sequently directed towards the axis. In the latter case, the 

 polar or proximal margins of the resulting elements will 

 constitute the convex side of the couple, and will therefore be 

 turned away from the axis. These two forms of articular 

 couples are therefore also respectively distinguished by the 

 direction of their secondary or transverse gliding ; when the 

 polar side of the couple is concave, or towards the axis, the 

 transverse gliding of the elements, during their passage from 

 the positive to the negative phase, is such as that the inclina- 

 tions of the two portions of a right line passing perpendicularly 

 through the couple will be towards the axis, and away from 

 it on the return from the negative to the positive. When, on 

 the other hand, the polar margins of the elements are on the 

 convex side of the couple, the elements glide in such a 

 manner that the two halves of the perpendicular line incHne 

 away from the axis, and towards it on the return to the posi- 

 tive phase. As the collocation of these two forms of articular 

 couples constitutes an important feature in the construction 

 of certain joints, and as they demand therefore distinct 

 designations, I shall employ the term axial to indicate a 

 couple the proximal side of which is towards the axis, and 

 the term antaxial to indicate a couple the proximal side of 

 which is convex. 



32. In certain joints, each articular couple acts in concert 

 with a second articular couple, wliich is developed around the 

 same axis, but on the opposite side. For this arrangement I 

 shall employ, as in my former communication on the knee- 

 joint, the term articular combination. If we assume, as I 

 have done throughout the present communication, that the 



