250 CURVATUllES AND MOVEMENTS OF THE 



tions of maximum and minimum contact of their constituent 

 articular couples. In the positive position, an articular couple 

 is in a state of stable equilibrium. In the negative position it 

 is in the condition of unstable equilibrium. 



14. If the successive relative positions of the twin-ele- 

 ments of a couple are examined in series from their positive 

 to their negative phase, it will be observed that as the 

 proximal extremity of the one element glides towards the 

 distal extremity of the opposite element, the corresponding 

 margins of the opposite elements glide past one another ; so 

 that when the negative phase is attained it will be found that 

 the proximal portion only of the distal extremity of the one 

 element remains in contact with the distal portion only of the 

 proximal extremity of the other element. During the return 

 movement from the negative to the positive phase, the series 

 of successive relative positions is reversed ; thus the opposite 

 extremities and margins glide towards one another, so that 

 at the close of the movement the proximal portion of the 

 distal extremity of the one element, which in the negative 

 phase is in contact with the distal portion only of the 

 proximal extremity of the other element, now resumes its 

 original positive position, while the margins have also be- 

 come coincident, and the twin-elements consequently con- 

 gruent. The movement is thus of such a kind that in the 

 passage from the positive to the negative phase the cor- 

 responding extremities and margins of the twin-elements 

 recede from one another, until at last the proximal part of the 

 distal 'extremity, and the contiguous part of the proximal 

 margin of the one element, coincide with the distal part of 

 the proximal extremity and the contiguous part of the distal 

 margin of the opposite element. 



15. As already stated (13), when the couple is in the 

 positive phase its elements are in stable equilibrium ; so it 

 may now be observed that the only mode in which they can 



