BY W. J. McKAY. 977 



If now the anterior one be moved so that its front portion 

 turns to the left, while its hinder extremity goes to the right, we 

 shall observe the following order of events : — the postzygapo- 

 physis of the right side moves outwards and forwards ; this brings 

 the articular facet, with the projection on its posterior edge, (vide 

 supra) forward, so that the facet rests mainly on the projection 

 on the anterior edge of the prezygapophysis beneath. There is 

 no obstacle to the movement of this zygapophysis in an outward 

 direction beyond that offered by the ligaments. Meanwhile the 

 postzygapophysis on the left side has moved inwards and back- 

 wards. This brings the facet, with its projection on the posterior 

 edge, backwards and inward, and the projection now coming into 

 contact with the sides of the lamina prevents any further move- 

 ment in that direction. If we had had another vertebra in front 

 of our anterior one, we of course would have found that it was 

 the right anterior zygapophysis that was stopped in its motion by 

 coming into contact with the pedicle of the front vertebra. 



Thus if we represent the points of movement as taking 

 place at the angles of a square, we shall see that at the two 

 extremities of one diameter we have an obstacle to further 

 motion, while at the extremities of the other diameter we have 

 comparative freedom. 



To these considerations we must now add the movements of the 

 zygosphene and zygantrum. With regard to these, the same side 

 that received a check above, will receive one now. And if we add 

 to this the opposition afforded by the ball and socket joints of the 

 centrum, we shall see that whenever one vertebra of a series moves 

 from side to side, its movement becomes limited by bone in four 

 places, and by ligaments in ten, or in other words we have disloca- 

 tion opposed at fourteen points. 



Verticcd moveme^it. 



The middle one of three vertebrae is prevented from moving in a 



vertical direction to any great extent by the following surfaces. 



Anteriorly we have the zygosphene in its firm zygantrum, and 



also the prezygapophyses lying beneath the postzygapophyses of 



