68 



will be formed for each S two arches in the perpen- 

 dicular plane, one for each C. The upper lines of 

 the arch repesents the opposed convexities which have 

 met in their passage over (§ 60). The loiver lines the 

 corresponding concaves. 



§ 72. We will now follow out the details of 

 forcing, or attempting to repeat, the various move- 

 ments of the head-ball and socket — assuming the 

 usual order of the actions, and that the gathering 

 from the diagonal left anterior and right posterior 

 winding lines has been carried to the j^oiut of dis- 

 charge, which implies that it can be carried no 

 further. We shall also speak of only a single S. 



As regards the ball and the socket themselves, 

 the attempt at forcing the left anterior point of ap- 

 plication to repeat its course along the circumfer- 

 ence of the left side of the socket will cause the 

 pressure of the ball to slij) over to the right side 

 of the socket, by the rear — and when the rigid pos- 

 terior winding line is subsequently forced, an at- 

 tempt at a repetition of the effects of that line at 

 its head-end will take place in the movement of 

 the right side of the socket, producing an analgous 

 result, viz. : the withdraAval of the socket on the left 

 side by the rear, the countei-part of its previous ac- 

 tion on the right side. Thus a cross-strain from 



