62 



mentioned, and consisting (5 ' ) of the bringing back 

 of the point of pressure to the front centre, and 

 that moving forward of the socket which prepares 

 the liberation of the strain forward (b). The fur- 

 ther movement of the socket by rotation outward 

 in the opposite direction to that of the head-ball. 

 This is more particularly accompanied by retrac- 

 tion of the moving side of the socket, and by rais- 

 ing the other side it redoubles the pressure of the 

 head-ball. 



We have called these actions "virtual," because 

 they may, during a considerable part of their 

 movement, only represent thfe tractions of the 

 twisted lines of the spine. 



§ 68. The actions in the tody of the snake which 

 accompany these actions of the head joint, are 



Corresponding with (a), those of the secondary 

 sections of the anterior winding line (§ 25), causing 

 the prehminary part of the formation of a 

 greater or less number of S S curves, of which 

 the posterior C C are longer than the anterior, and 

 also the posterior part of each C longer than its 

 anterior part. In this way the posterior curvature 

 of each C carries more ribs than the anterior, and 

 thus, in each C the "propellers" preponderate over 



