57 



§ 60. So soon as the continued advancement of the 

 'potential change of curvature extends its effects actual- 

 ly to the spine, a discharge is prepared in which the 

 posterior C is so far discharged and hfted as to crowd 

 forward the effect of propulsion on to the anterior 

 C, and their united discharge thrusts the body 

 forward by a movement in which the rear ribs of 

 an S really precede the front ones in leaving the 

 ground.^ 



The changing of the curvature of the convexes 

 from one side to the other has, owing to the con- 

 vex being the part which primarily foUows the 

 lines of traction, this characteristic, that iJie convex 

 passes over, the concave under, in making the 

 change. 



§ 61. The ribs on the concaves do not change their 

 facing until the actual change in the shape of the spine 

 brings this about by changing the direction of their 

 articulating heads. 



§ 62. The above stated conditions, when each 

 section of ribs has been twisted by the turns last 

 introduced into a position for discharging the pri- 

 mary ones, at once divide the ribs on any convex, 

 or concave (should such be in appui), into two sets, 



* The same crowdiug forward of the action would occur were there 

 a number of S S. 



