164 



prepare for the spring forward, by refusing the 

 discharge and continuing to develope the original 

 hues of gathering. While the alternate anterior 

 line is kept from forming (§70 snake's motion), the 

 head-joint, both as to its ophidian and separate 

 condyle (or limb) motion, will change to the alter- 

 nate bearings, and the effect of this will he—frst, 

 to change the appuis to the alternate legs ; and 

 second, on these concave appuis to discharge the 

 old lines and form the new ones. 



This delaying the spring until all the concaves 

 have actually changed to convexes, and thus mak- 

 ing that change on the ground entirely, instead of 

 finishing it in air, again brings the head condyles, 

 in advance, into their alternate position, and puts 

 the concaves in appui for the next step. 



We think that Jive points of difference from pro- 

 gression must be produced by that accumulation 

 of action at the head-joint which, in retrogression, 

 at each step keeps the balance of gravity on the 

 concaves. (1.) The filling of the alternating lung 

 lobes must be in advance of that in progression. 

 (2.) The passage of the convexes over the con- 

 caves (§ 60) must be reduced to a minimum, if, in- 

 deed, the contrary action do not have place. (3.) 

 The thiTist of the feet of appui, depending now on 



