184 



that, in quadrupeds and man (and, we suppose, 

 in most species of the snake), the normal position 

 of the ribs for an outer bearing so interferes with 

 their course as to bring the first development of the 

 posterior lines also into the rear C C C, this being, 

 however, a reflex action from the suppressed 

 movements of their secondary sections in the front 

 C C C. Thus it is not until the working in the 

 front C C C is carried out that the posterior point 

 of application can act directly in its primary sec- 

 tion and move the posterior cross-line end. 



This peculiarity in the action of the posterior 

 winding lines is what secures progressive or (chang- 

 ing the appuis) retrogressive locomotion instead of 

 two springs, one to the rear and the other forward 

 and centering in the cross-line (§§ 31, 32). 



Third. — As every stage in the process of halting 

 is formed by induction from the continuance of the 

 first action, it follows that this first action, whether 

 of an anterior "point of application" by the head- 

 condyle movement, or of a posterior "point" by its 

 socket movement, must also be the last action, so 

 far as induction can carry the movement. Thus, 

 in forcing the left head condyle and its anterior 

 Hue (§ 71) the equahzing of the condyles must be 

 its final action ; and in forcing the right socket 



