u 



the manner we are about to describe, one of tbe 

 turns will give force, forward or backward, to the 

 ensuing spring, and, as in the case of the spine, or 

 rod, the other turn will control the direction of the 

 spring. 



§ 47. We may consider the cylinder formed by 

 the spine, the ribs, and the ligamentous connec- 

 tions between the rib ends, as a compound spine, in 

 which the idea that the elastic rod of our previous 

 discussion should be able to release its twists of 

 one form, so as to accept those of the replacing 

 form, is carried out. 



If this be allowed, we see that the fundamental 

 action of every portion of the machinery for loco- 

 motion is the action of the double twisty viz., a turn 

 in one direction met bv a turn in the contrary 

 direction, and under the rule that one of these 

 turns being Hberated it is guided as to the direc- 

 tion of its discharge by the constraining influence 

 of the other. 



For example, a rib faced with its concave for- 

 ward, by reason of the contour of the spine, under 

 its articulation, and, becoming twisted by a turn 

 in fche contrary direction, will, finally, with the 

 reversing action of the cross-line of its spinal S, 

 spring at the moment when the shape of the spine 



