43 



does not move in its articulation, then, when, by 

 the formation of torsion curvatures in the spine, 

 the facings of the protuberances on the vertebrae 

 are changed, viz., to the front, by coming on the 

 anterior portion of a convex, or the posterior por- 

 tion of a concave, and to the rear by coming on 

 the posterior portion of a convex, or the anterior 

 of a concave — it is obvious, that the facings of the 

 ribs will be changed correspondingly. The con- 

 cave inner surface of each rib will be in the snake 

 turned toward the rear when the ball of its articu- 

 lation is turned to the front, and to the front when 

 the ball is turned to the rear. 



We shall ahvays speak of a rib as ^^ facing forward 

 or hackiuard,'' with reference to its concave surface. 

 Thus the ribs on the anterior half of convexities,, 

 and posterior half of concavities, face backward, 

 the ribs on the remaining halves forward. 



§ 46. If next, the ribs in any facing be pressed 

 against the ground, so that their ground ends are 

 firmly fixed ; then an altered facing of their articu- 

 lations, such as would be caused by the commence- 

 ment of a change to the opposite curvatures in the 

 spine, will introduce a twist into the C shape of 

 each rib, thus changing it into a twisted S ; and 

 when this twist is discharged at the articulation, in 



