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They have shown that where, for lower animals, 

 certain parts of the higher ones disappear, repre- 

 sentative pieces may yet often be found, as for ex- 

 ample, pieces of bone, apparently useless, but 

 answering in position to the hip-bones of higher 

 animals, on the bodies of some serpents. They 

 have also made it very probable that Ugs and arms 

 are hut metamorphosed ribs. 



§ 11. From this close analogy of structure, we 

 may infer a close analogy as to the principles 

 of locomotion among these animals, and we may, 

 therefore, look to those in which the motions of 

 the spine are the most obvious, and in which these 

 motions most immediately produce locomotion, for 

 a clue to its more obscure action in quadrupeds 

 and man. 



Assuming, then, that the spine is the true basis 

 of all movement ; that its deformity, brought about 

 by permanent abnormal flexures, arising from an 

 unequal counter-action between muscles intended 

 to balance each other, is (where the frame is 

 otherwise healthy) the true cause of distorted ac- 

 tion ; and, that, to redress the shape of the spine, in 

 these cases, ivill be to redress the faults of motion in 

 all parts dependent on the spine, we proceed, in 

 the first place, to inquire how the spine acts. 

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