ON DESIGN IN ANIMAL BODIES. . 205 



of the cylinder — that is, that the material is re- 

 moved from the centre, and accumulated on the 

 circumference, thus : — 



We find a spine or ridge, A, running along the 

 bone, B C, which, when divided by the saw in 

 a transverse direction, exhibits the irregularity, 

 whereof A is the section. 



The section of this spine shows a surface as 

 dense as ivory ; that part is, therefore, much more 

 capable of resisting compression than the other 

 part of the cylinder, which is common bone. This 

 declares what the spine is, and the anatomists 

 must be wrong who imagine that the bone is 

 moulded by the action of the muscle, or that the 

 spine is a mere ridge, arising by accident among 

 the muscles. It is, on the contrary, a strengthen- 

 ing of the bone in the direction on which the 

 weight bears. If we resume the experiment with 

 the piece of timber, we shall learn why the spine 

 is harder than the rest of the bone. If a portion 

 of the upper part of the timber be cut away, and 

 a harder wood inserted in its place, the insertion 



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