THE SHAPES OF VERTEBRATE SKELETONS. 195 



only, of alternating intensities, becoming at the centre small 

 and equal ; and from the centre we advance, through a 

 reverse series of changes, to the other side. 



Thus it is demonstrable that any substance in which the 

 power of resisting compression is unequal to the power of 

 resisting tension, cannot be subject to alternating transverse 

 strains, without having a central portion differentiated in its 

 conditions from the outer portions, and consequently dif- 

 ferentiated in its structure. This conclusion may easily be 

 verified by experiment. If something having a certain tough- 

 ness but not difficult to break, as a thick piece of sheet lead, 

 be bent from side to side till it is broken, the surface of frac- 

 ture will exhibit an unlikeness of texture between the inner 

 and outer parts. 



§ 255. And now for the application of this seemingly-irre- 

 levant truth. Though it has no obvious connection with the 

 interpretation of vertebral structure, we shall soon see that it 

 fundamentally concerns us. 



The simplest type of vertebrate animal, the fish, has a 

 mode of locomotion which involves alternating transverse 

 strains. It is not, indeed, subjected to alternating transverse 

 strains by some outer agency, as in the case we have been 

 investigating : it subjects itself to them. But though the 

 strains are here internally produced instead of externally 

 produced, the case is not therefore removed into a wholly 



different category. For sup- ^ ~~_^^ ^#4 



posing Fig. 284 to represent £ll^^£^^^3^^^^^H =2£ « aM -r- 



the outline of a fish when *>- --""" 



bent on one side (the dotted lines representing its outline 

 when the bend is reversed), it is clear that part of the sub- 

 stance forming the convex half must be in a state of tension. 

 This state of tension implies the existence in the other half 

 of some counter-balancing compression. And between the 

 two there must be a neutral axis. The way in which this 

 conclusion is reconcilable with the fact that there is tension 



