CHAPTER XV. 



THE SHAPES OF VERTEBKATE SKELETONS. 



254. WHEN an elongated mass of any substance is 

 transversely strained, different parts of the mass are ex- 

 posed to forces of opposite kinds. If, for example, a bar 

 of metal or wood is supported at its two ends, as shown in 

 Fig. 281, and has to bear a weight on its centre, its lower 



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part is thrown into a state of tension, while its upper part is 

 thrown into a state of compression. As will be manifest to 

 any one who observes what happens on breaking a stick 

 across his knee, the greatest degree of tension falls on the 

 fibres forming the convex surface, while the fibres forming 

 the concave surface are subject to the greatest degree of 

 compression. Between these extremes the fibres at different 

 depths are subject to different forces. Progressing upwards 

 from the under surface of the bar shown in Fig. 281, the 

 tension of the fibres becomes less; and progressing down- 

 wards from the upper surface, the compression of the fibres 

 becomes less; until, at a certain distance between the two 

 surfaces, there is a place at which the fibres are neither ex- 

 tended nor compressed. This, shown by the dotted line in 

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