THE SHAPES OF VERTEBRATE SKELETONS. 211 



angle G F M, the parallel lines of which it is composed (here 

 dotted for the sake of distinction) will express the amount 

 and distribution of the compressions between E and G. 



Similarly, when the deflecting force is in the direction of the 

 arrow F, the compressions and tensions will be quantitatively 

 symbolized by the triangles K F 0, and K E P. And 

 thus the several spaces occupied by full lines and by dotted 

 lines and by the two together, will represent the different 

 actions to which different parts of the transverse section are 

 subject by alternating transverse strains. Here, then, it is 

 made manifest to the eye that the central space between G 

 and K, is differently conditioned from the spaces above and 

 below it; and that the difference of condition is sharply 

 marked off. The fibres forming the outer surface C D, are 

 subject to violent tensions and violent compressions. Pro- 

 gressing inwards the tensions and compressions decrease 

 the tensions the more rapidly. As we approach the point G, 

 the tensions to which the fibres are alternately subject, bear 

 smaller and smaller ratios to the compressions, and disappear 

 at the point G. Thence to the centre occur compressions 

 only, of alternating intensities, becoming at the centre small 



