THE SHAPES OF VERTEBRATE SKELETONS. 193 



the figure, is called in mechanical language the "neutral 

 axis." It varies in position with the nature of the substance 

 strained : being, in common pine-wood, at a distance of about 

 five eighths of the depth from the upper surface or three 

 eighths from the under surface. Clearly, if such a piece of 

 wood instead of being subject to a downward force is secured 

 at its ends and subject to an upward force, the distribution 

 of the compressions and tensions will be reversed, and the 

 neutral axis will be nearest to the upper surface. Fig. 282 

 represents these opposite attitudes of the bar and the changed 



£8% 



position of its neutral axis : the arrow indicating the direc- 

 tion of the force producing the upward bend, and the faint 

 dotted line a, showing the previous position of the neutral axis. 

 Between the two neutral axes will be seen a central space . 

 and it is obvious that when the bar has its strain from time 

 to time reversed, the repeated changes of its molecular con- 

 dition must affect the central space in a way different from 

 that in which they affect the two outer spaces. Fig. 283 is 

 a diagram conveying some idea of these contrasts in molecular 

 condition. If A B C D be the middle part of a bar thus 

 treated, while Gr H and K L are the alternating neutral 

 axes ; then the forces to which the bar is in each case subject, 

 may be readily shown. Supposing the deflecting force to 

 be acting in the direction of the arrow E, then the tensions 

 to which the fibres between Gr and F are exposed, will be 

 represented by a series of lines increasing in length as the 

 distance from Gr increases ; so that the triangle Gr F M, will 

 express the amount and distribution of all the molecular 

 tensions. But the molecular compressions throughout the 

 space from Gf to E, must balance the molecular tensions ; 

 and hence, if the triangle Gr E N be made equal to the tri- 



