112 THE CANCELLI OF BONES 



force be applied, when it yields it will bend into 

 an an<rle on the concave side, but the convex 



O ' 



side still retaining its curve. The tenacity of the 

 material being greater than its rigidity, it yields 

 to pressure rather than tension, the concave side 

 of the tube being compressed, while the convex 

 stretches. The same effect will be still better 

 seen in bending the branch of a tree, when the 

 bark, if it yield on the convex side, will be torn 

 asunder, whereas on the concave side it is thrown 

 into folds. The shell of the neck of the thigh 

 may be regarded as a bent tube, and is adapted 

 to resist pressure by its oval form, the longest 

 axis being vertical ; and secondly, by the greater 

 thickness of the concave side of the neck, to 

 which the weight is more directly transmitted, 

 and which in consequence of its curved form is 

 more likely to yield to compression than the con- 

 vex surface on the opposite side to traction. 



The walls of the bone are still further sup- 

 ported by the disposition of the cancelli, which 

 act as so many braces within. In addition to 

 this, however, these last form a segment of an 

 arch, and themselves support directly a portion of 

 the weight of the body, and transmit it to the 

 walls of the neck. If, on the application of weight 

 to the head of the bone, the neck yield at all, the 

 effect will be tension of the fibres (b b) ; and in 

 consequence of their resting beneath upon the 

 fibres (a a), compression of these last. 



