LAWS OF BONE ARCHITECTURE 233 



Table 5 gives the area of spongy and of compact bone, the 

 equivalent area of compact bone, the moment of inertia, and 

 other data for each of the transverse sections analyzed. These 

 data are called the i^roperties of the sections. From the infor- 

 mation given in this table the effects produced at every section 

 b}^ the external load may be computed. The moments of in- 

 ertia are computed about both principal axes, A-A and B-B, 

 for completeness; but only those about axis A-A are required 

 to compute the effects of the load on the femur-head. 



6. Bending strength. The bending strength of the femur at 

 each section is measured by the section modulus of the section. 

 The general formula for the determination of the section modulus 

 has been given in Part II (par. 4.^)- The values of the section 

 moduli are given in table 5, columns 14 and 21 (p. 235). 



The bending strength of the femur about any other plane than 

 that of A-A may be determined, since the moment of inertia is 

 computed about the two principal axes A-A and B-B which are 

 at right angles and these are principal moments of inertia. 



7. Torsional strength. The resistance to twisting action is 

 known as torsional strength, and it is measured by the sum of the 

 moments of inertia about the two principal axes, A-A and B-B 

 in the sections shown in plates 1-5. 



Practical^ all muscles, being attached to the surface of the 

 bones, must produce twisting to a certain degree when exerting 

 a pull upon any bone. The extent of the twisting or torsional 

 stress depends upon the amount of the pull and the angle be- 

 tween the direction of the pull and that of the longitudinal axis 

 of the bone. Such stresses combine with the maximum tensile 

 and compressive stresses in the femur (and other weight-bearing 

 bones) to produce still greater stresses than those due solely to 

 the weight borne by the bone. 



A further consideration of the manner of attachment of muscle 

 to bone makes clear that in addition to the torsion in the bone 

 produced by a muscle pull there is a tensile force acting in a 

 direction opposite to the pull and a compression in the bone, 

 in the fibers, between the attachment of the muscle and the 

 joint about which the force is exerted. 



