LAWS OF BONE ARCHITECTURE 229 



The neutral axis BCDE, figure 14, is drawn to pass through 

 the niaxnnuni number of centers of gravity of the transverse 

 sections. The few exceptions where it fails to pass through the 

 center of gravity occur where the cross section undergoes a 

 marked change in shape and size with consequent distortion of 

 the position of the neutral axis. 



4. Moment of inertia. The graphical method of determining 

 the moment of inertia has been used throughout as described in 

 Part II (46-48)- The two axes about which the moments of 

 inertia have been calculated are at right angles. The axis which 

 is of chief importance in the analysis of loads on the femur-head 

 is indicated by A-A in Plates 1-5, and in all tables and diagrams. 

 This axis lies in the neutral plane. 



The difference in the consistency of the bone in the various 

 sections is properly allowed for in the following manner : Where 

 the section is" entirely composed of cancellated bone the moment 

 of inertia is computed as for compact bone having the same size 

 and shape. The section is then carefully weighed and the ratio 

 of the weight of the cancellated bone to compact bone of the 

 same area and thickness is determined, this ratio being called 

 the ratio of consistency of the cancellated bone. The effective 

 moment of inertia is then found by multiptying the moment of 

 inertia of compact bone by the ratio of consistency of the can- 

 cellated bone. This is the effective moment of inertia of that 

 section in terms of compact bone. 



Where a section is composed of both compact and cancellated 

 bone, the area of each is determined, and the moment of inertia 

 is computed separately. The section is carefully weighed. 

 The weight of the compact bone is computed for a section of the 

 same area and thickness, and this is subtracted from the total 

 weight of the section, giving the weight of the cancellated bone 

 in the section. The ratio of the weight of the cancellated bone 

 to the weight of compact bone of the same area and thickness is 

 found, and is the I'atio of consistency as previously defined. 

 The effective moment of inertia of the cancellated bone in the 

 section is equal to the product of the moment of inertia of the 

 cancellated bone bv its ratio of consistency. The total effective 



