LAWS OF BONE ARCHITECTURE 277 



for the stresses due to running is 5.68, which is also the factor 

 of safety of the entire femur as it is the factor of safety of the 

 weakest section of the femur. 



The middle third of the femur is more exposed to injury from 

 blows, falls and other accidents than other parts of the femur. 

 For this reason the factors of safety of the bone in this region 

 will be of considerable interest. Referring to figure 17, it will 

 be seen that in sections 20 to 44, comprising the middle third, 

 the imit-stresses are relatively high and at sections 20, 28 and 

 36 approach the maximiun values reached at section 8. Based 

 on the ultimate strength of compact bone at 24,000 and 17,700 

 pounds per square inch for compression and tension, respectively, 

 the following factors of safety are found for the various sections 

 of the middle third for the stresses due to running: 



The minimum factor of safety is 5.98 at section 28; the maxi- 

 mum factor of safety in this region is 7.68 at section 44: the 

 average value of the factor of safety is 6.60. Thus the factor of 

 safet}^ in the middle third is from 6 to 35 per cent greater than 

 at the weakest section in the neck of the femur. These factors 

 of safety are all based on the stresses due to running, with the 

 load on the femiu'-head as in life. 



Blows striking the femur will be at an angle to its axis, the 

 maximum destructive effect being produced when the blow is 

 directed at right angles to the axis of the bone. In the standing, 

 walking and running positions nearly all blows will be received 

 from a direction other than medial. Hence, such blows will tend 

 to produce bending in the femur in a direction opposite to that 

 produced by the load carred on the femur-head, so that these 

 bending stresses of opposite character tend to balance each 

 other. 



In figure 27 the factors of safety in successive sections of 

 the femur are shown for the stresses due to running. Those 

 due to walking and to the standing position are proportionally 

 larger and therefore need not be shown. 



In the figure the positions of the heavier concentrations of 

 stress in the femur due to muscle action are shown as follows: 

 at the greater trochanter, where the glutei, quadratus femoris 



