LAWS OF BONE ARCHITECTURE 287 



Eveo^vhere the inner structure of the femur agrees closely 

 with that required by the static conditions imposed upon the 

 bone by the preponderant load on the femur-head. In running 

 the factor of safety of the normal femur analyzed is a minimum 

 at the narrowest part of the neck, where it is 5.71 for compres- 

 sion: for tension, the factor of safety at this section is 5.68. 

 These factors of safety depend upon the maximum strength of 

 bone as determined by tests which give the ultimate strength 

 of bone in compression at 24,000 pounds, and in tension 17,700 

 pounds per square inch. The factor of safety in the middle third, 

 which is most exposed to violence, is somewhat greater than in 

 the neck of the femur, being between 5.98 and 7.68, with an 

 average value of 6.60. These factors of safety are based upon 

 the maximum stresses that occur normally, which is assumed to 

 be when running. 



The factor of safety of the femur as a single structural ele- 

 ment of the skeleton is determined by the strength of the weak- 

 est section with respect to the normal maximum stresses at that 

 section. The weakest section of the femur with respect to the 

 maximum stresses that occur at the various sections is in the 

 narrowest part of the neck of the femur, about 2 inches from the 

 head measured along the axis of the bone. At this section, 

 which, therefore, determines the factor of safety of the entire 

 femur, the factor of safety for the stresses due to running is 

 5.68; for the stresses due to walking the factor of safety is 11.36 

 and for the standing position it is 30.30. 



The analysis of strength as outlined agrees with the actual 

 breaking strengths of femurs made by Messerer, for the loads 

 applied to the femur-head in the same direction as in life. The 

 analysis is also in agreement with Messerer's breaking-tests on 

 femurs by applying loads to the middle of the bone, which is 

 supported at both ends. 



Compression tests were made of the compact bone of one of 

 the femurs analyzed in this paper and the breaking strength 

 was found to be 25,100 pounds per square inch. 



Statistics of the location of fractures of the femur in two inde- 

 pendent series of 241 and 152 fractures, as given by Hyde and 



