274 JOHN C. KOCH 



2. The inner architecture of jiormal bone is determined by the 

 definite and exact reqiiironionts of mathematical and mechanical 

 laws to ])rodiice a maximum of strengtli with a tnininmm of 

 material. 



Further, the observations here recorded for the femur, the 

 largest and heaviest bone of the body, must in a general way 

 hold true for all the bones of the body: else we must assume 

 the absurd conclusion that the structure of the femur is in con- 

 formity with mechanical laws and the other bones are based 

 upon other unknown laws. The numerous experiments by Rau- 

 ber and Messerer in testing human bone to destruction have 

 shown that the physical properties of all the larger bones of the 

 body are substantially the same. Hence, the laws formulated 

 above for the femur must hold in general for all the bones of the 

 body. 



PART VI. MISCELLANEOUS AND CONCLUSIONS 



The factor of safety in the human femur 



Factor of safety, working stress. To assure the safety of any 

 structure there must be no danger of breaking in any part un- 

 der the heaviest loads for which the structure is designed. The 

 greatest stress induced in any part of a structure by the heavi- 

 est loads which it will be called upon to carry must never ap- 

 proach the breaking strength of the material. The working 

 stress for any material is that unit-stress (pounds per square 

 inch, kilograms per square centimeter) which has been found safe 

 to permit in that material and provide a necessary degree of 

 security against breaking. Usually working stresses are de- 

 termined by experiment. The working stress is the unit-stress 

 employed in determining the sizes of structural members of 

 any given material. The factor of safety is the ratio of the 

 ultimate strength of the material to the working stress. 



In the design of structures the purpose of the factor of safety 

 is to guard against the unavoidable defects in the structural 

 material employed which might reduce the ultimate strength 

 of the material, and also to provide for the possibility of an 

 unforeseen increase in the load to be carried. In wooden struc- 

 tures the factor of safety is usually about 10, in steel from 4 to 5. 



