280 JOHN C. KOCH 



of safety are in every case greater than in tlie adjacent sections. 

 The increase in the factor of safety to about 10 in the region 

 of tlie lesser trochanter is small as compared with the increase 

 at the other points of concentration, and is to be accounted for 

 by the fact that the muscle action here produces a tensile stress 

 in the bone which is opposite in character to the compressive 

 internal stress produced by the load on the femur-head. Hence, 

 the two stresses tend to balance each other. 



In the region of the greater trochanter the internal stress 

 due to the muscle action is tensile as are also the internal stresses 

 in this region due to the load on the femur-head: in this region 

 it is seen that the factor of safety varies from 10.00 to 15.85, 

 although in the neck where no muscles are attached the factor 

 of safety is only 5.68. 



At the distal end of the femur the factor of safety progres- 

 sively increases as the extremity is approached. The increase 

 follows approximately the magnitude of the muscles attached. 



From a study of figure 27 it is evident that the larger 

 muscles exert considerable influence on the structure of the 

 bone as shown in the discussion above, and it is clear that ad- 

 ditional strength is provided at the points where the larger mus- 

 cular loadings occur. The actual factor of safety, therefore, is 

 much more uniform for the normal load transmitted through 

 the femur-head than is indicated by the curve plotted for the 

 factor of safety in this figure. 



It may be concluded that blows received with the femur carry- 

 ing a load as in standing, walking or running, the maximum 

 effect will be produced by a blow from the medial side and the 

 minimum effect by a blow from the lateral side of the loaded 

 femur. 



It is evident from the larger factors of safety in the middle 

 third that there is greater relative strength to resist blows at 

 the middle third than in the upper femur. 



The factor of safety of the femur for the greatest stress due 

 to running has been shown to be 5.68; for walking it will be 

 11.36 and for the standing position it will be 30.30: these values 

 are based on maximum values of the ultimate strength of bone. 



