224 JOHN C. KOCH 



111 this lij;-uro no ntteiu])! is made to show the internal struc- 

 ture, merely the outline of the femur in the plane described is 

 given. 



In order to compute the internal stresses in such a structure 

 it will be necessary first to consider the external loads that 

 produce these stresses. 



2. External loads. A. The effect of muscle action. It may 

 be taken as axiomatic that muscles act almost entirely in ten- 

 sion. On the contrary, bone has great strength in resisting 

 both tension and compression. The chief function of the bones 

 from the mechanical point of view is to serve as supporting 

 structures, the stresses in bones being chiefly compressive. 



In the femur, the difficulty of taking into consideration the 

 effect of the action of the muscles is, first : the wide variations in 

 the amount of the forces exerted by them, and second, the rela- 

 tive effect of muscle action is small as compared with the load 

 on the femur when the weight of the body rests on the femurs 

 either in the standing or w^alking positions. The tensile strength 

 of compact bone is about 230 times as great as that of muscle 

 for similar areas of cross section. The net area of cross sec- 

 tion of a normal femur may be taken roughly as about 0.80 

 square inches of compact bone. To produce sufficient tension 

 by the action of the muscles to develop the full strength of the 

 femur there would be required about 170 square inches of cross 

 section of muscle. As the actual cross sectional area of the 

 muscles of the thigh at about the middle of the shaft is about 

 25 square inches, the greatest possible action of the muscles 

 could develop only one-seventh of the strength of the femur. 

 Furthermore, maximum tension in the muscles of the thigh 

 takes place in general groups, the flexors alternating in action 

 with the extensors, so that in reality only about half of the area 

 of the muscles is in maximum tension simultaneously. This 

 would develop only about one-fourteenth of the maximum 

 strength of the femur. 



From the foregoing it is evident that, though it is recognized 

 that the action of the muscles exerts an appreciable effect on 

 the stresses in the femur, it is relatively small and very com- 



