LAWS OF BONE AKCHITECTURE 267 



It has been shown that, hi general, the trabeculae of the ten- 

 sile system are lighter in structui'e than those of the compression 

 system in corresponding positions. The significance of the dif- 

 ference in thickness of these two systems is that the thickness 

 of the trabeculae varies with the intensity of the stresses at any 

 given point. Comparison of figure 21 (also see fig. 28) with 

 figure 19 a, will show that the trabeculae of the compression system 

 carry heavier stresses than those of the tensile system in corre- 

 sponding positions. For example, the maximum tensile stress at 

 section 8 (fig. 19 a) in the outermost fiber is 771 pounds per 

 square inch, and at the corresponding point on the compression 

 side the compressive stress is 954 pounds per square inch. Simi- 

 lar comparisons may be made at other points, which will confirm 

 the conclusion that the thickness and closeness of spacing of the 

 trabeculae varies in proportion to the intensity of the stresses 

 carried by them. 



As the trabeculae of both systems approach the level of the 

 lesser trochanter, they become thinner and farther apart. The 

 explanation for this lighter structure is seen in the much lower 

 stresses in both systems in this region, as shown in figure 19 a. 



It will be seen that the trabeculae lie exactly in the paths of 

 the maximum tensile and compression stresses (compare figs. 21, 

 19 and 19 a), and hence these trabeculae carry these stresses 

 in the most economical manner. This is in accordance with the 

 well-recognized principle of mechanics that the most direct 

 manner of transmitting stress is in the direction in which the 

 stress acts. 



Further significance of the spongy structure of the upper femur 



The question may well be asked. Why should the head and 

 neck of the femur be composed so largely of spongy bone, with 

 a gradual transition to compact bone in the hollow shaft? Re- 

 ferring to figure 16, it is seen that the amount of the vertical 

 shear varies almost uniformly from a maximum of 90 pounds 

 (90 per cent of the load on the femur-head) midway between 

 sections 4 and 6, to a minimum of— 5.7 pounds at section 18. 



