LAWS OF BONE ARCHITECTURE 265 



tern which is transverse to the first system, and which is arranged 

 hirgely in the form of radial Hnes. 



4. Structural features. The essential featm'es of the distal 

 part of the femur are the following: 



1. Gradual increase in the total area of the cross section with 

 practically no increase in the total equivalent area as compact 

 bone, from sections 52 to 70 inclusive, as shown in table 5, 

 column 7 (p. 235). 



2. The system of longitudinal trabeculae parallel to the longi- 

 tudinal axis of the femur. These trabeculae are braced by two 

 other systems of transverse trabeculae, which are practically at 

 right angles to each other and to the longitudinal system. 



3. With practically no increase in the amount of bony ma- 

 terial used, there is a greatly increased stability produced by the 

 expansion of the lower femur from a hollow shaft of compact 

 bone to a structure of much larger cross section almost entirely 

 composed of spongy bone. 



PART V. SIGNIFICANCE OF THE INNER ARCHITECTURE OF THE 



FEMUR 



General 



In Part III the femur was analyzed in detail by the methods 

 of applied mechanics for an assumed load of 100 pounds on 

 the femur-head. Within the elastic limit of materials the 

 stresses produced by any given load are proportional to the 

 load: a load of 100 pounds will produce stresses one hundred 

 times as great as those produced by a load of one pound. Hence 

 the analysis based on the assumed load can be readily adapted 

 by simple proportion to yield correct values of the stresses pro- 

 duced by other loads, provided the elastic limit is not exceeded. 



For these reasons the lines of maximum stress within the 

 femur are in the same position for a load of one pound or of 

 100 pounds, or any other load that does not stress the bone 

 beyond the elastic limit. As the elastic limit for almost all 

 materials is about one-half of the ultimate or breaking strength 

 of the material, these conclusions will apply to all loads ordi- 



