LAWS OF BONE ARCHITECTURE 211 



solid lines and those of maxinunn tension are indicated by 

 dotted lines. Fioure 12a is a similar drawing to represent the 

 lines of maxinunn stress in a cantilever beam; the same notation 

 is used in both figures. 



The lines of maximum stress shown in figure 12b exist in every 

 loaded beam on two supports; while in loaded cantilever beams 

 somewhat similar lines also exist. The direction of the lines 

 depends somewhat on the manner in which the loads are applied 

 to the beam, and the amount of these maximum stresses, of 

 course, varies with the amount of the load. The general for- 

 mulas given for the determination of the amounts and directions 

 of these maximum stresses hold true for all beams. The inter- 

 nal strength of the beam must be exerted in the paths of these 

 maximum stresses in order to oppose and balance them. There 

 is an infinite number of these lines of stress, of which those 

 drawn in figs. 12a, h are only a small part, but whose shape and 

 position is characteristic. The cohesion of the fibers of the 

 beam, its shearing strength and its resistance to compression 

 and to tension combine in the beam to resist the maximum 

 stresses acting in these peculiar curved paths. The beam 

 would act a great deal more efficiently if the material were 

 laid down in curved lines pursuing the same paths as the lines 

 of maximum stress: in this manner considerably less material 

 would be required to support the same external load. 



In Part III, the lines of maximum tensile and compressive 

 stress will be drawn in a similar manner in the femur, after com- 

 putation of their magnitude and direction, by applying the 

 principles and methods of mechanics discussed in this section. 



Properties of hone 



In order to investigate the structure of bone from a mechani- 

 cal point of view, it is necessary to determine the physical prop- 

 erties of this material and to have proof that it obeys similar 

 laws to those that obtain for other materials such as wood, iron, 

 steel, concrete, etc. 



Through experiment and the testing of sections of animal and 

 human bones in testing machines it has been fully demon- 



