LAWS OF BONE ARCHITECTURE 271 



to the longitudinal axis, transmitting it eventually to the articu- 

 lar surface, in a direction perpendicular to that surface. 



Referring to figure 25, which shows the essential features of 

 the lower fenuu* in longitudinal frontal section, it is seen that 

 the large ex])ansi()n of the bone is produced by the gradual transi- 

 tion of the hollow shaft of compact bone to cancellated bone, 

 resulting in the production of a much larger volume. The tra- 

 becule are given off from the shaft in lines parallel to the longi- 

 tudinal axis, and are braced transversely by two series of tra- 

 beculae at right angles to each other, in the same manner as 

 required theoretically for economy. This construction is an 

 excellent illustration of the principle that a long, slender column 

 (as a single trabecula) braced at frequent intervals, acts as a 

 short column whose height is the distance between the braces. 

 The strength of the femur where the spongy bone forms a large 

 part of the cross section, is somewhat reduced, because the 

 trabeculae are not capable of carrying quite as large stresses as 

 an equal amount of bone in a compact mass. The material in 

 spongy bone comprising the system of transverse bracing is not 

 effective in carrying the stresses in a longitudinal direction, 

 while in the compact bone practically all of the material is effec- 

 tive in transmitting stress in a longitudinal direction. 



Referring to figure 17 (also see fig. 18), where the maximum 

 unit-stresses in the femur are given in diagrammatic form, for a 

 load of 100 pounds on the femur-head, it will be seen that the 

 maximum unit-stresses at section 52 are 817 pounds compres- 

 sion and 465 pounds per square inch tension, on the medial and 

 lateral sides, respectively. It will be seen that the intensity of 

 these maximum unit-stresses decreases rapidly to 356 pounds 

 compression and 8 pounds per square inch tension, respectivel}^ 

 at section 64. Distal to this section (64) the stresses on both 

 medial and lateral sides of the neutral axis becomes compressive 

 and gradually approach the value of 65 pounds per square inch 

 uniformly distributed over section 75. 



The actual compressive strength of the lower femur decreases 

 gradually as the compact bone is replaced by spongy bone. This 

 decrease in strength roughly parallels the decrease in the inten- 



