LAWS OF BONE ARCHITECTURE 261 



architecture of the shaft without distortion and ofTer the most 

 accurate basis for determining the true strength of this part of 

 the femur. 



The shape of section 20 is almost circular, with a similar hol- 

 low space within. Distal to this section the shape of the shaft 

 gradually changes, becoming somewhat elliptical in section 28. 

 The spongy bone in the sections between 20 and 28 becomes 

 markedly smaller in amount, in section 28 only part of the area 

 within the compact shell being occupied by spongy bone. Be- 

 j'ond section 28 the area of spongy bone is so small as to be 

 negligible in computing the strength of the bone, until the distal 

 part of the femur is reached. 



The outline of the shaft from section 32 to section 52 is pear- 

 shaped, with a fairly symmetrical hollow space within, which 

 produces an annular figure of variable thickness. 



The chief value of the transverse sections is that they give 

 an accurate basis for computing the strength of the shaft, and 

 they give the true outlines of the femur at successive sections. 



The distal portion of the femur 



This portion includes all of the femur distal to section 52 

 where the shaft begins to increase in gross area of cross section. 



1. Frontal section. Figure 25 shows the middle frontal ; ec- 

 tion through the femur. There are seen to be two main systems 

 of trabeculae, a longitudinal and a transverse system. The 

 trabeculae of the former rise from the inner wall of the shaft 

 and continue in perfectly straight lines parallel to the axis of the 

 shaft and proceed to the epiphyseal line, whence they continue 

 in more or less curved lines to meet the articular surface of the 

 knee-joint at right angles at every point. Near the center 

 there are a few thin, delicate, longitudinal trabeculae which 

 spring from the longitudinal trabeculae just described, to which 

 they are joined by fine transverse filaments that lie in planes 

 parallel to the sagittal plane. 



The trabeculae of the transverse system are somewhat lighter 

 in structure than those of the longitudinal system, and consist 

 of numerous trabeculae at right angles to the latter. 



