STRESS LINES 



177 



overhung by the loaded head, is the compression member. The 

 head of the femur is a little more complicated than Fairbairn's 

 crane, in that the load is applied on two points, i.e. on the head of 

 the bone and on the great trochanter. This entails a division in 

 the distribution of the stress lines corresponding to the incidence 

 of the loads. In the compact tissue of the shaft the tension and 

 compression lines run parallel. The lines of stress are closest 

 together at the point of greatest strain, i.e. in midshaft. This 

 place has to be thickened to prevent the bone from snapping 

 (a walking stick pressed vertically against the floor breaks half- 



INCIDENCE OF LOAD 



FIG. 31. To show the stress lines in the head of the Femur, A, in section, andjU, 

 on the surface. The central diaaram gives an idea of the location of the lines of 

 stress in the head of a crane. (After Culmann, Meyer and Dixon.) 



way up). The central portion of the shaft has to bear no strain, 

 and therefore is hollow. It may be considered as a large mesh 

 between the tension and compression lines. In the cancellous 

 tissue the tension lines cross the compression lines at right 

 angles. 



The same phenomenon may be seen in any bone which undergoes 

 tension and compression. It is very noticeable in the human foot, 

 especially in the heel bone (calcaneus). It is roughly triangular, 

 having three bearing surfaces. The upper surface is compressed 

 by the weight of the body applied from the ankle bone. There- 

 fore, compression lines start from it and run downwards. The 

 lower surface rests on the ground, i.e. has to bear an upward 



B.B. 12 



