STRESS LINES 



2 15 



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- 

 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 conipression lines. In the cancellous 



INCIDENCE OF lOAD 



Fig. 51. — To show the stress lines in tlie head of the Femur, A, in section, and B, mi 

 the surface. The central diagram gives an idea of the location of the lines of stress in the 

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



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 

 thrust, and so compression lines run upwards from this surface 

 to meet the compression lines from the upper surface. The third 

 or anterior surface is in contact with the bones of the arch of the 

 foot and transmits the ankle pressure forwards to them. This 

 gives rise to a second system of compression lines running obliquely 

 forwards. These two systems correspond to the two beams in 



