178 



THE CIVIL ENGINEERS OF THE BODY 



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 

 Fig. 30. Now the application of a load at the apex would 

 cause the beams to diverge at their lower ends if they were 

 not tied together by the girder. So tension lines must exist 

 to prevent the fracture of the bone between the two systems 

 of compression lines. These tension lines will be seen in Fig. 32 



TIBIA 



CAPSULE 



FIG. 32. Diagram showing some of the stress linea in the arch of the foot. (After 

 Hermann Meyer.) 

 (The diagram is not strictly a section, and the stress lines are not all in one plane.) 



forming curves with their concavities upwards and orthogonal 

 to the compression lines. It will be noticed that these ties are 

 closer together at the arch of the bone between the two struts, 

 i.e. at the point where fracture is most likely to take place. Just 

 above this point no stress lines can be seen, i.e. there is a neutral 

 region. Examination of the bone makes clear the fact that in, 

 this neutral zone trabeculae are almost entirely absent. Where 

 there are no stress lines it would be a waste of material to build 

 struts or ties. As Sir Donald MacAlistcr puts it, " any mass 

 of bone put there would not ' row its weight,' and it has been 

 4 turned out '." 



