216 



THE CIVIL ENGINEERS OE THE BODY 



Fig. 50. 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 tw^o systems of compression 

 lines. These tension lines will be seen in Fig. 52 forming curves 

 with their concavities upwards and orthogonal to the compression 

 lines. They are continued forward by the fibres of the plantar 

 ligaments which act as the tie-bar of the arch. It will be noticed 

 that these ties in the calcaneus are closer together at the arch 

 of the bone between the two struts, i.e. at the point where 



-TIBIA 



ARTICULAR CARTILAGE 



CAPSULE 



Fig. 52. — Diagram .sliowing some of the stress lines in the arch of the foot. (After 

 Hermann Meyer.) 



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



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 

 MacAlister puts it, " any mass of bone put there tvould not ' row its 

 weight,' and it has been ' turned out '." 



This internal structure is altered to meet alterations in the 

 incidence of stress. For example, during the first twenty years 

 of life when the body is growing and the bone lengthening, constant 

 alterations in internal and external structure have to be made. 

 The unnecessary parts are decalcified and the fibrous tissue under- 

 goes alteration. During this process some of the fibrous tissue 



