MOTION AND LOCOMOTION 89 



weight to be supported is at all likely to bear too 

 heavily on the walls, the " struts " which meet at 

 the apex of the roof, and which would, at their free 

 ends, tend to force the walls outwards are connected 

 by a " tie beam." The struts are obviously in a 



FIG. 41. Transverse section of a root, showing aggregation 

 of vascular and skeletal tissue in the centre. ( x 50. ) 



state of compression and the tie beam in a state of 

 tension. When these strains are equal, the rafter 

 is a rigid system, and will then bear down vertically 

 on the walls without exerting any tendency to force 

 the walls outwards. Now the trunk of the body 

 bears down through the long bones of the legs on 

 the arch of the ankle. The ankle has, on the one side, 



