90 A PRIMER OF BIOLOGY 



the bones of the foot, on the other, the heel bone 

 for its two struts, while the tie beam is the muscle 



FIG. 42. Human ankle. 



FIG. 43. Raf er. 



and sinew of the sole. Obviously, the tie beam 

 must not in this case be permanently rigid, but capable 



of being made either 

 rigid or flexible as 

 need requires. In 

 the plant, struts are 

 frequently adopted 

 by tall, top-heavy 

 trees, the earth form- 

 ing the tie beam. 



The principle of 

 the crane, again, is 

 very well exempli- 

 fied in the human 

 thigh bone (Fig. 39). 

 When the body 

 is bent forward, 

 the weight rests on 

 the knuckle of the 

 of the iple thigh bone, revolving in the socket of the hip bone, 

 crane. and is liable to " sheering." How is this avoided in a 



FIG. 44. Crane showing lattice (girder) 

 shaft and solid head. 



