THE BONES. 41 



Herons, Cranes, and certain other wading birds, which sleep 

 standing on one foot, the leg being kept perfectly straight; 

 and in these birds, as the foot rests upon the ground, any 

 grasping power acting upon the toes would be useless. In this 

 case, therefore, what is required is some contrivance to keep 

 the leg firm, that it may not give way, in consequence of any 

 swinging or balancing motion of the body, wliich would 

 inevitably overset the sleeper. Accordingly, the upper part of 

 what we shall call the heel-bone, at c, is fitted in a very 

 curious manner into the lower part of the leg-bone above it, 

 and precisely answers the purpose required of the bird, afford- 

 ing a sufficient degree of resistance by the bending of the leg, 

 to counteract the effect of any overbalancing of the body. The 

 bird, moreover, has a power over this well-contrived prop, and 

 thus bends its leg or keeps it perfectly stiff, as best suits its 

 convenience. 



