THE BONES. 41 



of birds, which deserve attention. Look at a bird fast asleep 

 with its head under its wing, resting usually on one leg, 

 and that again resting upon a perch hot thicker than one's 

 finger. How apparently difficult and uneasy is such a 

 position ! and yet it is all made easy and safe to the bird 

 by an admirable piece of internal mechanism, which may be 

 briefly thus explained. Connected with the thigh-bone and 

 leg, a set of muscles run down to the very extremity of the 

 toes, so contrived and placed, that when, by pressure down- 

 wards, the limb bends, these fine muscles are pulled in, and 

 therefore contract the toes, thus making them grasp more 

 firmly whatever the bird is resting upon; just as if a set of 

 fine strings ran over pulleys to certain hooks, and were acted 

 upon at the other end by a weight or pressure, and thereby 

 made to draw in the hooks. 



But there are some birds, and very large ones too, such 

 as 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, which would inevitably overset the sleeper. Accord- 

 ingly, 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, affording a sufficient degree of resist- 

 ance 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. 



