THE BONES. 43 



There are some other beautiful contrivances in the 

 limbs 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 not 

 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. Con- 

 nected 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 downwards, 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. 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, affording 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 conve- 

 nience. 



