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 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. 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. 
