THE BONES. 
49 
placed, that when, by pressure downwards, the limb 
bends, these fine muscles are pulled in, and there- 
fore 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 he useless. In 
this case, therefore, what is required, is some con- 
trivance 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 hone, at c, is fitted in a very 
curious manner into the lower part of the leg hone 
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, more- 
over, has a power over this well-contrived prop, and 
thus bends its leg or keeps it perfectly stiff, as best 
suits its convenience. 
VOL. i. 
E 
